- Scan ID:
- 54b2a126-e1d0-4b89-b6e6-acedff1138d2Finished
- Submitted URL:
- https://react.dev/learn/lifecycle-of-reactive-effects
- Report Finished:
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| Link | Text |
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| https://github.com/facebook/react/releases | Open on GitHub |
| https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/is | Object.is |
| https://developer.mozilla.org/en-US/docs/Web/API/Location/pathname | location.pathname |
| https://opensource.fb.com/ | Meta Open Source |
| https://twitter.com/sawaratsuki1004 | @sawaratsuki1004 |
| https://github.com/facebook/react/blob/main/CODE_OF_CONDUCT.md | Code of Conduct |
| https://reactnative.dev/ | React Native |
| https://opensource.facebook.com/legal/privacy | Privacy |
| https://opensource.fb.com/legal/terms/ | Terms |
| https://www.facebook.com/react | React on Facebook |
JavaScript Variables · 26 found
Global JavaScript variables loaded on the window object of a page, are variables declared outside of functions and accessible from anywhere in the code within the current scope
| Name | Type |
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| 0 | object |
| 1 | object |
| 2 | object |
| 3 | object |
| 4 | object |
| onbeforetoggle | object |
| documentPictureInPicture | object |
| onscrollend | object |
| gtag | function |
| dataLayer | object |
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Messages logged to the web console
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class="w-full pt-6 scrolling-touch lg:h-auto grow pe-0 lg:pe-5 lg:pb-16 md:pt-4 lg:pt-4 scrolling-gpu"><!--$--><ul><h3 class="mb-1 text-sm font-bold ms-5 text-tertiary dark:text-tertiary-dark">GET STARTED</h3><li><a title="Quick Start" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between ps-5 text-base font-bold text-primary dark:text-primary-dark" href="/learn"><div>Quick Start<!-- --> </div><span class="pe-1 text-tertiary dark:text-tertiary-dark"><svg xmlns="http://www.w3.org/2000/svg" width="20" height="20" viewBox="0 0 20 20" class="duration-100 ease-in transition -rotate-90 rtl:rotate-90" style="min-width:20px;min-height:20px"><g fill="none" fill-rule="evenodd" transform="translate(-446 -398)"><path fill="currentColor" fill-rule="nonzero" d="M95.8838835,240.366117 C95.3957281,239.877961 94.6042719,239.877961 94.1161165,240.366117 C93.6279612,240.854272 93.6279612,241.645728 94.1161165,242.133883 L98.6161165,246.633883 C99.1042719,247.122039 99.8957281,247.122039 100.383883,246.633883 L104.883883,242.133883 C105.372039,241.645728 105.372039,240.854272 104.883883,240.366117 C104.395728,239.877961 103.604272,239.877961 103.116117,240.366117 L99.5,243.982233 L95.8838835,240.366117 Z" transform="translate(356.5 164.5)"></path><polygon points="446 418 466 418 466 398 446 398"></polygon></g></svg></span></a><div class="opacity-50" style="transition:opacity 250ms ease-in-out"><div id="react-collapsed-panel-:R8im6:" aria-hidden="true" role="region" style="box-sizing:border-box;display:none;height:0px;overflow:hidden"><ul><li><a title="Tutorial: Tic-Tac-Toe" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/tutorial-tic-tac-toe"><div>Tutorial: Tic-Tac-Toe<!-- --> </div></a></li><li><a title="Thinking in React" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/thinking-in-react"><div>Thinking in React<!-- --> </div></a></li></ul></div></div></li><li><a title="Installation" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between ps-5 text-base font-bold text-primary dark:text-primary-dark" href="/learn/installation"><div>Installation<!-- --> </div><span class="pe-1 text-tertiary dark:text-tertiary-dark"><svg xmlns="http://www.w3.org/2000/svg" width="20" height="20" viewBox="0 0 20 20" class="duration-100 ease-in transition -rotate-90 rtl:rotate-90" style="min-width:20px;min-height:20px"><g fill="none" fill-rule="evenodd" transform="translate(-446 -398)"><path fill="currentColor" fill-rule="nonzero" d="M95.8838835,240.366117 C95.3957281,239.877961 94.6042719,239.877961 94.1161165,240.366117 C93.6279612,240.854272 93.6279612,241.645728 94.1161165,242.133883 L98.6161165,246.633883 C99.1042719,247.122039 99.8957281,247.122039 100.383883,246.633883 L104.883883,242.133883 C105.372039,241.645728 105.372039,240.854272 104.883883,240.366117 C104.395728,239.877961 103.604272,239.877961 103.116117,240.366117 L99.5,243.982233 L95.8838835,240.366117 Z" transform="translate(356.5 164.5)"></path><polygon points="446 418 466 418 466 398 446 398"></polygon></g></svg></span></a><div class="opacity-50" style="transition:opacity 250ms ease-in-out"><div id="react-collapsed-panel-:R8qm6:" aria-hidden="true" role="region" style="box-sizing:border-box;display:none;height:0px;overflow:hidden"><ul><li><a title="Start a New React Project" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/start-a-new-react-project"><div>Start a New React Project<!-- --> </div></a></li><li><a title="Add React to an Existing Project" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/add-react-to-an-existing-project"><div>Add React to an Existing Project<!-- --> </div></a></li><li><a title="Editor Setup" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/editor-setup"><div>Editor Setup<!-- --> </div></a></li><li><a title="Using TypeScript" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/typescript"><div>Using TypeScript<!-- --> </div></a></li><li><a title="React Developer Tools" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/react-developer-tools"><div>React Developer Tools<!-- --> </div></a></li><li><a title="React Compiler" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/react-compiler"><div>React Compiler<!-- --> <svg class="ms-2 text-gray-30 dark:text-gray-60 inline-block w-4 h-4 align-[-3px]" width="20px" height="20px" viewBox="0 0 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13.4856467,17.5000531 C12.6833967,17.7332867 11.0855382,17.9999981 7.99965585,17.9999981 Z" id="Shape"></path></g></g></svg></div></a></li></ul></div></div></li><li role="separator" class="mt-4 mb-2 ms-5 border-b border-border dark:border-border-dark"></li><h3 class="mb-1 text-sm font-bold ms-5 text-tertiary dark:text-tertiary-dark mt-2">LEARN REACT</h3><li><a title="Describing the UI" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between ps-5 text-base font-bold text-primary dark:text-primary-dark" href="/learn/describing-the-ui"><div>Describing the UI<!-- --> </div><span class="pe-1 text-tertiary dark:text-tertiary-dark"><svg xmlns="http://www.w3.org/2000/svg" width="20" height="20" viewBox="0 0 20 20" class="duration-100 ease-in transition -rotate-90 rtl:rotate-90" style="min-width:20px;min-height:20px"><g fill="none" fill-rule="evenodd" transform="translate(-446 -398)"><path fill="currentColor" fill-rule="nonzero" d="M95.8838835,240.366117 C95.3957281,239.877961 94.6042719,239.877961 94.1161165,240.366117 C93.6279612,240.854272 93.6279612,241.645728 94.1161165,242.133883 L98.6161165,246.633883 C99.1042719,247.122039 99.8957281,247.122039 100.383883,246.633883 L104.883883,242.133883 C105.372039,241.645728 105.372039,240.854272 104.883883,240.366117 C104.395728,239.877961 103.604272,239.877961 103.116117,240.366117 L99.5,243.982233 L95.8838835,240.366117 Z" transform="translate(356.5 164.5)"></path><polygon points="446 418 466 418 466 398 446 398"></polygon></g></svg></span></a><div class="opacity-50" style="transition:opacity 250ms ease-in-out"><div id="react-collapsed-panel-:R9am6:" aria-hidden="true" role="region" style="box-sizing:border-box;display:none;height:0px;overflow:hidden"><ul><li><a title="Your First Component" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/your-first-component"><div>Your First Component<!-- --> </div></a></li><li><a title="Importing and Exporting Components" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/importing-and-exporting-components"><div>Importing and Exporting Components<!-- --> </div></a></li><li><a title="Writing Markup with JSX" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/writing-markup-with-jsx"><div>Writing Markup with JSX<!-- --> </div></a></li><li><a title="JavaScript in JSX with Curly Braces" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/javascript-in-jsx-with-curly-braces"><div>JavaScript in JSX with Curly Braces<!-- --> </div></a></li><li><a title="Passing Props to a Component" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/passing-props-to-a-component"><div>Passing Props to a Component<!-- --> </div></a></li><li><a title="Conditional Rendering" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/conditional-rendering"><div>Conditional Rendering<!-- --> </div></a></li><li><a title="Rendering Lists" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/rendering-lists"><div>Rendering Lists<!-- --> </div></a></li><li><a title="Keeping Components Pure" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/keeping-components-pure"><div>Keeping Components Pure<!-- --> </div></a></li><li><a title="Your UI as a Tree" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/understanding-your-ui-as-a-tree"><div>Your UI as a Tree<!-- --> </div></a></li></ul></div></div></li><li><a title="Adding Interactivity" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between ps-5 text-base font-bold text-primary dark:text-primary-dark" href="/learn/adding-interactivity"><div>Adding Interactivity<!-- --> </div><span class="pe-1 text-tertiary dark:text-tertiary-dark"><svg xmlns="http://www.w3.org/2000/svg" width="20" height="20" viewBox="0 0 20 20" class="duration-100 ease-in transition -rotate-90 rtl:rotate-90" style="min-width:20px;min-height:20px"><g fill="none" fill-rule="evenodd" transform="translate(-446 -398)"><path fill="currentColor" fill-rule="nonzero" d="M95.8838835,240.366117 C95.3957281,239.877961 94.6042719,239.877961 94.1161165,240.366117 C93.6279612,240.854272 93.6279612,241.645728 94.1161165,242.133883 L98.6161165,246.633883 C99.1042719,247.122039 99.8957281,247.122039 100.383883,246.633883 L104.883883,242.133883 C105.372039,241.645728 105.372039,240.854272 104.883883,240.366117 C104.395728,239.877961 103.604272,239.877961 103.116117,240.366117 L99.5,243.982233 L95.8838835,240.366117 Z" transform="translate(356.5 164.5)"></path><polygon points="446 418 466 418 466 398 446 398"></polygon></g></svg></span></a><div class="opacity-50" style="transition:opacity 250ms ease-in-out"><div id="react-collapsed-panel-:R9im6:" aria-hidden="true" role="region" style="box-sizing:border-box;display:none;height:0px;overflow:hidden"><ul><li><a title="Responding to Events" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/responding-to-events"><div>Responding to Events<!-- --> </div></a></li><li><a title="State: A Component's Memory" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/state-a-components-memory"><div>State: A Component's Memory<!-- --> </div></a></li><li><a title="Render and Commit" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/render-and-commit"><div>Render and Commit<!-- --> </div></a></li><li><a title="State as a Snapshot" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/state-as-a-snapshot"><div>State as a Snapshot<!-- --> </div></a></li><li><a title="Queueing a Series of State Updates" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/queueing-a-series-of-state-updates"><div>Queueing a Series of State Updates<!-- --> </div></a></li><li><a title="Updating Objects in State" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/updating-objects-in-state"><div>Updating Objects in State<!-- --> </div></a></li><li><a title="Updating Arrays in State" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/updating-arrays-in-state"><div>Updating Arrays in State<!-- --> </div></a></li></ul></div></div></li><li><a title="Managing State" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between ps-5 text-base font-bold text-primary dark:text-primary-dark" href="/learn/managing-state"><div>Managing State<!-- --> </div><span class="pe-1 text-tertiary dark:text-tertiary-dark"><svg xmlns="http://www.w3.org/2000/svg" width="20" height="20" viewBox="0 0 20 20" class="duration-100 ease-in transition -rotate-90 rtl:rotate-90" style="min-width:20px;min-height:20px"><g fill="none" fill-rule="evenodd" transform="translate(-446 -398)"><path fill="currentColor" fill-rule="nonzero" d="M95.8838835,240.366117 C95.3957281,239.877961 94.6042719,239.877961 94.1161165,240.366117 C93.6279612,240.854272 93.6279612,241.645728 94.1161165,242.133883 L98.6161165,246.633883 C99.1042719,247.122039 99.8957281,247.122039 100.383883,246.633883 L104.883883,242.133883 C105.372039,241.645728 105.372039,240.854272 104.883883,240.366117 C104.395728,239.877961 103.604272,239.877961 103.116117,240.366117 L99.5,243.982233 L95.8838835,240.366117 Z" transform="translate(356.5 164.5)"></path><polygon points="446 418 466 418 466 398 446 398"></polygon></g></svg></span></a><div class="opacity-50" style="transition:opacity 250ms ease-in-out"><div id="react-collapsed-panel-:R9qm6:" aria-hidden="true" role="region" style="box-sizing:border-box;display:none;height:0px;overflow:hidden"><ul><li><a title="Reacting to Input with State" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/reacting-to-input-with-state"><div>Reacting to Input with State<!-- --> </div></a></li><li><a title="Choosing the State Structure" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/choosing-the-state-structure"><div>Choosing the State Structure<!-- --> </div></a></li><li><a title="Sharing State Between Components" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/sharing-state-between-components"><div>Sharing State Between Components<!-- --> </div></a></li><li><a title="Preserving and Resetting State" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/preserving-and-resetting-state"><div>Preserving and Resetting State<!-- --> </div></a></li><li><a title="Extracting State Logic into a Reducer" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/extracting-state-logic-into-a-reducer"><div>Extracting State Logic into a Reducer<!-- --> </div></a></li><li><a title="Passing Data Deeply with Context" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/passing-data-deeply-with-context"><div>Passing Data Deeply with Context<!-- --> </div></a></li><li><a title="Scaling Up with Reducer and Context" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/scaling-up-with-reducer-and-context"><div>Scaling Up with Reducer and Context<!-- --> </div></a></li></ul></div></div></li><li><a title="Escape Hatches" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between ps-5 text-base font-bold text-primary dark:text-primary-dark" href="/learn/escape-hatches"><div>Escape Hatches<!-- --> </div><span class="pe-1 text-link dark:text-link-dark"><svg xmlns="http://www.w3.org/2000/svg" width="20" height="20" viewBox="0 0 20 20" class="duration-100 ease-in transition rotate-0" style="min-width:20px;min-height:20px"><g fill="none" fill-rule="evenodd" transform="translate(-446 -398)"><path fill="currentColor" fill-rule="nonzero" d="M95.8838835,240.366117 C95.3957281,239.877961 94.6042719,239.877961 94.1161165,240.366117 C93.6279612,240.854272 93.6279612,241.645728 94.1161165,242.133883 L98.6161165,246.633883 C99.1042719,247.122039 99.8957281,247.122039 100.383883,246.633883 L104.883883,242.133883 C105.372039,241.645728 105.372039,240.854272 104.883883,240.366117 C104.395728,239.877961 103.604272,239.877961 103.116117,240.366117 L99.5,243.982233 L95.8838835,240.366117 Z" transform="translate(356.5 164.5)"></path><polygon points="446 418 466 418 466 398 446 398"></polygon></g></svg></span></a><div class="opacity-100" style="transition:opacity 250ms ease-in-out"><div id="react-collapsed-panel-:Ra2m6:" aria-hidden="false" role="region" style="box-sizing:border-box"><ul><li><a title="Referencing Values with Refs" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/referencing-values-with-refs"><div>Referencing Values with Refs<!-- --> </div></a></li><li><a title="Manipulating the DOM with Refs" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/manipulating-the-dom-with-refs"><div>Manipulating the DOM with Refs<!-- --> </div></a></li><li><a title="Synchronizing with Effects" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/synchronizing-with-effects"><div>Synchronizing with Effects<!-- --> </div></a></li><li><a title="You Might Not Need an Effect" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/you-might-not-need-an-effect"><div>You Might Not Need an Effect<!-- --> </div></a></li><li><a title="Lifecycle of Reactive Effects" target="" aria-current="page" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-link dark:text-link-dark bg-highlight dark:bg-highlight-dark border-blue-40 hover:bg-highlight hover:text-link dark:hover:bg-highlight-dark dark:hover:text-link-dark" href="/learn/lifecycle-of-reactive-effects"><div>Lifecycle of Reactive Effects<!-- --> </div></a></li><li><a title="Separating Events from Effects" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/separating-events-from-effects"><div>Separating Events from Effects<!-- --> </div></a></li><li><a title="Removing Effect Dependencies" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/removing-effect-dependencies"><div>Removing Effect Dependencies<!-- --> </div></a></li><li><a title="Reusing Logic with Custom Hooks" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/reusing-logic-with-custom-hooks"><div>Reusing Logic with Custom Hooks<!-- --> </div></a></li></ul></div></div></li></ul><!--/$--><div class="h-20"></div></nav><div class="fixed bottom-0 hidden lg:block"><div class="max-w-custom-xs w-80 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dark:text-link-dark text-sm tracking-wide font-bold uppercase me-1 hover:underline" href="/learn">Learn React</a><span class="inline-block me-1 text-link dark:text-link-dark text-lg rtl:rotate-180"><svg width="20" height="20" viewBox="0 0 20 20" fill="none" xmlns="http://www.w3.org/2000/svg"><path d="M6.86612 13.6161C6.37796 14.1043 6.37796 14.8957 6.86612 15.3839C7.35427 15.872 8.14572 15.872 8.63388 15.3839L13.1339 10.8839C13.622 10.3957 13.622 9.60428 13.1339 9.11612L8.63388 4.61612C8.14572 4.12797 7.35427 4.12797 6.86612 4.61612C6.37796 5.10428 6.37796 5.89573 6.86612 6.38388L10.4822 10L6.86612 13.6161Z" fill="currentColor"></path></svg></span></div><div class="flex mb-3 mt-0.5 items-center"><a class="text-link dark:text-link-dark text-sm tracking-wide font-bold uppercase me-1 hover:underline" href="/learn/escape-hatches">Escape Hatches</a><span class="inline-block me-1 text-link dark:text-link-dark text-lg rtl:rotate-180"><svg width="20" height="20" viewBox="0 0 20 20" fill="none" xmlns="http://www.w3.org/2000/svg"><path d="M6.86612 13.6161C6.37796 14.1043 6.37796 14.8957 6.86612 15.3839C7.35427 15.872 8.14572 15.872 8.63388 15.3839L13.1339 10.8839C13.622 10.3957 13.622 9.60428 13.1339 9.11612L8.63388 4.61612C8.14572 4.12797 7.35427 4.12797 6.86612 4.61612C6.37796 5.10428 6.37796 5.89573 6.86612 6.38388L10.4822 10L6.86612 13.6161Z" fill="currentColor"></path></svg></span></div></div><h1 class="mdx-heading mt-0 text-primary dark:text-primary-dark -mx-.5 break-words text-5xl font-display font-bold leading-tight">Lifecycle of Reactive Effects<a href="#undefined" aria-label="Link for this heading" title="Link for this heading" class="mdx-header-anchor hidden"><svg width="1em" height="1em" viewBox="0 0 13 13" xmlns="http://www.w3.org/2000/svg" class="text-gray-70 ms-2 h-5 w-5"><g fill="currentColor" fill-rule="evenodd"><path d="M7.778 7.975a2.5 2.5 0 0 0 .347-3.837L6.017 2.03a2.498 2.498 0 0 0-3.542-.007 2.5 2.5 0 0 0 .006 3.543l1.153 1.15c.07-.29.154-.563.25-.773.036-.077.084-.16.14-.25L3.18 4.85a1.496 1.496 0 0 1 .002-2.12 1.496 1.496 0 0 1 2.12 0l2.124 2.123a1.496 1.496 0 0 1-.333 2.37c.16.246.42.504.685.752z"></path><path d="M5.657 4.557a2.5 2.5 0 0 0-.347 3.837l2.108 2.108a2.498 2.498 0 0 0 3.542.007 2.5 2.5 0 0 0-.006-3.543L9.802 5.815c-.07.29-.154.565-.25.774-.036.076-.084.16-.14.25l.842.84c.585.587.59 1.532 0 2.122-.587.585-1.532.59-2.12 0L6.008 7.68a1.496 1.496 0 0 1 .332-2.372c-.16-.245-.42-.503-.685-.75z"></path></g></svg></a></h1></div></div></div><div class="px-5 sm:px-12"><div class="max-w-7xl mx-auto"><div class="max-w-4xl ms-0 2xl:mx-auto"><div class="font-display text-xl text-primary dark:text-primary-dark leading-relaxed"><p class="whitespace-pre-wrap my-4">Effects have a different lifecycle from components. Components may mount, update, or unmount. An Effect can only do two things: to start synchronizing something, and later to stop synchronizing it. This cycle can happen multiple times if your Effect depends on props and state that change over time. React provides a linter rule to check that you’ve specified your Effect’s dependencies correctly. This keeps your Effect synchronized to the latest props and state.</p></div>
<div class="p-6 xl:p-8 pb-4 xl:pb-6 bg-card dark:bg-card-dark rounded-2xl shadow-inner-border dark:shadow-inner-border-dark text-base text-secondary dark:text-secondary-dark my-8"><h3 class="mdx-heading text-primary dark:text-primary-dark mt-0 mb-3 leading-tight text-2xl font-display leading-9 text-primary dark:text-primary-dark font-bold my-6">You will learn</h3><ul class="ms-6 my-3 list-disc">
<li class="leading-relaxed mb-1">How an Effect’s lifecycle is different from a component’s lifecycle</li>
<li class="leading-relaxed mb-1">How to think about each individual Effect in isolation</li>
<li class="leading-relaxed mb-1">When your Effect needs to re-synchronize, and why</li>
<li class="leading-relaxed mb-1">How your Effect’s dependencies are determined</li>
<li class="leading-relaxed mb-1">What it means for a value to be reactive</li>
<li class="leading-relaxed mb-1">What an empty dependency array means</li>
<li class="leading-relaxed mb-1">How React verifies your dependencies are correct with a linter</li>
<li class="leading-relaxed mb-1">What to do when you disagree with the linter</li>
</ul></div>
<h2 id="the-lifecycle-of-an-effect" class="mdx-heading text-3xl font-display leading-10 text-primary dark:text-primary-dark font-bold my-6">The lifecycle of an Effect <a href="#the-lifecycle-of-an-effect" aria-label="Link for The lifecycle of an Effect " title="Link for The lifecycle of an Effect " class="mdx-header-anchor inline-block"><svg width="1em" height="1em" viewBox="0 0 13 13" xmlns="http://www.w3.org/2000/svg" class="text-gray-70 ms-2 h-5 w-5"><g fill="currentColor" fill-rule="evenodd"><path d="M7.778 7.975a2.5 2.5 0 0 0 .347-3.837L6.017 2.03a2.498 2.498 0 0 0-3.542-.007 2.5 2.5 0 0 0 .006 3.543l1.153 1.15c.07-.29.154-.563.25-.773.036-.077.084-.16.14-.25L3.18 4.85a1.496 1.496 0 0 1 .002-2.12 1.496 1.496 0 0 1 2.12 0l2.124 2.123a1.496 1.496 0 0 1-.333 2.37c.16.246.42.504.685.752z"></path><path d="M5.657 4.557a2.5 2.5 0 0 0-.347 3.837l2.108 2.108a2.498 2.498 0 0 0 3.542.007 2.5 2.5 0 0 0-.006-3.543L9.802 5.815c-.07.29-.154.565-.25.774-.036.076-.084.16-.14.25l.842.84c.585.587.59 1.532 0 2.122-.587.585-1.532.59-2.12 0L6.008 7.68a1.496 1.496 0 0 1 .332-2.372c-.16-.245-.42-.503-.685-.75z"></path></g></svg></a></h2>
<p class="whitespace-pre-wrap my-4">Every React component goes through the same lifecycle:</p>
<ul class="ms-6 my-3 list-disc">
<li class="leading-relaxed mb-1">A component <em>mounts</em> when it’s added to the screen.</li>
<li class="leading-relaxed mb-1">A component <em>updates</em> when it receives new props or state, usually in response to an interaction.</li>
<li class="leading-relaxed mb-1">A component <em>unmounts</em> when it’s removed from the screen.</li>
</ul>
<p class="whitespace-pre-wrap my-4"><strong class="font-bold">It’s a good way to think about components, but <em>not</em> about Effects.</strong> Instead, try to think about each Effect independently from your component’s lifecycle. An Effect describes how to <a class="inline text-link dark:text-link-dark border-b border-link border-opacity-0 hover:border-opacity-100 duration-100 ease-in transition leading-normal" href="/learn/synchronizing-with-effects">synchronize an external system</a> to the current props and state. As your code changes, synchronization will need to happen more or less often.</p>
<p class="whitespace-pre-wrap my-4">To illustrate this point, consider this Effect connecting your component to a chat server:</p>
<!--$--><div dir="ltr" class="sandpack sandpack--codeblock rounded-2xl h-full w-full overflow-x-auto flex items-center bg-wash dark:bg-gray-95 shadow-lg my-8" style="contain:content"><div class="sp-wrapper"><div class="sp-stack"><div class="sp-code-editor"><pre class="sp-cm sp-pristine sp-javascript flex align-start"><code class="sp-pre-placeholder grow-[2]"><div class="cm-line "><span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">serverUrl</span> = <span class="sp-syntax-string">'https://localhost:1234'</span><span class="sp-syntax-punctuation">;</span><br></div><div class="cm-line "><br></div><div class="cm-line "><span class="sp-syntax-keyword">function</span> <span class="sp-syntax-definition">ChatRoom</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-property">roomId</span> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span><br></div><div class="cm-line "> <span class="sp-syntax-definition">useEffect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br></div><div class="cm-line "> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">connection</span> = <span class="sp-syntax-definition">createConnection</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">serverUrl</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br></div><div class="cm-line "> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">connect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br></div><div class="cm-line "> <span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br></div><div class="cm-line "> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">disconnect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br></div><div class="cm-line "> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">;</span><br></div><div class="cm-line "> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">]</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br></div><div class="cm-line "> <span class="sp-syntax-comment">// ...</span><br></div><div class="cm-line "><span class="sp-syntax-punctuation">}</span></div></code></pre></div></div></div></div><!--/$-->
<p class="whitespace-pre-wrap my-4">Your Effect’s body specifies how to <strong class="font-bold">start synchronizing:</strong></p>
<!--$--><div dir="ltr" class="sandpack sandpack--codeblock rounded-2xl h-full w-full overflow-x-auto flex items-center bg-wash dark:bg-gray-95 shadow-lg my-8" style="contain:content"><div class="sp-wrapper"><div class="sp-stack"><div class="sp-code-editor"><pre class="sp-cm sp-pristine sp-javascript flex align-start"><code class="sp-pre-placeholder grow-[2]"><div class="cm-line "> <span class="sp-syntax-comment">// ...</span><br></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">connection</span> = <span class="sp-syntax-definition">createConnection</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">serverUrl</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">connect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br></div><div class="cm-line "> <span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br></div><div class="cm-line "> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">disconnect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br></div><div class="cm-line "> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">;</span><br></div><div class="cm-line "> <span class="sp-syntax-comment">// ...</span></div></code></pre></div></div></div></div><!--/$-->
<p class="whitespace-pre-wrap my-4">The cleanup function returned by your Effect specifies how to <strong class="font-bold">stop synchronizing:</strong></p>
<!--$--><div dir="ltr" class="sandpack sandpack--codeblock rounded-2xl h-full w-full overflow-x-auto flex items-center bg-wash dark:bg-gray-95 shadow-lg my-8" style="contain:content"><div class="sp-wrapper"><div class="sp-stack"><div class="sp-code-editor"><pre class="sp-cm sp-pristine sp-javascript flex align-start"><code class="sp-pre-placeholder grow-[2]"><div class="cm-line "> <span class="sp-syntax-comment">// ...</span><br></div><div class="cm-line "> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">connection</span> = <span class="sp-syntax-definition">createConnection</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">serverUrl</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br></div><div class="cm-line "> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">connect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br></div><div class="cm-line "> <span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">disconnect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br></div><div class="cm-line "> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">;</span><br></div><div class="cm-line "> <span class="sp-syntax-comment">// ...</span></div></code></pre></div></div></div></div><!--/$-->
<p class="whitespace-pre-wrap my-4">Intuitively, you might think that React would <strong class="font-bold">start synchronizing</strong> when your component mounts and <strong class="font-bold">stop synchronizing</strong> when your component unmounts. However, this is not the end of the story! Sometimes, it may also be necessary to <strong class="font-bold">start and stop synchronizing multiple times</strong> while the component remains mounted.</p>
<p class="whitespace-pre-wrap my-4">Let’s look at <em>why</em> this is necessary, <em>when</em> it happens, and <em>how</em> you can control this behavior.</p>
<div class="expandable-callout pt-8 pb-4 px-5 sm:px-8 my-8 relative rounded-none shadow-inner-border -mx-5 sm:mx-auto sm:rounded-2xl bg-green-5 dark:bg-green-60 dark:bg-opacity-20 text-primary dark:text-primary-dark text-lg"><h3 class="text-2xl font-display font-bold text-green-60 dark:text-green-40"><svg class="inline me-3 mb-1 text-lg text-green-60 dark:text-green-40" width="2em" height="2em" viewBox="0 0 72 72" fill="none" xmlns="http://www.w3.org/2000/svg"><g clip-path="url(#clip0_40_48064)"><path d="M24 27C24 25.3431 25.3431 24 27 24H45C46.6569 24 48 25.3431 48 27C48 28.6569 46.6569 30 45 30H27C25.3431 30 24 28.6569 24 27Z" fill="currentColor"></path><path d="M24 39C24 37.3431 25.3431 36 27 36H39C40.6569 36 42 37.3431 42 39C42 40.6569 40.6569 42 39 42H27C25.3431 42 24 40.6569 24 39Z" fill="currentColor"></path><path fill-rule="evenodd" clip-rule="evenodd" d="M12 18C12 13.0294 16.0294 9 21 9H51C55.9706 9 60 13.0294 60 18V54C60 58.9706 55.9706 63 51 63H21C16.0294 63 12 58.9706 12 54V18ZM21 15H51C52.6569 15 54 16.3431 54 18V54C54 55.6569 52.6569 57 51 57H21C19.3431 57 18 55.6569 18 54V18C18 16.3431 19.3431 15 21 15Z" fill="currentColor"></path></g><defs><clipPath id="clip0_40_48064"><rect width="72" height="72" fill="white"></rect></clipPath></defs></svg>Note</h3><div class="relative"><div class="py-2"><p class="whitespace-pre-wrap my-4">Some Effects don’t return a cleanup function at all. <a class="inline text-link dark:text-link-dark border-b border-link border-opacity-0 hover:border-opacity-100 duration-100 ease-in transition leading-normal" href="/learn/synchronizing-with-effects#how-to-handle-the-effect-firing-twice-in-development">More often than not,</a> you’ll want to return one—but if you don’t, React will behave as if you returned an empty cleanup function.</p></div></div></div>
<h3 id="why-synchronization-may-need-to-happen-more-than-once" class="mdx-heading text-2xl font-display leading-9 text-primary dark:text-primary-dark font-bold my-6">Why synchronization may need to happen more than once <a href="#why-synchronization-may-need-to-happen-more-than-once" aria-label="Link for Why synchronization may need to happen more than once " title="Link for Why synchronization may need to happen more than once " class="mdx-header-anchor inline-block"><svg width="1em" height="1em" viewBox="0 0 13 13" xmlns="http://www.w3.org/2000/svg" class="text-gray-70 ms-2 h-5 w-5"><g fill="currentColor" fill-rule="evenodd"><path d="M7.778 7.975a2.5 2.5 0 0 0 .347-3.837L6.017 2.03a2.498 2.498 0 0 0-3.542-.007 2.5 2.5 0 0 0 .006 3.543l1.153 1.15c.07-.29.154-.563.25-.773.036-.077.084-.16.14-.25L3.18 4.85a1.496 1.496 0 0 1 .002-2.12 1.496 1.496 0 0 1 2.12 0l2.124 2.123a1.496 1.496 0 0 1-.333 2.37c.16.246.42.504.685.752z"></path><path d="M5.657 4.557a2.5 2.5 0 0 0-.347 3.837l2.108 2.108a2.498 2.498 0 0 0 3.542.007 2.5 2.5 0 0 0-.006-3.543L9.802 5.815c-.07.29-.154.565-.25.774-.036.076-.084.16-.14.25l.842.84c.585.587.59 1.532 0 2.122-.587.585-1.532.59-2.12 0L6.008 7.68a1.496 1.496 0 0 1 .332-2.372c-.16-.245-.42-.503-.685-.75z"></path></g></svg></a></h3>
<p class="whitespace-pre-wrap my-4">Imagine this <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">ChatRoom</code> component receives a <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">roomId</code> prop that the user picks in a dropdown. Let’s say that initially the user picks the <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">"general"</code> room as the <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">roomId</code>. Your app displays the <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">"general"</code> chat room:</p>
<!--$--><div dir="ltr" class="sandpack sandpack--codeblock rounded-2xl h-full w-full overflow-x-auto flex items-center bg-wash dark:bg-gray-95 shadow-lg my-8" style="contain:content"><div class="sp-wrapper"><div class="sp-stack"><div class="sp-code-editor"><pre class="sp-cm sp-pristine sp-javascript flex align-start"><code class="sp-pre-placeholder grow-[2]"><div class="cm-line "><span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">serverUrl</span> = <span class="sp-syntax-string">'https://localhost:1234'</span><span class="sp-syntax-punctuation">;</span><br></div><div class="cm-line "><br></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"><span class="sp-syntax-keyword">function</span> <span class="sp-syntax-definition">ChatRoom</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-property">roomId</span> <span class="sp-syntax-comment">/* "general" */</span> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span><br></div><div class="cm-line "> <span class="sp-syntax-comment">// ...</span><br></div><div class="cm-line "> <span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">h1</span><span class="sp-syntax-punctuation">></span>Welcome to the <span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">}</span> room!<span class="sp-syntax-punctuation"></</span><span class="sp-syntax-tag">h1</span><span class="sp-syntax-punctuation">></span><span class="sp-syntax-punctuation">;</span><br></div><div class="cm-line "><span class="sp-syntax-punctuation">}</span></div></code></pre></div></div></div></div><!--/$-->
<p class="whitespace-pre-wrap my-4">After the UI is displayed, React will run your Effect to <strong class="font-bold">start synchronizing.</strong> It connects to the <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">"general"</code> room:</p>
<!--$--><div dir="ltr" class="sandpack sandpack--codeblock rounded-2xl h-full w-full overflow-x-auto flex items-center bg-wash dark:bg-gray-95 shadow-lg my-8" style="contain:content"><div class="sp-wrapper"><div class="sp-stack"><div class="sp-code-editor"><pre class="sp-cm sp-pristine sp-javascript flex align-start"><code class="sp-pre-placeholder grow-[2]"><div class="cm-line "><span class="sp-syntax-keyword">function</span> <span class="sp-syntax-definition">ChatRoom</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-property">roomId</span> <span class="sp-syntax-comment">/* "general" */</span> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span><br></div><div class="cm-line "> <span class="sp-syntax-definition">useEffect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">connection</span> = <span class="sp-syntax-definition">createConnection</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">serverUrl</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-comment">// Connects to the "general" room</span><br></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">connect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br></div><div class="cm-line "> <span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br></div><div class="cm-line "> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">disconnect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-comment">// Disconnects from the "general" room</span><br></div><div class="cm-line "> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">;</span><br></div><div class="cm-line "> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">]</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br></div><div class="cm-line "> <span class="sp-syntax-comment">// ...</span></div></code></pre></div></div></div></div><!--/$-->
<p class="whitespace-pre-wrap my-4">So far, so good.</p>
<p class="whitespace-pre-wrap my-4">Later, the user picks a different room in the dropdown (for example, <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">"travel"</code>). First, React will update the UI:</p>
<!--$--><div dir="ltr" class="sandpack sandpack--codeblock rounded-2xl h-full w-full overflow-x-auto flex items-center bg-wash dark:bg-gray-95 shadow-lg my-8" style="contain:content"><div class="sp-wrapper"><div class="sp-stack"><div class="sp-code-editor"><pre class="sp-cm sp-pristine sp-javascript flex align-start"><code class="sp-pre-placeholder grow-[2]"><div class="cm-line bg-github-highlight dark:bg-opacity-10"><span class="sp-syntax-keyword">function</span> <span class="sp-syntax-definition">ChatRoom</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-property">roomId</span> <span class="sp-syntax-comment">/* "travel" */</span> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span><br></div><div class="cm-line "> <span class="sp-syntax-comment">// ...</span><br></div><div class="cm-line "> <span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">h1</span><span class="sp-syntax-punctuation">></span>Welcome to the <span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">}</span> room!<span class="sp-syntax-punctuation"></</span><span class="sp-syntax-tag">h1</span><span class="sp-syntax-punctuation">></span><span class="sp-syntax-punctuation">;</span><br></div><div class="cm-line "><span class="sp-syntax-punctuation">}</span></div></code></pre></div></div></div></div><!--/$-->
<p class="whitespace-pre-wrap my-4">Think about what should happen next. The user sees that <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">"travel"</code> is the selected chat room in the UI. However, the Effect that ran the last time is still connected to the <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">"general"</code> room. <strong class="font-bold">The <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">roomId</code> prop has changed, so what your Effect did back then (connecting to the <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">"general"</code> room) no longer matches the UI.</strong></p>
<p class="whitespace-pre-wrap my-4">At this point, you want React to do two things:</p>
<ol class="ms-6 my-3 list-decimal">
<li class="leading-relaxed mb-1">Stop synchronizing with the old <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">roomId</code> (disconnect from the <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">"general"</code> room)</li>
<li class="leading-relaxed mb-1">Start synchronizing with the new <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">roomId</code> (connect to the <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">"travel"</code> room)</li>
</ol>
<p class="whitespace-pre-wrap my-4"><strong class="font-bold">Luckily, you’ve already taught React how to do both of these things!</strong> Your Effect’s body specifies how to start synchronizing, and your cleanup function specifies how to stop synchronizing. All that React needs to do now is to call them in the correct order and with the correct props and state. Let’s see how exactly that happens.</p>
<h3 id="how-react-re-synchronizes-your-effect" class="mdx-heading text-2xl font-display leading-9 text-primary dark:text-primary-dark font-bold my-6">How React re-synchronizes your Effect <a href="#how-react-re-synchronizes-your-effect" aria-label="Link for How React re-synchronizes your Effect " title="Link for How React re-synchronizes your Effect " class="mdx-header-anchor inline-block"><svg width="1em" height="1em" viewBox="0 0 13 13" xmlns="http://www.w3.org/2000/svg" class="text-gray-70 ms-2 h-5 w-5"><g fill="currentColor" fill-rule="evenodd"><path d="M7.778 7.975a2.5 2.5 0 0 0 .347-3.837L6.017 2.03a2.498 2.498 0 0 0-3.542-.007 2.5 2.5 0 0 0 .006 3.543l1.153 1.15c.07-.29.154-.563.25-.773.036-.077.084-.16.14-.25L3.18 4.85a1.496 1.496 0 0 1 .002-2.12 1.496 1.496 0 0 1 2.12 0l2.124 2.123a1.496 1.496 0 0 1-.333 2.37c.16.246.42.504.685.752z"></path><path d="M5.657 4.557a2.5 2.5 0 0 0-.347 3.837l2.108 2.108a2.498 2.498 0 0 0 3.542.007 2.5 2.5 0 0 0-.006-3.543L9.802 5.815c-.07.29-.154.565-.25.774-.036.076-.084.16-.14.25l.842.84c.585.587.59 1.532 0 2.122-.587.585-1.532.59-2.12 0L6.008 7.68a1.496 1.496 0 0 1 .332-2.372c-.16-.245-.42-.503-.685-.75z"></path></g></svg></a></h3>
<p class="whitespace-pre-wrap my-4">Recall that your <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">ChatRoom</code> component has received a new value for its <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">roomId</code> prop. It used to be <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">"general"</code>, and now it is <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">"travel"</code>. React needs to re-synchronize your Effect to re-connect you to a different room.</p>
<p class="whitespace-pre-wrap my-4">To <strong class="font-bold">stop synchronizing,</strong> React will call the cleanup function that your Effect returned after connecting to the <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">"general"</code> room. Since <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">roomId</code> was <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">"general"</code>, the cleanup function disconnects from the <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">"general"</code> room:</p>
<!--$--><div dir="ltr" class="sandpack sandpack--codeblock rounded-2xl h-full w-full overflow-x-auto flex items-center bg-wash dark:bg-gray-95 shadow-lg my-8" style="contain:content"><div class="sp-wrapper"><div class="sp-stack"><div class="sp-code-editor"><pre class="sp-cm sp-pristine sp-javascript flex align-start"><code class="sp-pre-placeholder grow-[2]"><div class="cm-line "><span class="sp-syntax-keyword">function</span> <span class="sp-syntax-definition">ChatRoom</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-property">roomId</span> <span class="sp-syntax-comment">/* "general" */</span> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span><br></div><div class="cm-line "> <span class="sp-syntax-definition">useEffect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br></div><div class="cm-line "> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">connection</span> = <span class="sp-syntax-definition">createConnection</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">serverUrl</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-comment">// Connects to the "general" room</span><br></div><div class="cm-line "> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">connect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br></div><div class="cm-line "> <span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">disconnect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-comment">// Disconnects from the "general" room</span><br></div><div class="cm-line "> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">;</span><br></div><div class="cm-line "> <span class="sp-syntax-comment">// ...</span></div></code></pre></div></div></div></div><!--/$-->
<p class="whitespace-pre-wrap my-4">Then React will run the Effect that you’ve provided during this render. This time, <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">roomId</code> is <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">"travel"</code> so it will <strong class="font-bold">start synchronizing</strong> to the <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">"travel"</code> chat room (until its cleanup function is eventually called too):</p>
<!--$--><div dir="ltr" class="sandpack sandpack--codeblock rounded-2xl h-full w-full overflow-x-auto flex items-center bg-wash dark:bg-gray-95 shadow-lg my-8" style="contain:content"><div class="sp-wrapper"><div class="sp-stack"><div class="sp-code-editor"><pre class="sp-cm sp-pristine sp-javascript flex align-start"><code class="sp-pre-placeholder grow-[2]"><div class="cm-line "><span class="sp-syntax-keyword">function</span> <span class="sp-syntax-definition">ChatRoom</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-property">roomId</span> <span class="sp-syntax-comment">/* "travel" */</span> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span><br></div><div class="cm-line "> <span class="sp-syntax-definition">useEffect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">connection</span> = <span class="sp-syntax-definition">createConnection</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">serverUrl</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-comment">// Connects to the "travel" room</span><br></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">connect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br></div><div class="cm-line "> <span class="sp-syntax-comment">// ...</span></div></code></pre></div></div></div></div><!--/$-->
<p class="whitespace-pre-wrap my-4">Thanks to this, you’re now connected to the same room that the user chose in the UI. Disaster averted!</p>
<p class="whitespace-pre-wrap my-4">Every time after your component re-renders with a different <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">roomId</code>, your Effect will re-synchronize. For example, let’s say the user changes <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">roomId</code> from <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">"travel"</code> to <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">"music"</code>. React will again <strong class="font-bold">stop synchronizing</strong> your Effect by calling its cleanup function (disconnecting you from the <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">"travel"</code> room). Then it will <strong class="font-bold">start synchronizing</strong> again by running its body with the new <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">roomId</code> prop (connecting you to the <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">"music"</code> room).</p>
<p class="whitespace-pre-wrap my-4">Finally, when the user goes to a different screen, <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">ChatRoom</code> unmounts. Now there is no need to stay connected at all. React will <strong class="font-bold">stop synchronizing</strong> your Effect one last time and disconnect you from the <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">"music"</code> chat room.</p>
<h3 id="thinking-from-the-effects-perspective" class="mdx-heading text-2xl font-display leading-9 text-primary dark:text-primary-dark font-bold my-6">Thinking from the Effect’s perspective <a href="#thinking-from-the-effects-perspective" aria-label="Link for Thinking from the Effect’s perspective " title="Link for Thinking from the Effect’s perspective " class="mdx-header-anchor inline-block"><svg width="1em" height="1em" viewBox="0 0 13 13" xmlns="http://www.w3.org/2000/svg" class="text-gray-70 ms-2 h-5 w-5"><g fill="currentColor" fill-rule="evenodd"><path d="M7.778 7.975a2.5 2.5 0 0 0 .347-3.837L6.017 2.03a2.498 2.498 0 0 0-3.542-.007 2.5 2.5 0 0 0 .006 3.543l1.153 1.15c.07-.29.154-.563.25-.773.036-.077.084-.16.14-.25L3.18 4.85a1.496 1.496 0 0 1 .002-2.12 1.496 1.496 0 0 1 2.12 0l2.124 2.123a1.496 1.496 0 0 1-.333 2.37c.16.246.42.504.685.752z"></path><path d="M5.657 4.557a2.5 2.5 0 0 0-.347 3.837l2.108 2.108a2.498 2.498 0 0 0 3.542.007 2.5 2.5 0 0 0-.006-3.543L9.802 5.815c-.07.29-.154.565-.25.774-.036.076-.084.16-.14.25l.842.84c.585.587.59 1.532 0 2.122-.587.585-1.532.59-2.12 0L6.008 7.68a1.496 1.496 0 0 1 .332-2.372c-.16-.245-.42-.503-.685-.75z"></path></g></svg></a></h3>
<p class="whitespace-pre-wrap my-4">Let’s recap everything that’s happened from the <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">ChatRoom</code> component’s perspective:</p>
<ol class="ms-6 my-3 list-decimal">
<li class="leading-relaxed mb-1"><code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">ChatRoom</code> mounted with <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">roomId</code> set to <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">"general"</code></li>
<li class="leading-relaxed mb-1"><code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">ChatRoom</code> updated with <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">roomId</code> set to <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">"travel"</code></li>
<li class="leading-relaxed mb-1"><code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">ChatRoom</code> updated with <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">roomId</code> set to <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">"music"</code></li>
<li class="leading-relaxed mb-1"><code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">ChatRoom</code> unmounted</li>
</ol>
<p class="whitespace-pre-wrap my-4">During each of these points in the component’s lifecycle, your Effect did different things:</p>
<ol class="ms-6 my-3 list-decimal">
<li class="leading-relaxed mb-1">Your Effect connected to the <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">"general"</code> room</li>
<li class="leading-relaxed mb-1">Your Effect disconnected from the <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">"general"</code> room and connected to the <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">"travel"</code> room</li>
<li class="leading-relaxed mb-1">Your Effect disconnected from the <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">"travel"</code> room and connected to the <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">"music"</code> room</li>
<li class="leading-relaxed mb-1">Your Effect disconnected from the <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">"music"</code> room</li>
</ol>
<p class="whitespace-pre-wrap my-4">Now let’s think about what happened from the perspective of the Effect itself:</p>
<!--$--><div dir="ltr" class="sandpack sandpack--codeblock rounded-2xl h-full w-full overflow-x-auto flex items-center bg-wash dark:bg-gray-95 shadow-lg my-8" style="contain:content"><div class="sp-wrapper"><div class="sp-stack"><div class="sp-code-editor"><pre class="sp-cm sp-pristine sp-javascript flex align-start"><code class="sp-pre-placeholder grow-[2]"><div class="cm-line "> <span class="sp-syntax-definition">useEffect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br></div><div class="cm-line "> <span class="sp-syntax-comment">// Your Effect connected to the room specified with roomId...</span><br></div><div class="cm-line "> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">connection</span> = <span class="sp-syntax-definition">createConnection</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">serverUrl</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br></div><div class="cm-line "> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">connect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br></div><div class="cm-line "> <span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br></div><div class="cm-line "> <span class="sp-syntax-comment">// ...until it disconnected</span><br></div><div class="cm-line "> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">disconnect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br></div><div class="cm-line "> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">;</span><br></div><div class="cm-line "> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">]</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span></div></code></pre></div></div></div></div><!--/$-->
<p class="whitespace-pre-wrap my-4">This code’s structure might inspire you to see what happened as a sequence of non-overlapping time periods:</p>
<ol class="ms-6 my-3 list-decimal">
<li class="leading-relaxed mb-1">Your Effect connected to the <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">"general"</code> room (until it disconnected)</li>
<li class="leading-relaxed mb-1">Your Effect connected to the <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">"travel"</code> room (until it disconnected)</li>
<li class="leading-relaxed mb-1">Your Effect connected to the <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">"music"</code> room (until it disconnected)</li>
</ol>
<p class="whitespace-pre-wrap my-4">Previously, you were thinking from the component’s perspective. When you looked from the component’s perspective, it was tempting to think of Effects as “callbacks” or “lifecycle events” that fire at a specific time like “after a render” or “before unmount”. This way of thinking gets complicated very fast, so it’s best to avoid.</p>
<p class="whitespace-pre-wrap my-4"><strong class="font-bold">Instead, always focus on a single start/stop cycle at a time. It shouldn’t matter whether a component is mounting, updating, or unmounting. All you need to do is to describe how to start synchronization and how to stop it. If you do it well, your Effect will be resilient to being started and stopped as many times as it’s needed.</strong></p>
<p class="whitespace-pre-wrap my-4">This might remind you how you don’t think whether a component is mounting or updating when you write the rendering logic that creates JSX. You describe what should be on the screen, and React <a class="inline text-link dark:text-link-dark border-b border-link border-opacity-0 hover:border-opacity-100 duration-100 ease-in transition leading-normal" href="/learn/reacting-to-input-with-state">figures out the rest.</a></p>
<h3 id="how-react-verifies-that-your-effect-can-re-synchronize" class="mdx-heading text-2xl font-display leading-9 text-primary dark:text-primary-dark font-bold my-6">How React verifies that your Effect can re-synchronize <a href="#how-react-verifies-that-your-effect-can-re-synchronize" aria-label="Link for How React verifies that your Effect can re-synchronize " title="Link for How React verifies that your Effect can re-synchronize " class="mdx-header-anchor inline-block"><svg width="1em" height="1em" viewBox="0 0 13 13" xmlns="http://www.w3.org/2000/svg" class="text-gray-70 ms-2 h-5 w-5"><g fill="currentColor" fill-rule="evenodd"><path d="M7.778 7.975a2.5 2.5 0 0 0 .347-3.837L6.017 2.03a2.498 2.498 0 0 0-3.542-.007 2.5 2.5 0 0 0 .006 3.543l1.153 1.15c.07-.29.154-.563.25-.773.036-.077.084-.16.14-.25L3.18 4.85a1.496 1.496 0 0 1 .002-2.12 1.496 1.496 0 0 1 2.12 0l2.124 2.123a1.496 1.496 0 0 1-.333 2.37c.16.246.42.504.685.752z"></path><path d="M5.657 4.557a2.5 2.5 0 0 0-.347 3.837l2.108 2.108a2.498 2.498 0 0 0 3.542.007 2.5 2.5 0 0 0-.006-3.543L9.802 5.815c-.07.29-.154.565-.25.774-.036.076-.084.16-.14.25l.842.84c.585.587.59 1.532 0 2.122-.587.585-1.532.59-2.12 0L6.008 7.68a1.496 1.496 0 0 1 .332-2.372c-.16-.245-.42-.503-.685-.75z"></path></g></svg></a></h3>
<p class="whitespace-pre-wrap my-4">Here is a live example that you can play with. Press “Open chat” to mount the <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">ChatRoom</code> component:</p>
</div><!--$--><div class="sandpack sandpack--playground w-full my-8" dir="ltr"><div class="sp-wrapper"><div class="shadow-lg dark:shadow-lg-dark rounded-lg" style="contain:content"><div class="bg-wash dark:bg-card-dark flex justify-between items-center relative z-10 border-b border-border dark:border-border-dark rounded-t-lg text-lg"><div class="flex-1 grow min-w-0 px-4 lg:px-6"><div><div class="relative overflow-hidden"><div class="w-[fit-content]"><div class=" sp-tabs" translate="no"><div aria-label="Select active file" class=" sp-tabs-scrollable-container" role="tablist"><button aria-selected="true" class=" sp-tab-button" data-active="true" role="tab" title="/src/App.js" type="button">App.js</button><button aria-selected="false" class=" sp-tab-button" data-active="false" role="tab" title="/src/chat.js" type="button">chat.js</button></div></div></div><button class="absolute top-0 start-[2px] invisible" id="headlessui-listbox-button-:R9iicq6:" aria-haspopup="true" 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8.85355C21.9474 8.75979 22.0001 8.63261 22.0001 8.5V3.5C22.0001 3.10218 21.8421 2.72064 21.5608 2.43934C21.2795 2.15804 20.8979 2 20.5001 2V2Z" fill="currentColor"></path><path d="M21.5 13H20.5C20.3674 13 20.2402 13.0527 20.1464 13.1464C20.0527 13.2402 20 13.3674 20 13.5V20H4V4H10.5C10.6326 4 10.7598 3.94732 10.8536 3.85355C10.9473 3.75979 11 3.63261 11 3.5V2.5C11 2.36739 10.9473 2.24021 10.8536 2.14645C10.7598 2.05268 10.6326 2 10.5 2H3.5C3.10218 2 2.72064 2.15804 2.43934 2.43934C2.15804 2.72064 2 3.10218 2 3.5V20.5C2 20.8978 2.15804 21.2794 2.43934 21.5607C2.72064 21.842 3.10218 22 3.5 22H20.5C20.8978 22 21.2794 21.842 21.5607 21.5607C21.842 21.2794 22 20.8978 22 20.5V13.5C22 13.3674 21.9473 13.2402 21.8536 13.1464C21.7598 13.0527 21.6326 13 21.5 13Z" fill="currentColor"></path></svg><span class="hidden md:block">Fork</span></button></div></div><div class=" sp-layout"><div class=" sp-editor sp-stack"><div class=" sp-code-editor"><div aria-autocomplete="list" aria-label="Code Editor for App.js" aria-multiline="true" class="sp-pristine sp-javascript sp-cm" role="textbox" tabindex="0" translate="no"><pre class=" sp-pre-placeholder" style="margin-left:var(--sp-space-11)"><span class="sp-syntax-keyword">import</span> <span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-plain">useState</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">useEffect</span> <span class="sp-syntax-punctuation">}</span> <span class="sp-syntax-keyword">from</span> <span class="sp-syntax-string">'react'</span><span class="sp-syntax-punctuation">;</span>
<span class="sp-syntax-keyword">import</span> <span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-plain">createConnection</span> <span class="sp-syntax-punctuation">}</span> <span class="sp-syntax-keyword">from</span> <span class="sp-syntax-string">'./chat.js'</span><span class="sp-syntax-punctuation">;</span>
<span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">serverUrl</span> = <span class="sp-syntax-string">'https://localhost:1234'</span><span class="sp-syntax-punctuation">;</span>
<span class="sp-syntax-keyword">function</span> <span class="sp-syntax-definition">ChatRoom</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-property">roomId</span> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span>
<span class="sp-syntax-definition">useEffect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span>
<span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">connection</span> = <span class="sp-syntax-definition">createConnection</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">serverUrl</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span>
<span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">connect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span>
<span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">disconnect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span>
<span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">]</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span>
<span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">h1</span><span class="sp-syntax-punctuation">></span>Welcome to the <span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">}</span> room!<span class="sp-syntax-punctuation"></</span><span class="sp-syntax-tag">h1</span><span class="sp-syntax-punctuation">></span><span class="sp-syntax-punctuation">;</span>
<span class="sp-syntax-punctuation">}</span>
<span class="sp-syntax-keyword">export</span> <span class="sp-syntax-keyword">default</span> <span class="sp-syntax-keyword">function</span> <span class="sp-syntax-definition">App</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span>
<span class="sp-syntax-keyword">const</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">setRoomId</span><span class="sp-syntax-punctuation">]</span> = <span class="sp-syntax-definition">useState</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-string">'general'</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span>
<span class="sp-syntax-keyword">const</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">show</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">setShow</span><span class="sp-syntax-punctuation">]</span> = <span class="sp-syntax-definition">useState</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-static">false</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span>
<span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation">(</span>
<span class="sp-syntax-punctuation"><</span><span class="sp-syntax-punctuation">></span>
<span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">label</span><span class="sp-syntax-punctuation">></span>
Choose the chat room:<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-string">' '</span><span class="sp-syntax-punctuation">}</span>
<span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">select</span>
<span class="sp-syntax-property">value</span>=<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">}</span>
<span class="sp-syntax-property">onChange</span>=<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">e</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-definition">setRoomId</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">e</span>.<span class="sp-syntax-property">target</span>.<span class="sp-syntax-property">value</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">}</span>
<span class="sp-syntax-punctuation">></span>
<span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">option</span> <span class="sp-syntax-property">value</span>=<span class="sp-syntax-string">"general"</span><span class="sp-syntax-punctuation">></span>general<span class="sp-syntax-punctuation"></</span><span class="sp-syntax-tag">option</span><span class="sp-syntax-punctuation">></span>
<span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">option</span> <span class="sp-syntax-property">value</span>=<span class="sp-syntax-string">"travel"</span><span class="sp-syntax-punctuation">></span>travel<span class="sp-syntax-punctuation"></</span><span class="sp-syntax-tag">option</span><span class="sp-syntax-punctuation">></span>
<span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">option</span> <span class="sp-syntax-property">value</span>=<span class="sp-syntax-string">"music"</span><span class="sp-syntax-punctuation">></span>music<span class="sp-syntax-punctuation"></</span><span class="sp-syntax-tag">option</span><span class="sp-syntax-punctuation">></span>
<span class="sp-syntax-punctuation"></</span><span class="sp-syntax-tag">select</span><span class="sp-syntax-punctuation">></span>
<span class="sp-syntax-punctuation"></</span><span class="sp-syntax-tag">label</span><span class="sp-syntax-punctuation">></span>
<span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">button</span> <span class="sp-syntax-property">onClick</span>=<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-definition">setShow</span><span class="sp-syntax-punctuation">(</span>!<span class="sp-syntax-plain">show</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">></span>
<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">show</span> ? <span class="sp-syntax-string">'Close chat'</span> : <span class="sp-syntax-string">'Open chat'</span><span class="sp-syntax-punctuation">}</span>
<span class="sp-syntax-punctuation"></</span><span class="sp-syntax-tag">button</span><span class="sp-syntax-punctuation">></span>
<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">show</span> && <span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">hr</span> <span class="sp-syntax-punctuation">/></span><span class="sp-syntax-punctuation">}</span>
<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">show</span> && <span class="sp-syntax-punctuation"><</span><span class="sp-syntax-definition">ChatRoom</span> <span class="sp-syntax-property">roomId</span>=<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">}</span> <span class="sp-syntax-punctuation">/></span><span class="sp-syntax-punctuation">}</span>
<span class="sp-syntax-punctuation"></</span><span class="sp-syntax-punctuation">></span>
<span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span>
<span class="sp-syntax-punctuation">}</span>
</pre></div></div></div><div class=" order-last xl:order-2 sp-stack"><div class="p-0 sm:p-2 md:p-4 lg:p-8 bg-card dark:bg-wash-dark h-full relative md:rounded-b-lg lg:rounded-b-none"><div style="position:relative"><iframe class="rounded-t-none bg-white md:shadow-md sm:rounded-lg w-full max-w-full transition-opacity absolute opacity-0 pointer-events-none duration-75" title="Sandbox Preview" style="height:15px;z-index:-1"></iframe></div></div></div><button translate="yes" class="sandpack-expand flex text-base justify-between dark:border-card-dark bg-wash dark:bg-card-dark items-center z-10 p-1 w-full order-2 xl:order-last border-b-1 relative top-0"><span class="flex p-2 focus:outline-none text-primary dark:text-primary-dark leading-[20px]"><svg class="rotate-0 inline me-1.5 text-xl" xmlns="http://www.w3.org/2000/svg" width="20" height="20" viewBox="0 0 20 20"><g fill="none" fill-rule="evenodd" transform="translate(-446 -398)"><path fill="currentColor" fill-rule="nonzero" d="M95.8838835,240.366117 C95.3957281,239.877961 94.6042719,239.877961 94.1161165,240.366117 C93.6279612,240.854272 93.6279612,241.645728 94.1161165,242.133883 L98.6161165,246.633883 C99.1042719,247.122039 99.8957281,247.122039 100.383883,246.633883 L104.883883,242.133883 C105.372039,241.645728 105.372039,240.854272 104.883883,240.366117 C104.395728,239.877961 103.604272,239.877961 103.116117,240.366117 L99.5,243.982233 L95.8838835,240.366117 Z" transform="translate(356.5 164.5)"></path><polygon points="446 418 466 418 466 398 446 398"></polygon></g></svg>Show more</span></button></div></div></div></div><!--/$--><div class="max-w-4xl ms-0 2xl:mx-auto">
<p class="whitespace-pre-wrap my-4">Notice that when the component mounts for the first time, you see three logs:</p>
<ol class="ms-6 my-3 list-decimal">
<li class="leading-relaxed mb-1"><code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">✅ Connecting to "general" room at https://localhost:1234...</code> <em>(development-only)</em></li>
<li class="leading-relaxed mb-1"><code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">❌ Disconnected from "general" room at https://localhost:1234.</code> <em>(development-only)</em></li>
<li class="leading-relaxed mb-1"><code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">✅ Connecting to "general" room at https://localhost:1234...</code></li>
</ol>
<p class="whitespace-pre-wrap my-4">The first two logs are development-only. In development, React always remounts each component once.</p>
<p class="whitespace-pre-wrap my-4"><strong class="font-bold">React verifies that your Effect can re-synchronize by forcing it to do that immediately in development.</strong> This might remind you of opening a door and closing it an extra time to check if the door lock works. React starts and stops your Effect one extra time in development to check <a class="inline text-link dark:text-link-dark border-b border-link border-opacity-0 hover:border-opacity-100 duration-100 ease-in transition leading-normal" href="/learn/synchronizing-with-effects#how-to-handle-the-effect-firing-twice-in-development">you’ve implemented its cleanup well.</a></p>
<p class="whitespace-pre-wrap my-4">The main reason your Effect will re-synchronize in practice is if some data it uses has changed. In the sandbox above, change the selected chat room. Notice how, when the <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">roomId</code> changes, your Effect re-synchronizes.</p>
<p class="whitespace-pre-wrap my-4">However, there are also more unusual cases in which re-synchronization is necessary. For example, try editing the <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">serverUrl</code> in the sandbox above while the chat is open. Notice how the Effect re-synchronizes in response to your edits to the code. In the future, React may add more features that rely on re-synchronization.</p>
<h3 id="how-react-knows-that-it-needs-to-re-synchronize-the-effect" class="mdx-heading text-2xl font-display leading-9 text-primary dark:text-primary-dark font-bold my-6">How React knows that it needs to re-synchronize the Effect <a href="#how-react-knows-that-it-needs-to-re-synchronize-the-effect" aria-label="Link for How React knows that it needs to re-synchronize the Effect " title="Link for How React knows that it needs to re-synchronize the Effect " class="mdx-header-anchor inline-block"><svg width="1em" height="1em" viewBox="0 0 13 13" xmlns="http://www.w3.org/2000/svg" class="text-gray-70 ms-2 h-5 w-5"><g fill="currentColor" fill-rule="evenodd"><path d="M7.778 7.975a2.5 2.5 0 0 0 .347-3.837L6.017 2.03a2.498 2.498 0 0 0-3.542-.007 2.5 2.5 0 0 0 .006 3.543l1.153 1.15c.07-.29.154-.563.25-.773.036-.077.084-.16.14-.25L3.18 4.85a1.496 1.496 0 0 1 .002-2.12 1.496 1.496 0 0 1 2.12 0l2.124 2.123a1.496 1.496 0 0 1-.333 2.37c.16.246.42.504.685.752z"></path><path d="M5.657 4.557a2.5 2.5 0 0 0-.347 3.837l2.108 2.108a2.498 2.498 0 0 0 3.542.007 2.5 2.5 0 0 0-.006-3.543L9.802 5.815c-.07.29-.154.565-.25.774-.036.076-.084.16-.14.25l.842.84c.585.587.59 1.532 0 2.122-.587.585-1.532.59-2.12 0L6.008 7.68a1.496 1.496 0 0 1 .332-2.372c-.16-.245-.42-.503-.685-.75z"></path></g></svg></a></h3>
<p class="whitespace-pre-wrap my-4">You might be wondering how React knew that your Effect needed to re-synchronize after <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">roomId</code> changes. It’s because <em>you told React</em> that its code depends on <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">roomId</code> by including it in the <a class="inline text-link dark:text-link-dark border-b border-link border-opacity-0 hover:border-opacity-100 duration-100 ease-in transition leading-normal" href="/learn/synchronizing-with-effects#step-2-specify-the-effect-dependencies">list of dependencies:</a></p>
<!--$--><div dir="ltr" class="sandpack sandpack--codeblock rounded-2xl h-full w-full overflow-x-auto flex items-center bg-wash dark:bg-gray-95 shadow-lg my-8" style="contain:content"><div class="sp-wrapper"><div class="sp-stack"><div class="sp-code-editor"><pre class="sp-cm sp-pristine sp-javascript flex align-start"><code class="sp-pre-placeholder grow-[2]"><div class="cm-line bg-github-highlight dark:bg-opacity-10"><span class="sp-syntax-keyword">function</span> <span class="sp-syntax-definition">ChatRoom</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-property">roomId</span> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-comment">// The roomId prop may change over time</span><br></div><div class="cm-line "> <span class="sp-syntax-definition">useEffect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">connection</span> = <span class="sp-syntax-definition">createConnection</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">serverUrl</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-comment">// This Effect reads roomId </span><br></div><div class="cm-line "> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">connect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br></div><div class="cm-line "> <span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br></div><div class="cm-line "> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">disconnect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br></div><div class="cm-line "> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">;</span><br></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">]</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-comment">// So you tell React that this Effect "depends on" roomId</span><br></div><div class="cm-line "> <span class="sp-syntax-comment">// ...</span></div></code></pre></div></div></div></div><!--/$-->
<p class="whitespace-pre-wrap my-4">Here’s how this works:</p>
<ol class="ms-6 my-3 list-decimal">
<li class="leading-relaxed mb-1">You knew <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">roomId</code> is a prop, which means it can change over time.</li>
<li class="leading-relaxed mb-1">You knew that your Effect reads <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">roomId</code> (so its logic depends on a value that may change later).</li>
<li class="leading-relaxed mb-1">This is why you specified it as your Effect’s dependency (so that it re-synchronizes when <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">roomId</code> changes).</li>
</ol>
<p class="whitespace-pre-wrap my-4">Every time after your component re-renders, React will look at the array of dependencies that you have passed. If any of the values in the array is different from the value at the same spot that you passed during the previous render, React will re-synchronize your Effect.</p>
<p class="whitespace-pre-wrap my-4">For example, if you passed <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">["general"]</code> during the initial render, and later you passed <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">["travel"]</code> during the next render, React will compare <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">"general"</code> and <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">"travel"</code>. These are different values (compared with <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/is" target="_blank" rel="nofollow noopener noreferrer" class="inline text-link dark:text-link-dark border-b border-link border-opacity-0 hover:border-opacity-100 duration-100 ease-in transition leading-normal"><code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">Object.is</code></a>), so React will re-synchronize your Effect. On the other hand, if your component re-renders but <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">roomId</code> has not changed, your Effect will remain connected to the same room.</p>
<h3 id="each-effect-represents-a-separate-synchronization-process" class="mdx-heading text-2xl font-display leading-9 text-primary dark:text-primary-dark font-bold my-6">Each Effect represents a separate synchronization process <a href="#each-effect-represents-a-separate-synchronization-process" aria-label="Link for Each Effect represents a separate synchronization process " title="Link for Each Effect represents a separate synchronization process " class="mdx-header-anchor inline-block"><svg width="1em" height="1em" viewBox="0 0 13 13" xmlns="http://www.w3.org/2000/svg" class="text-gray-70 ms-2 h-5 w-5"><g fill="currentColor" fill-rule="evenodd"><path d="M7.778 7.975a2.5 2.5 0 0 0 .347-3.837L6.017 2.03a2.498 2.498 0 0 0-3.542-.007 2.5 2.5 0 0 0 .006 3.543l1.153 1.15c.07-.29.154-.563.25-.773.036-.077.084-.16.14-.25L3.18 4.85a1.496 1.496 0 0 1 .002-2.12 1.496 1.496 0 0 1 2.12 0l2.124 2.123a1.496 1.496 0 0 1-.333 2.37c.16.246.42.504.685.752z"></path><path d="M5.657 4.557a2.5 2.5 0 0 0-.347 3.837l2.108 2.108a2.498 2.498 0 0 0 3.542.007 2.5 2.5 0 0 0-.006-3.543L9.802 5.815c-.07.29-.154.565-.25.774-.036.076-.084.16-.14.25l.842.84c.585.587.59 1.532 0 2.122-.587.585-1.532.59-2.12 0L6.008 7.68a1.496 1.496 0 0 1 .332-2.372c-.16-.245-.42-.503-.685-.75z"></path></g></svg></a></h3>
<p class="whitespace-pre-wrap my-4">Resist adding unrelated logic to your Effect only because this logic needs to run at the same time as an Effect you already wrote. For example, let’s say you want to send an analytics event when the user visits the room. You already have an Effect that depends on <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">roomId</code>, so you might feel tempted to add the analytics call there:</p>
<!--$--><div dir="ltr" class="sandpack sandpack--codeblock rounded-2xl h-full w-full overflow-x-auto flex items-center bg-wash dark:bg-gray-95 shadow-lg my-8" style="contain:content"><div class="sp-wrapper"><div class="sp-stack"><div class="sp-code-editor"><pre class="sp-cm sp-pristine sp-javascript flex align-start"><code class="sp-pre-placeholder grow-[2]"><div class="cm-line "><span class="sp-syntax-keyword">function</span> <span class="sp-syntax-definition">ChatRoom</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-property">roomId</span> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span><br></div><div class="cm-line "> <span class="sp-syntax-definition">useEffect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-definition">logVisit</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br></div><div class="cm-line "> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">connection</span> = <span class="sp-syntax-definition">createConnection</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">serverUrl</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br></div><div class="cm-line "> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">connect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br></div><div class="cm-line "> <span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br></div><div class="cm-line "> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">disconnect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br></div><div class="cm-line "> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">;</span><br></div><div class="cm-line "> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">]</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br></div><div class="cm-line "> <span class="sp-syntax-comment">// ...</span><br></div><div class="cm-line "><span class="sp-syntax-punctuation">}</span></div></code></pre></div></div></div></div><!--/$-->
<p class="whitespace-pre-wrap my-4">But imagine you later add another dependency to this Effect that needs to re-establish the connection. If this Effect re-synchronizes, it will also call <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">logVisit(roomId)</code> for the same room, which you did not intend. Logging the visit <strong class="font-bold">is a separate process</strong> from connecting. Write them as two separate Effects:</p>
<!--$--><div dir="ltr" class="sandpack sandpack--codeblock rounded-2xl h-full w-full overflow-x-auto flex items-center bg-wash dark:bg-gray-95 shadow-lg my-8" style="contain:content"><div class="sp-wrapper"><div class="sp-stack"><div class="sp-code-editor"><pre class="sp-cm sp-pristine sp-javascript flex align-start"><code class="sp-pre-placeholder grow-[2]"><div class="cm-line "><span class="sp-syntax-keyword">function</span> <span class="sp-syntax-definition">ChatRoom</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-property">roomId</span> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span><br></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-definition">useEffect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-definition">logVisit</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">]</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br></div><div class="cm-line "><br></div><div class="cm-line "> <span class="sp-syntax-definition">useEffect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br></div><div class="cm-line "> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">connection</span> = <span class="sp-syntax-definition">createConnection</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">serverUrl</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br></div><div class="cm-line "> <span class="sp-syntax-comment">// ...</span><br></div><div class="cm-line "> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">]</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br></div><div class="cm-line "> <span class="sp-syntax-comment">// ...</span><br></div><div class="cm-line "><span class="sp-syntax-punctuation">}</span></div></code></pre></div></div></div></div><!--/$-->
<p class="whitespace-pre-wrap my-4"><strong class="font-bold">Each Effect in your code should represent a separate and independent synchronization process.</strong></p>
<p class="whitespace-pre-wrap my-4">In the above example, deleting one Effect wouldn’t break the other Effect’s logic. This is a good indication that they synchronize different things, and so it made sense to split them up. On the other hand, if you split up a cohesive piece of logic into separate Effects, the code may look “cleaner” but will be <a class="inline text-link dark:text-link-dark border-b border-link border-opacity-0 hover:border-opacity-100 duration-100 ease-in transition leading-normal" href="/learn/you-might-not-need-an-effect#chains-of-computations">more difficult to maintain.</a> This is why you should think whether the processes are same or separate, not whether the code looks cleaner.</p>
<h2 id="effects-react-to-reactive-values" class="mdx-heading text-3xl font-display leading-10 text-primary dark:text-primary-dark font-bold my-6">Effects “react” to reactive values <a href="#effects-react-to-reactive-values" aria-label="Link for Effects “react” to reactive values " title="Link for Effects “react” to reactive values " class="mdx-header-anchor inline-block"><svg width="1em" height="1em" viewBox="0 0 13 13" xmlns="http://www.w3.org/2000/svg" class="text-gray-70 ms-2 h-5 w-5"><g fill="currentColor" fill-rule="evenodd"><path d="M7.778 7.975a2.5 2.5 0 0 0 .347-3.837L6.017 2.03a2.498 2.498 0 0 0-3.542-.007 2.5 2.5 0 0 0 .006 3.543l1.153 1.15c.07-.29.154-.563.25-.773.036-.077.084-.16.14-.25L3.18 4.85a1.496 1.496 0 0 1 .002-2.12 1.496 1.496 0 0 1 2.12 0l2.124 2.123a1.496 1.496 0 0 1-.333 2.37c.16.246.42.504.685.752z"></path><path d="M5.657 4.557a2.5 2.5 0 0 0-.347 3.837l2.108 2.108a2.498 2.498 0 0 0 3.542.007 2.5 2.5 0 0 0-.006-3.543L9.802 5.815c-.07.29-.154.565-.25.774-.036.076-.084.16-.14.25l.842.84c.585.587.59 1.532 0 2.122-.587.585-1.532.59-2.12 0L6.008 7.68a1.496 1.496 0 0 1 .332-2.372c-.16-.245-.42-.503-.685-.75z"></path></g></svg></a></h2>
<p class="whitespace-pre-wrap my-4">Your Effect reads two variables (<code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">serverUrl</code> and <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">roomId</code>), but you only specified <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">roomId</code> as a dependency:</p>
<!--$--><div dir="ltr" class="sandpack sandpack--codeblock rounded-2xl h-full w-full overflow-x-auto flex items-center bg-wash dark:bg-gray-95 shadow-lg my-8" style="contain:content"><div class="sp-wrapper"><div class="sp-stack"><div class="sp-code-editor"><pre class="sp-cm sp-pristine sp-javascript flex align-start"><code class="sp-pre-placeholder grow-[2]"><div class="cm-line "><span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">serverUrl</span> = <span class="sp-syntax-string">'https://localhost:1234'</span><span class="sp-syntax-punctuation">;</span><br></div><div class="cm-line "><br></div><div class="cm-line "><span class="sp-syntax-keyword">function</span> <span class="sp-syntax-definition">ChatRoom</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-property">roomId</span> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span><br></div><div class="cm-line "> <span class="sp-syntax-definition">useEffect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">connection</span> = <span class="sp-syntax-definition">createConnection</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">serverUrl</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br></div><div class="cm-line "> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">connect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br></div><div class="cm-line "> <span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br></div><div class="cm-line "> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">disconnect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br></div><div class="cm-line "> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">;</span><br></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">]</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br></div><div class="cm-line "> <span class="sp-syntax-comment">// ...</span><br></div><div class="cm-line "><span class="sp-syntax-punctuation">}</span></div></code></pre></div></div></div></div><!--/$-->
<p class="whitespace-pre-wrap my-4">Why doesn’t <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">serverUrl</code> need to be a dependency?</p>
<p class="whitespace-pre-wrap my-4">This is because the <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">serverUrl</code> never changes due to a re-render. It’s always the same no matter how many times the component re-renders and why. Since <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">serverUrl</code> never changes, it wouldn’t make sense to specify it as a dependency. After all, dependencies only do something when they change over time!</p>
<p class="whitespace-pre-wrap my-4">On the other hand, <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">roomId</code> may be different on a re-render. <strong class="font-bold">Props, state, and other values declared inside the component are <em>reactive</em> because they’re calculated during rendering and participate in the React data flow.</strong></p>
<p class="whitespace-pre-wrap my-4">If <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">serverUrl</code> was a state variable, it would be reactive. Reactive values must be included in dependencies:</p>
<!--$--><div dir="ltr" class="sandpack sandpack--codeblock rounded-2xl h-full w-full overflow-x-auto flex items-center bg-wash dark:bg-gray-95 shadow-lg my-8" style="contain:content"><div class="sp-wrapper"><div class="sp-stack"><div class="sp-code-editor"><pre class="sp-cm sp-pristine sp-javascript flex align-start"><code class="sp-pre-placeholder grow-[2]"><div class="cm-line "><span class="sp-syntax-keyword">function</span> <span class="sp-syntax-definition">ChatRoom</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-property">roomId</span> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-comment">// Props change over time</span><br></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">serverUrl</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">setServerUrl</span><span class="sp-syntax-punctuation">]</span> = <span class="sp-syntax-definition">useState</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-string">'https://localhost:1234'</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-comment">// State may change over time</span><br></div><div class="cm-line "><br></div><div class="cm-line "> <span class="sp-syntax-definition">useEffect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">connection</span> = <span class="sp-syntax-definition">createConnection</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">serverUrl</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-comment">// Your Effect reads props and state</span><br></div><div class="cm-line "> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">connect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br></div><div class="cm-line "> <span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br></div><div class="cm-line "> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">disconnect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br></div><div class="cm-line "> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">;</span><br></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">serverUrl</span><span class="sp-syntax-punctuation">]</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-comment">// So you tell React that this Effect "depends on" on props and state</span><br></div><div class="cm-line "> <span class="sp-syntax-comment">// ...</span><br></div><div class="cm-line "><span class="sp-syntax-punctuation">}</span></div></code></pre></div></div></div></div><!--/$-->
<p class="whitespace-pre-wrap my-4">By including <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">serverUrl</code> as a dependency, you ensure that the Effect re-synchronizes after it changes.</p>
<p class="whitespace-pre-wrap my-4">Try changing the selected chat room or edit the server URL in this sandbox:</p>
</div><!--$--><div class="sandpack sandpack--playground w-full my-8" dir="ltr"><div class="sp-wrapper"><div class="shadow-lg dark:shadow-lg-dark rounded-lg" style="contain:content"><div class="bg-wash dark:bg-card-dark flex justify-between items-center relative z-10 border-b border-border dark:border-border-dark rounded-t-lg text-lg"><div class="flex-1 grow min-w-0 px-4 lg:px-6"><div><div class="relative overflow-hidden"><div class="w-[fit-content]"><div class=" sp-tabs" translate="no"><div aria-label="Select active file" class=" sp-tabs-scrollable-container" role="tablist"><button aria-selected="true" class=" sp-tab-button" data-active="true" role="tab" title="/src/App.js" type="button">App.js</button><button aria-selected="false" class=" sp-tab-button" data-active="false" role="tab" title="/src/chat.js" type="button">chat.js</button></div></div></div><button class="absolute top-0 start-[2px] invisible" id="headlessui-listbox-button-:R9ikcq6:" aria-haspopup="true" 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8.85355C21.9474 8.75979 22.0001 8.63261 22.0001 8.5V3.5C22.0001 3.10218 21.8421 2.72064 21.5608 2.43934C21.2795 2.15804 20.8979 2 20.5001 2V2Z" fill="currentColor"></path><path d="M21.5 13H20.5C20.3674 13 20.2402 13.0527 20.1464 13.1464C20.0527 13.2402 20 13.3674 20 13.5V20H4V4H10.5C10.6326 4 10.7598 3.94732 10.8536 3.85355C10.9473 3.75979 11 3.63261 11 3.5V2.5C11 2.36739 10.9473 2.24021 10.8536 2.14645C10.7598 2.05268 10.6326 2 10.5 2H3.5C3.10218 2 2.72064 2.15804 2.43934 2.43934C2.15804 2.72064 2 3.10218 2 3.5V20.5C2 20.8978 2.15804 21.2794 2.43934 21.5607C2.72064 21.842 3.10218 22 3.5 22H20.5C20.8978 22 21.2794 21.842 21.5607 21.5607C21.842 21.2794 22 20.8978 22 20.5V13.5C22 13.3674 21.9473 13.2402 21.8536 13.1464C21.7598 13.0527 21.6326 13 21.5 13Z" fill="currentColor"></path></svg><span class="hidden md:block">Fork</span></button></div></div><div class=" sp-layout"><div class=" sp-editor sp-stack"><div class=" sp-code-editor"><div aria-autocomplete="list" aria-label="Code Editor for App.js" aria-multiline="true" class="sp-pristine sp-javascript sp-cm" role="textbox" tabindex="0" translate="no"><pre class=" sp-pre-placeholder" style="margin-left:var(--sp-space-11)"><span class="sp-syntax-keyword">import</span> <span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-plain">useState</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">useEffect</span> <span class="sp-syntax-punctuation">}</span> <span class="sp-syntax-keyword">from</span> <span class="sp-syntax-string">'react'</span><span class="sp-syntax-punctuation">;</span>
<span class="sp-syntax-keyword">import</span> <span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-plain">createConnection</span> <span class="sp-syntax-punctuation">}</span> <span class="sp-syntax-keyword">from</span> <span class="sp-syntax-string">'./chat.js'</span><span class="sp-syntax-punctuation">;</span>
<span class="sp-syntax-keyword">function</span> <span class="sp-syntax-definition">ChatRoom</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-property">roomId</span> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span>
<span class="sp-syntax-keyword">const</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">serverUrl</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">setServerUrl</span><span class="sp-syntax-punctuation">]</span> = <span class="sp-syntax-definition">useState</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-string">'https://localhost:1234'</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span>
<span class="sp-syntax-definition">useEffect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span>
<span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">connection</span> = <span class="sp-syntax-definition">createConnection</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">serverUrl</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span>
<span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">connect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span>
<span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">disconnect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span>
<span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">serverUrl</span><span class="sp-syntax-punctuation">]</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span>
<span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation">(</span>
<span class="sp-syntax-punctuation"><</span><span class="sp-syntax-punctuation">></span>
<span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">label</span><span class="sp-syntax-punctuation">></span>
Server URL:<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-string">' '</span><span class="sp-syntax-punctuation">}</span>
<span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">input</span>
<span class="sp-syntax-property">value</span>=<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">serverUrl</span><span class="sp-syntax-punctuation">}</span>
<span class="sp-syntax-property">onChange</span>=<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">e</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-definition">setServerUrl</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">e</span>.<span class="sp-syntax-property">target</span>.<span class="sp-syntax-property">value</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">}</span>
<span class="sp-syntax-punctuation">/></span>
<span class="sp-syntax-punctuation"></</span><span class="sp-syntax-tag">label</span><span class="sp-syntax-punctuation">></span>
<span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">h1</span><span class="sp-syntax-punctuation">></span>Welcome to the <span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">}</span> room!<span class="sp-syntax-punctuation"></</span><span class="sp-syntax-tag">h1</span><span class="sp-syntax-punctuation">></span>
<span class="sp-syntax-punctuation"></</span><span class="sp-syntax-punctuation">></span>
<span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span>
<span class="sp-syntax-punctuation">}</span>
<span class="sp-syntax-keyword">export</span> <span class="sp-syntax-keyword">default</span> <span class="sp-syntax-keyword">function</span> <span class="sp-syntax-definition">App</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span>
<span class="sp-syntax-keyword">const</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">setRoomId</span><span class="sp-syntax-punctuation">]</span> = <span class="sp-syntax-definition">useState</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-string">'general'</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span>
<span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation">(</span>
<span class="sp-syntax-punctuation"><</span><span class="sp-syntax-punctuation">></span>
<span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">label</span><span class="sp-syntax-punctuation">></span>
Choose the chat room:<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-string">' '</span><span class="sp-syntax-punctuation">}</span>
<span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">select</span>
<span class="sp-syntax-property">value</span>=<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">}</span>
<span class="sp-syntax-property">onChange</span>=<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">e</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-definition">setRoomId</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">e</span>.<span class="sp-syntax-property">target</span>.<span class="sp-syntax-property">value</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">}</span>
<span class="sp-syntax-punctuation">></span>
<span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">option</span> <span class="sp-syntax-property">value</span>=<span class="sp-syntax-string">"general"</span><span class="sp-syntax-punctuation">></span>general<span class="sp-syntax-punctuation"></</span><span class="sp-syntax-tag">option</span><span class="sp-syntax-punctuation">></span>
<span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">option</span> <span class="sp-syntax-property">value</span>=<span class="sp-syntax-string">"travel"</span><span class="sp-syntax-punctuation">></span>travel<span class="sp-syntax-punctuation"></</span><span class="sp-syntax-tag">option</span><span class="sp-syntax-punctuation">></span>
<span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">option</span> <span class="sp-syntax-property">value</span>=<span class="sp-syntax-string">"music"</span><span class="sp-syntax-punctuation">></span>music<span class="sp-syntax-punctuation"></</span><span class="sp-syntax-tag">option</span><span class="sp-syntax-punctuation">></span>
<span class="sp-syntax-punctuation"></</span><span class="sp-syntax-tag">select</span><span class="sp-syntax-punctuation">></span>
<span class="sp-syntax-punctuation"></</span><span class="sp-syntax-tag">label</span><span class="sp-syntax-punctuation">></span>
<span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">hr</span> <span class="sp-syntax-punctuation">/></span>
<span class="sp-syntax-punctuation"><</span><span class="sp-syntax-definition">ChatRoom</span> <span class="sp-syntax-property">roomId</span>=<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">}</span> <span class="sp-syntax-punctuation">/></span>
<span class="sp-syntax-punctuation"></</span><span class="sp-syntax-punctuation">></span>
<span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span>
<span class="sp-syntax-punctuation">}</span>
</pre></div></div></div><div class=" order-last xl:order-2 sp-stack"><div class="p-0 sm:p-2 md:p-4 lg:p-8 bg-card dark:bg-wash-dark h-full relative md:rounded-b-lg lg:rounded-b-none"><div style="position:relative"><iframe class="rounded-t-none bg-white md:shadow-md sm:rounded-lg w-full max-w-full transition-opacity absolute opacity-0 pointer-events-none duration-75" title="Sandbox Preview" style="height:15px;z-index:-1"></iframe></div></div></div><button translate="yes" class="sandpack-expand flex text-base justify-between dark:border-card-dark bg-wash dark:bg-card-dark items-center z-10 p-1 w-full order-2 xl:order-last border-b-1 relative top-0"><span class="flex p-2 focus:outline-none text-primary dark:text-primary-dark leading-[20px]"><svg class="rotate-0 inline me-1.5 text-xl" xmlns="http://www.w3.org/2000/svg" width="20" height="20" viewBox="0 0 20 20"><g fill="none" fill-rule="evenodd" transform="translate(-446 -398)"><path fill="currentColor" fill-rule="nonzero" d="M95.8838835,240.366117 C95.3957281,239.877961 94.6042719,239.877961 94.1161165,240.366117 C93.6279612,240.854272 93.6279612,241.645728 94.1161165,242.133883 L98.6161165,246.633883 C99.1042719,247.122039 99.8957281,247.122039 100.383883,246.633883 L104.883883,242.133883 C105.372039,241.645728 105.372039,240.854272 104.883883,240.366117 C104.395728,239.877961 103.604272,239.877961 103.116117,240.366117 L99.5,243.982233 L95.8838835,240.366117 Z" transform="translate(356.5 164.5)"></path><polygon points="446 418 466 418 466 398 446 398"></polygon></g></svg>Show more</span></button></div></div></div></div><!--/$--><div class="max-w-4xl ms-0 2xl:mx-auto">
<p class="whitespace-pre-wrap my-4">Whenever you change a reactive value like <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">roomId</code> or <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">serverUrl</code>, the Effect re-connects to the chat server.</p>
<h3 id="what-an-effect-with-empty-dependencies-means" class="mdx-heading text-2xl font-display leading-9 text-primary dark:text-primary-dark font-bold my-6">What an Effect with empty dependencies means <a href="#what-an-effect-with-empty-dependencies-means" aria-label="Link for What an Effect with empty dependencies means " title="Link for What an Effect with empty dependencies means " class="mdx-header-anchor inline-block"><svg width="1em" height="1em" viewBox="0 0 13 13" xmlns="http://www.w3.org/2000/svg" class="text-gray-70 ms-2 h-5 w-5"><g fill="currentColor" fill-rule="evenodd"><path d="M7.778 7.975a2.5 2.5 0 0 0 .347-3.837L6.017 2.03a2.498 2.498 0 0 0-3.542-.007 2.5 2.5 0 0 0 .006 3.543l1.153 1.15c.07-.29.154-.563.25-.773.036-.077.084-.16.14-.25L3.18 4.85a1.496 1.496 0 0 1 .002-2.12 1.496 1.496 0 0 1 2.12 0l2.124 2.123a1.496 1.496 0 0 1-.333 2.37c.16.246.42.504.685.752z"></path><path d="M5.657 4.557a2.5 2.5 0 0 0-.347 3.837l2.108 2.108a2.498 2.498 0 0 0 3.542.007 2.5 2.5 0 0 0-.006-3.543L9.802 5.815c-.07.29-.154.565-.25.774-.036.076-.084.16-.14.25l.842.84c.585.587.59 1.532 0 2.122-.587.585-1.532.59-2.12 0L6.008 7.68a1.496 1.496 0 0 1 .332-2.372c-.16-.245-.42-.503-.685-.75z"></path></g></svg></a></h3>
<p class="whitespace-pre-wrap my-4">What happens if you move both <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">serverUrl</code> and <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">roomId</code> outside the component?</p>
<!--$--><div dir="ltr" class="sandpack sandpack--codeblock rounded-2xl h-full w-full overflow-x-auto flex items-center bg-wash dark:bg-gray-95 shadow-lg my-8" style="contain:content"><div class="sp-wrapper"><div class="sp-stack"><div class="sp-code-editor"><pre class="sp-cm sp-pristine sp-javascript flex align-start"><code class="sp-pre-placeholder grow-[2]"><div class="cm-line bg-github-highlight dark:bg-opacity-10"><span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">serverUrl</span> = <span class="sp-syntax-string">'https://localhost:1234'</span><span class="sp-syntax-punctuation">;</span><br></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"><span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">roomId</span> = <span class="sp-syntax-string">'general'</span><span class="sp-syntax-punctuation">;</span><br></div><div class="cm-line "><br></div><div class="cm-line "><span class="sp-syntax-keyword">function</span> <span class="sp-syntax-definition">ChatRoom</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span><br></div><div class="cm-line "> <span class="sp-syntax-definition">useEffect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br></div><div class="cm-line "> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">connection</span> = <span class="sp-syntax-definition">createConnection</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">serverUrl</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br></div><div class="cm-line "> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">connect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br></div><div class="cm-line "> <span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br></div><div class="cm-line "> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">disconnect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br></div><div class="cm-line "> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">;</span><br></div><div class="cm-line "> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-punctuation">]</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-comment">// ✅ All dependencies declared</span><br></div><div class="cm-line "> <span class="sp-syntax-comment">// ...</span><br></div><div class="cm-line "><span class="sp-syntax-punctuation">}</span></div></code></pre></div></div></div></div><!--/$-->
<p class="whitespace-pre-wrap my-4">Now your Effect’s code does not use <em>any</em> reactive values, so its dependencies can be empty (<code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">[]</code>).</p>
<p class="whitespace-pre-wrap my-4">Thinking from the component’s perspective, the empty <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">[]</code> dependency array means this Effect connects to the chat room only when the component mounts, and disconnects only when the component unmounts. (Keep in mind that React would still <a class="inline text-link dark:text-link-dark border-b border-link border-opacity-0 hover:border-opacity-100 duration-100 ease-in transition leading-normal" href="#how-react-verifies-that-your-effect-can-re-synchronize">re-synchronize it an extra time</a> in development to stress-test your logic.)</p>
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for App.js" aria-multiline="true" class="sp-pristine sp-javascript sp-cm" role="textbox" tabindex="0" translate="no"><pre class=" sp-pre-placeholder" style="margin-left:var(--sp-space-11)"><span class="sp-syntax-keyword">import</span> <span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-plain">useState</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">useEffect</span> <span class="sp-syntax-punctuation">}</span> <span class="sp-syntax-keyword">from</span> <span class="sp-syntax-string">'react'</span><span class="sp-syntax-punctuation">;</span>
<span class="sp-syntax-keyword">import</span> <span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-plain">createConnection</span> <span class="sp-syntax-punctuation">}</span> <span class="sp-syntax-keyword">from</span> <span class="sp-syntax-string">'./chat.js'</span><span class="sp-syntax-punctuation">;</span>
<span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">serverUrl</span> = <span class="sp-syntax-string">'https://localhost:1234'</span><span class="sp-syntax-punctuation">;</span>
<span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">roomId</span> = <span class="sp-syntax-string">'general'</span><span class="sp-syntax-punctuation">;</span>
<span class="sp-syntax-keyword">function</span> <span class="sp-syntax-definition">ChatRoom</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span>
<span class="sp-syntax-definition">useEffect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span>
<span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">connection</span> = <span class="sp-syntax-definition">createConnection</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">serverUrl</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span>
<span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">connect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span>
<span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">disconnect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span>
<span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-punctuation">]</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span>
<span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">h1</span><span class="sp-syntax-punctuation">></span>Welcome to the <span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">}</span> room!<span class="sp-syntax-punctuation"></</span><span class="sp-syntax-tag">h1</span><span class="sp-syntax-punctuation">></span><span class="sp-syntax-punctuation">;</span>
<span class="sp-syntax-punctuation">}</span>
<span class="sp-syntax-keyword">export</span> <span class="sp-syntax-keyword">default</span> <span class="sp-syntax-keyword">function</span> <span class="sp-syntax-definition">App</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span>
<span class="sp-syntax-keyword">const</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">show</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">setShow</span><span class="sp-syntax-punctuation">]</span> = <span class="sp-syntax-definition">useState</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-static">false</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span>
<span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation">(</span>
<span class="sp-syntax-punctuation"><</span><span class="sp-syntax-punctuation">></span>
<span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">button</span> <span class="sp-syntax-property">onClick</span>=<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-definition">setShow</span><span class="sp-syntax-punctuation">(</span>!<span class="sp-syntax-plain">show</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">></span>
<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">show</span> ? <span class="sp-syntax-string">'Close chat'</span> : <span class="sp-syntax-string">'Open chat'</span><span class="sp-syntax-punctuation">}</span>
<span class="sp-syntax-punctuation"></</span><span class="sp-syntax-tag">button</span><span class="sp-syntax-punctuation">></span>
<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">show</span> && <span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">hr</span> <span class="sp-syntax-punctuation">/></span><span class="sp-syntax-punctuation">}</span>
<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">show</span> && <span class="sp-syntax-punctuation"><</span><span class="sp-syntax-definition">ChatRoom</span> <span class="sp-syntax-punctuation">/></span><span class="sp-syntax-punctuation">}</span>
<span class="sp-syntax-punctuation"></</span><span class="sp-syntax-punctuation">></span>
<span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span>
<span class="sp-syntax-punctuation">}</span>
</pre></div></div></div><div class=" order-last xl:order-2 sp-stack"><div class="p-0 sm:p-2 md:p-4 lg:p-8 bg-card dark:bg-wash-dark h-full relative md:rounded-b-lg lg:rounded-b-none"><div style="position:relative"><iframe class="rounded-t-none bg-white md:shadow-md sm:rounded-lg w-full max-w-full transition-opacity absolute opacity-0 pointer-events-none duration-75" title="Sandbox Preview" style="height:15px;z-index:-1"></iframe></div></div></div><button translate="yes" class="sandpack-expand flex text-base justify-between dark:border-card-dark bg-wash dark:bg-card-dark items-center z-10 p-1 w-full order-2 xl:order-last border-b-1 relative top-0"><span class="flex p-2 focus:outline-none text-primary dark:text-primary-dark leading-[20px]"><svg class="rotate-0 inline me-1.5 text-xl" xmlns="http://www.w3.org/2000/svg" width="20" height="20" viewBox="0 0 20 20"><g fill="none" fill-rule="evenodd" transform="translate(-446 -398)"><path fill="currentColor" fill-rule="nonzero" d="M95.8838835,240.366117 C95.3957281,239.877961 94.6042719,239.877961 94.1161165,240.366117 C93.6279612,240.854272 93.6279612,241.645728 94.1161165,242.133883 L98.6161165,246.633883 C99.1042719,247.122039 99.8957281,247.122039 100.383883,246.633883 L104.883883,242.133883 C105.372039,241.645728 105.372039,240.854272 104.883883,240.366117 C104.395728,239.877961 103.604272,239.877961 103.116117,240.366117 L99.5,243.982233 L95.8838835,240.366117 Z" transform="translate(356.5 164.5)"></path><polygon points="446 418 466 418 466 398 446 398"></polygon></g></svg>Show more</span></button></div></div></div></div><!--/$--><div class="max-w-4xl ms-0 2xl:mx-auto">
<p class="whitespace-pre-wrap my-4">However, if you <a class="inline text-link dark:text-link-dark border-b border-link border-opacity-0 hover:border-opacity-100 duration-100 ease-in transition leading-normal" href="#thinking-from-the-effects-perspective">think from the Effect’s perspective,</a> you don’t need to think about mounting and unmounting at all. What’s important is you’ve specified what your Effect does to start and stop synchronizing. Today, it has no reactive dependencies. But if you ever want the user to change <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">roomId</code> or <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">serverUrl</code> over time (and they would become reactive), your Effect’s code won’t change. You will only need to add them to the dependencies.</p>
<h3 id="all-variables-declared-in-the-component-body-are-reactive" class="mdx-heading text-2xl font-display leading-9 text-primary dark:text-primary-dark font-bold my-6">All variables declared in the component body are reactive <a href="#all-variables-declared-in-the-component-body-are-reactive" aria-label="Link for All variables declared in the component body are reactive " title="Link for All variables declared in the component body are reactive " class="mdx-header-anchor inline-block"><svg width="1em" height="1em" viewBox="0 0 13 13" xmlns="http://www.w3.org/2000/svg" class="text-gray-70 ms-2 h-5 w-5"><g fill="currentColor" fill-rule="evenodd"><path d="M7.778 7.975a2.5 2.5 0 0 0 .347-3.837L6.017 2.03a2.498 2.498 0 0 0-3.542-.007 2.5 2.5 0 0 0 .006 3.543l1.153 1.15c.07-.29.154-.563.25-.773.036-.077.084-.16.14-.25L3.18 4.85a1.496 1.496 0 0 1 .002-2.12 1.496 1.496 0 0 1 2.12 0l2.124 2.123a1.496 1.496 0 0 1-.333 2.37c.16.246.42.504.685.752z"></path><path d="M5.657 4.557a2.5 2.5 0 0 0-.347 3.837l2.108 2.108a2.498 2.498 0 0 0 3.542.007 2.5 2.5 0 0 0-.006-3.543L9.802 5.815c-.07.29-.154.565-.25.774-.036.076-.084.16-.14.25l.842.84c.585.587.59 1.532 0 2.122-.587.585-1.532.59-2.12 0L6.008 7.68a1.496 1.496 0 0 1 .332-2.372c-.16-.245-.42-.503-.685-.75z"></path></g></svg></a></h3>
<p class="whitespace-pre-wrap my-4">Props and state aren’t the only reactive values. Values that you calculate from them are also reactive. If the props or state change, your component will re-render, and the values calculated from them will also change. This is why all variables from the component body used by the Effect should be in the Effect dependency list.</p>
<p class="whitespace-pre-wrap my-4">Let’s say that the user can pick a chat server in the dropdown, but they can also configure a default server in settings. Suppose you’ve already put the settings state in a <a class="inline text-link dark:text-link-dark border-b border-link border-opacity-0 hover:border-opacity-100 duration-100 ease-in transition leading-normal" href="/learn/scaling-up-with-reducer-and-context">context</a> so you read the <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">settings</code> from that context. Now you calculate the <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">serverUrl</code> based on the selected server from props and the default server:</p>
<!--$--><div dir="ltr" class="sandpack sandpack--codeblock rounded-2xl h-full w-full overflow-x-auto flex items-center bg-wash dark:bg-gray-95 shadow-lg my-8" style="contain:content"><div class="sp-wrapper"><div class="sp-stack"><div class="sp-code-editor"><pre class="sp-cm sp-pristine sp-javascript flex align-start"><code class="sp-pre-placeholder grow-[2]"><div class="cm-line "><span class="sp-syntax-keyword">function</span> <span class="sp-syntax-definition">ChatRoom</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-property">roomId</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-property">selectedServerUrl</span> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-comment">// roomId is reactive</span><br></div><div class="cm-line "> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">settings</span> = <span class="sp-syntax-definition">useContext</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">SettingsContext</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-comment">// settings is reactive</span><br></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">serverUrl</span> = <span class="sp-syntax-plain">selectedServerUrl</span> ?? <span class="sp-syntax-plain">settings</span>.<span class="sp-syntax-property">defaultServerUrl</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-comment">// serverUrl is reactive</span><br></div><div class="cm-line "> <span class="sp-syntax-definition">useEffect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">connection</span> = <span class="sp-syntax-definition">createConnection</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">serverUrl</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-comment">// Your Effect reads roomId and serverUrl</span><br></div><div class="cm-line "> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">connect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br></div><div class="cm-line "> <span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br></div><div class="cm-line "> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">disconnect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br></div><div class="cm-line "> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">;</span><br></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">serverUrl</span><span class="sp-syntax-punctuation">]</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-comment">// So it needs to re-synchronize when either of them changes!</span><br></div><div class="cm-line "> <span class="sp-syntax-comment">// ...</span><br></div><div class="cm-line "><span class="sp-syntax-punctuation">}</span></div></code></pre></div></div></div></div><!--/$-->
<p class="whitespace-pre-wrap my-4">In this example, <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">serverUrl</code> is not a prop or a state variable. It’s a regular variable that you calculate during rendering. But it’s calculated during rendering, so it can change due to a re-render. This is why it’s reactive.</p>
<p class="whitespace-pre-wrap my-4"><strong class="font-bold">All values inside the component (including props, state, and variables in your component’s body) are reactive. Any reactive value can change on a re-render, so you need to include reactive values as Effect’s dependencies.</strong></p>
<p class="whitespace-pre-wrap my-4">In other words, Effects “react” to all values from the component body.</p>
<details class="my-12 rounded-2xl shadow-inner-border dark:shadow-inner-border-dark relative dark:bg-opacity-20 dark:bg-purple-60 bg-purple-5"><summary class="list-none p-8" tabindex="-1"><h5 class="mb-4 uppercase font-bold flex items-center text-sm dark:text-purple-30 text-purple-50"><svg class="inline me-2 dark:text-purple-30 text-purple-40" width="1.5em" height="1.5em" viewBox="0 0 72 72" fill="none" xmlns="http://www.w3.org/2000/svg"><path fill-rule="evenodd" clip-rule="evenodd" d="M34.7409 59.7228L32.9567 58.9094C27.2672 56.3157 20.7328 56.3157 15.0433 58.9094C12.6018 60.0224 9.39163 59.0275 8.44602 56.0621C7.45647 52.9589 5.99975 46.5898 6 35.9997C6.00029 23.5648 8.00803 18.3599 9.11099 16.4196C9.67795 15.4222 10.5255 14.8455 11.2254 14.5264L12.0179 14.1651C19.6351 10.6926 28.4011 10.6738 36 14.1733C43.5989 10.6738 52.3649 10.6926 59.9821 14.1651L60.7746 14.5264C61.4745 14.8455 62.3221 15.4222 62.889 16.4196C63.992 18.3599 65.9997 23.5648 66 35.9997C66.0002 46.5898 64.5435 52.9589 63.554 56.0621C62.6084 59.0275 59.3982 60.0224 56.9567 58.9094C51.2672 56.3157 44.7328 56.3157 39.0433 58.9094L37.2591 59.7228C37.1986 59.7508 37.1373 59.7767 37.0753 59.8004C36.4484 60.0411 35.7556 60.0653 35.1102 59.8648C34.9847 59.8258 34.8613 59.7784 34.7409 59.7228ZM14.5068 19.6246C20.3733 16.9501 27.0874 16.8775 33 19.4067V52.473C26.7613 50.32 19.9378 50.471 13.7811 52.9261C13.0005 49.9843 11.9998 44.547 12 35.9998C12.0002 25.5786 13.4879 21.1893 14.1179 19.8018L14.5068 19.6246ZM39 52.473C45.2387 50.32 52.0622 50.471 58.2189 52.9261C58.9995 49.9843 60.0002 44.547 60 35.9998C59.9998 25.5786 58.5121 21.1893 57.8821 19.8018L57.4932 19.6246C51.6267 16.9501 44.9126 16.8775 39 19.4067V52.473Z" fill="currentColor"></path></svg>Deep Dive</h5><div class="mb-4"><h4 id="can-global-or-mutable-values-be-dependencies" class="mdx-heading text-xl font-bold text-primary dark:text-primary-dark text-xl font-display font-bold leading-9 my-4">Can global or mutable values be dependencies? <a href="#can-global-or-mutable-values-be-dependencies" aria-label="Link for Can global or mutable values be dependencies? " title="Link for Can global or mutable values be dependencies? " class="mdx-header-anchor inline-block"><svg width="1em" height="1em" viewBox="0 0 13 13" xmlns="http://www.w3.org/2000/svg" class="text-gray-70 ms-2 h-5 w-5"><g fill="currentColor" fill-rule="evenodd"><path d="M7.778 7.975a2.5 2.5 0 0 0 .347-3.837L6.017 2.03a2.498 2.498 0 0 0-3.542-.007 2.5 2.5 0 0 0 .006 3.543l1.153 1.15c.07-.29.154-.563.25-.773.036-.077.084-.16.14-.25L3.18 4.85a1.496 1.496 0 0 1 .002-2.12 1.496 1.496 0 0 1 2.12 0l2.124 2.123a1.496 1.496 0 0 1-.333 2.37c.16.246.42.504.685.752z"></path><path d="M5.657 4.557a2.5 2.5 0 0 0-.347 3.837l2.108 2.108a2.498 2.498 0 0 0 3.542.007 2.5 2.5 0 0 0-.006-3.543L9.802 5.815c-.07.29-.154.565-.25.774-.036.076-.084.16-.14.25l.842.84c.585.587.59 1.532 0 2.122-.587.585-1.532.59-2.12 0L6.008 7.68a1.496 1.496 0 0 1 .332-2.372c-.16-.245-.42-.503-.685-.75z"></path></g></svg></a></h4></div><button class="bg-purple-50 border-purple-50 hover:bg-purple-40 focus:bg-purple-50 active:bg-purple-50 text-base leading-tight font-bold rounded-full py-2 px-4 focus:outline focus:outline-offset-2 focus:outline-link dark:focus:outline-link-dark inline-flex items-center my-1 bg-link border-link text-white hover:bg-link focus:bg-link active:bg-link"><span class="me-1"><svg class="rotate-0" xmlns="http://www.w3.org/2000/svg" width="20" height="20" viewBox="0 0 20 20"><g fill="none" fill-rule="evenodd" transform="translate(-446 -398)"><path fill="currentColor" fill-rule="nonzero" d="M95.8838835,240.366117 C95.3957281,239.877961 94.6042719,239.877961 94.1161165,240.366117 C93.6279612,240.854272 93.6279612,241.645728 94.1161165,242.133883 L98.6161165,246.633883 C99.1042719,247.122039 99.8957281,247.122039 100.383883,246.633883 L104.883883,242.133883 C105.372039,241.645728 105.372039,240.854272 104.883883,240.366117 C104.395728,239.877961 103.604272,239.877961 103.116117,240.366117 L99.5,243.982233 L95.8838835,240.366117 Z" transform="translate(356.5 164.5)"></path><polygon points="446 418 466 418 466 398 446 398"></polygon></g></svg></span>Show Details</button></summary><div class="p-8 border-t dark:border-purple-60 border-purple-10 "><p class="whitespace-pre-wrap my-4">Mutable values (including global variables) aren’t reactive.</p><p class="whitespace-pre-wrap my-4"><strong class="font-bold">A mutable value like <a href="https://developer.mozilla.org/en-US/docs/Web/API/Location/pathname" target="_blank" rel="nofollow noopener noreferrer" class="inline text-link dark:text-link-dark border-b border-link border-opacity-0 hover:border-opacity-100 duration-100 ease-in transition leading-normal"><code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">location.pathname</code></a> can’t be a dependency.</strong> It’s mutable, so it can change at any time completely outside of the React rendering data flow. Changing it wouldn’t trigger a re-render of your component. Therefore, even if you specified it in the dependencies, React <em>wouldn’t know</em> to re-synchronize the Effect when it changes. This also breaks the rules of React because reading mutable data during rendering (which is when you calculate the dependencies) breaks <a class="inline text-link dark:text-link-dark border-b border-link border-opacity-0 hover:border-opacity-100 duration-100 ease-in transition leading-normal" href="/learn/keeping-components-pure">purity of rendering.</a> Instead, you should read and subscribe to an external mutable value with <a class="inline text-link dark:text-link-dark border-b border-link border-opacity-0 hover:border-opacity-100 duration-100 ease-in transition leading-normal" href="/learn/you-might-not-need-an-effect#subscribing-to-an-external-store"><code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">useSyncExternalStore</code>.</a></p><p class="whitespace-pre-wrap my-4"><strong class="font-bold">A mutable value like <a class="inline text-link dark:text-link-dark border-b border-link border-opacity-0 hover:border-opacity-100 duration-100 ease-in transition leading-normal" href="/reference/react/useRef#reference"><code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">ref.current</code></a> or things you read from it also can’t be a dependency.</strong> The ref object returned by <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">useRef</code> itself can be a dependency, but its <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">current</code> property is intentionally mutable. It lets you <a class="inline text-link dark:text-link-dark border-b border-link border-opacity-0 hover:border-opacity-100 duration-100 ease-in transition leading-normal" href="/learn/referencing-values-with-refs">keep track of something without triggering a re-render.</a> But since changing it doesn’t trigger a re-render, it’s not a reactive value, and React won’t know to re-run your Effect when it changes.</p><p class="whitespace-pre-wrap my-4">As you’ll learn below on this page, a linter will check for these issues automatically.</p></div></details>
<h3 id="react-verifies-that-you-specified-every-reactive-value-as-a-dependency" class="mdx-heading text-2xl font-display leading-9 text-primary dark:text-primary-dark font-bold my-6">React verifies that you specified every reactive value as a dependency <a href="#react-verifies-that-you-specified-every-reactive-value-as-a-dependency" aria-label="Link for React verifies that you specified every reactive value as a dependency " title="Link for React verifies that you specified every reactive value as a dependency " class="mdx-header-anchor inline-block"><svg width="1em" height="1em" viewBox="0 0 13 13" xmlns="http://www.w3.org/2000/svg" class="text-gray-70 ms-2 h-5 w-5"><g fill="currentColor" fill-rule="evenodd"><path d="M7.778 7.975a2.5 2.5 0 0 0 .347-3.837L6.017 2.03a2.498 2.498 0 0 0-3.542-.007 2.5 2.5 0 0 0 .006 3.543l1.153 1.15c.07-.29.154-.563.25-.773.036-.077.084-.16.14-.25L3.18 4.85a1.496 1.496 0 0 1 .002-2.12 1.496 1.496 0 0 1 2.12 0l2.124 2.123a1.496 1.496 0 0 1-.333 2.37c.16.246.42.504.685.752z"></path><path d="M5.657 4.557a2.5 2.5 0 0 0-.347 3.837l2.108 2.108a2.498 2.498 0 0 0 3.542.007 2.5 2.5 0 0 0-.006-3.543L9.802 5.815c-.07.29-.154.565-.25.774-.036.076-.084.16-.14.25l.842.84c.585.587.59 1.532 0 2.122-.587.585-1.532.59-2.12 0L6.008 7.68a1.496 1.496 0 0 1 .332-2.372c-.16-.245-.42-.503-.685-.75z"></path></g></svg></a></h3>
<p class="whitespace-pre-wrap my-4">If your linter is <a class="inline text-link dark:text-link-dark border-b border-link border-opacity-0 hover:border-opacity-100 duration-100 ease-in transition leading-normal" href="/learn/editor-setup#linting">configured for React,</a> it will check that every reactive value used by your Effect’s code is declared as its dependency. For example, this is a lint error because both <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">roomId</code> and <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">serverUrl</code> are reactive:</p>
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for App.js" aria-multiline="true" class="sp-pristine sp-javascript sp-cm" role="textbox" tabindex="0" translate="no"><pre class=" sp-pre-placeholder" style="margin-left:var(--sp-space-11)"><span class="sp-syntax-keyword">import</span> <span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-plain">useState</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">useEffect</span> <span class="sp-syntax-punctuation">}</span> <span class="sp-syntax-keyword">from</span> <span class="sp-syntax-string">'react'</span><span class="sp-syntax-punctuation">;</span>
<span class="sp-syntax-keyword">import</span> <span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-plain">createConnection</span> <span class="sp-syntax-punctuation">}</span> <span class="sp-syntax-keyword">from</span> <span class="sp-syntax-string">'./chat.js'</span><span class="sp-syntax-punctuation">;</span>
<span class="sp-syntax-keyword">function</span> <span class="sp-syntax-definition">ChatRoom</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-property">roomId</span> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-comment">// roomId is reactive</span>
<span class="sp-syntax-keyword">const</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">serverUrl</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">setServerUrl</span><span class="sp-syntax-punctuation">]</span> = <span class="sp-syntax-definition">useState</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-string">'https://localhost:1234'</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-comment">// serverUrl is reactive</span>
<span class="sp-syntax-definition">useEffect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span>
<span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">connection</span> = <span class="sp-syntax-definition">createConnection</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">serverUrl</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span>
<span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">connect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span>
<span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">disconnect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span>
<span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-punctuation">]</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-comment">// <-- Something's wrong here!</span>
<span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation">(</span>
<span class="sp-syntax-punctuation"><</span><span class="sp-syntax-punctuation">></span>
<span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">label</span><span class="sp-syntax-punctuation">></span>
Server URL:<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-string">' '</span><span class="sp-syntax-punctuation">}</span>
<span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">input</span>
<span class="sp-syntax-property">value</span>=<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">serverUrl</span><span class="sp-syntax-punctuation">}</span>
<span class="sp-syntax-property">onChange</span>=<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">e</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-definition">setServerUrl</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">e</span>.<span class="sp-syntax-property">target</span>.<span class="sp-syntax-property">value</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">}</span>
<span class="sp-syntax-punctuation">/></span>
<span class="sp-syntax-punctuation"></</span><span class="sp-syntax-tag">label</span><span class="sp-syntax-punctuation">></span>
<span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">h1</span><span class="sp-syntax-punctuation">></span>Welcome to the <span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">}</span> room!<span class="sp-syntax-punctuation"></</span><span class="sp-syntax-tag">h1</span><span class="sp-syntax-punctuation">></span>
<span class="sp-syntax-punctuation"></</span><span class="sp-syntax-punctuation">></span>
<span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span>
<span class="sp-syntax-punctuation">}</span>
<span class="sp-syntax-keyword">export</span> <span class="sp-syntax-keyword">default</span> <span class="sp-syntax-keyword">function</span> <span class="sp-syntax-definition">App</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span>
<span class="sp-syntax-keyword">const</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">setRoomId</span><span class="sp-syntax-punctuation">]</span> = <span class="sp-syntax-definition">useState</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-string">'general'</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span>
<span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation">(</span>
<span class="sp-syntax-punctuation"><</span><span class="sp-syntax-punctuation">></span>
<span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">label</span><span class="sp-syntax-punctuation">></span>
Choose the chat room:<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-string">' '</span><span class="sp-syntax-punctuation">}</span>
<span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">select</span>
<span class="sp-syntax-property">value</span>=<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">}</span>
<span class="sp-syntax-property">onChange</span>=<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">e</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-definition">setRoomId</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">e</span>.<span class="sp-syntax-property">target</span>.<span class="sp-syntax-property">value</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">}</span>
<span class="sp-syntax-punctuation">></span>
<span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">option</span> <span class="sp-syntax-property">value</span>=<span class="sp-syntax-string">"general"</span><span class="sp-syntax-punctuation">></span>general<span class="sp-syntax-punctuation"></</span><span class="sp-syntax-tag">option</span><span class="sp-syntax-punctuation">></span>
<span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">option</span> <span class="sp-syntax-property">value</span>=<span class="sp-syntax-string">"travel"</span><span class="sp-syntax-punctuation">></span>travel<span class="sp-syntax-punctuation"></</span><span class="sp-syntax-tag">option</span><span class="sp-syntax-punctuation">></span>
<span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">option</span> <span class="sp-syntax-property">value</span>=<span class="sp-syntax-string">"music"</span><span class="sp-syntax-punctuation">></span>music<span class="sp-syntax-punctuation"></</span><span class="sp-syntax-tag">option</span><span class="sp-syntax-punctuation">></span>
<span class="sp-syntax-punctuation"></</span><span class="sp-syntax-tag">select</span><span class="sp-syntax-punctuation">></span>
<span class="sp-syntax-punctuation"></</span><span class="sp-syntax-tag">label</span><span class="sp-syntax-punctuation">></span>
<span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">hr</span> <span class="sp-syntax-punctuation">/></span>
<span class="sp-syntax-punctuation"><</span><span class="sp-syntax-definition">ChatRoom</span> <span class="sp-syntax-property">roomId</span>=<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">}</span> <span class="sp-syntax-punctuation">/></span>
<span class="sp-syntax-punctuation"></</span><span class="sp-syntax-punctuation">></span>
<span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span>
<span class="sp-syntax-punctuation">}</span>
</pre></div></div></div><div class=" order-last xl:order-2 sp-stack"><div class="p-0 sm:p-2 md:p-4 lg:p-8 bg-card dark:bg-wash-dark h-full relative md:rounded-b-lg lg:rounded-b-none"><div style="position:relative"><iframe class="rounded-t-none bg-white md:shadow-md sm:rounded-lg w-full max-w-full transition-opacity absolute opacity-0 pointer-events-none duration-75" title="Sandbox Preview" style="height:15px;z-index:-1"></iframe></div></div></div><button translate="yes" class="sandpack-expand flex text-base justify-between dark:border-card-dark bg-wash dark:bg-card-dark items-center z-10 p-1 w-full order-2 xl:order-last border-b-1 relative top-0"><span class="flex p-2 focus:outline-none text-primary dark:text-primary-dark leading-[20px]"><svg class="rotate-0 inline me-1.5 text-xl" xmlns="http://www.w3.org/2000/svg" width="20" height="20" viewBox="0 0 20 20"><g fill="none" fill-rule="evenodd" transform="translate(-446 -398)"><path fill="currentColor" fill-rule="nonzero" d="M95.8838835,240.366117 C95.3957281,239.877961 94.6042719,239.877961 94.1161165,240.366117 C93.6279612,240.854272 93.6279612,241.645728 94.1161165,242.133883 L98.6161165,246.633883 C99.1042719,247.122039 99.8957281,247.122039 100.383883,246.633883 L104.883883,242.133883 C105.372039,241.645728 105.372039,240.854272 104.883883,240.366117 C104.395728,239.877961 103.604272,239.877961 103.116117,240.366117 L99.5,243.982233 L95.8838835,240.366117 Z" transform="translate(356.5 164.5)"></path><polygon points="446 418 466 418 466 398 446 398"></polygon></g></svg>Show more</span></button></div></div></div></div><!--/$--><div class="max-w-4xl ms-0 2xl:mx-auto">
<p class="whitespace-pre-wrap my-4">This may look like a React error, but really React is pointing out a bug in your code. Both <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">roomId</code> and <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">serverUrl</code> may change over time, but you’re forgetting to re-synchronize your Effect when they change. You will remain connected to the initial <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">roomId</code> and <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">serverUrl</code> even after the user picks different values in the UI.</p>
<p class="whitespace-pre-wrap my-4">To fix the bug, follow the linter’s suggestion to specify <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">roomId</code> and <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">serverUrl</code> as dependencies of your Effect:</p>
<!--$--><div dir="ltr" class="sandpack sandpack--codeblock rounded-2xl h-full w-full overflow-x-auto flex items-center bg-wash dark:bg-gray-95 shadow-lg my-8" style="contain:content"><div class="sp-wrapper"><div class="sp-stack"><div class="sp-code-editor"><pre class="sp-cm sp-pristine sp-javascript flex align-start"><code class="sp-pre-placeholder grow-[2]"><div class="cm-line "><span class="sp-syntax-keyword">function</span> <span class="sp-syntax-definition">ChatRoom</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-property">roomId</span> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-comment">// roomId is reactive</span><br></div><div class="cm-line "> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">serverUrl</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">setServerUrl</span><span class="sp-syntax-punctuation">]</span> = <span class="sp-syntax-definition">useState</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-string">'https://localhost:1234'</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-comment">// serverUrl is reactive</span><br></div><div class="cm-line "> <span class="sp-syntax-definition">useEffect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br></div><div class="cm-line "> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">connection</span> = <span class="sp-syntax-definition">createConnection</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">serverUrl</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br></div><div class="cm-line "> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">connect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br></div><div class="cm-line "> <span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br></div><div class="cm-line "> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">disconnect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br></div><div class="cm-line "> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">;</span><br></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">serverUrl</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">]</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-comment">// ✅ All dependencies declared</span><br></div><div class="cm-line "> <span class="sp-syntax-comment">// ...</span><br></div><div class="cm-line "><span class="sp-syntax-punctuation">}</span></div></code></pre></div></div></div></div><!--/$-->
<p class="whitespace-pre-wrap my-4">Try this fix in the sandbox above. Verify that the linter error is gone, and the chat re-connects when needed.</p>
<div class="expandable-callout pt-8 pb-4 px-5 sm:px-8 my-8 relative rounded-none shadow-inner-border -mx-5 sm:mx-auto sm:rounded-2xl bg-green-5 dark:bg-green-60 dark:bg-opacity-20 text-primary dark:text-primary-dark text-lg"><h3 class="text-2xl font-display font-bold text-green-60 dark:text-green-40"><svg class="inline me-3 mb-1 text-lg text-green-60 dark:text-green-40" width="2em" height="2em" viewBox="0 0 72 72" fill="none" xmlns="http://www.w3.org/2000/svg"><g clip-path="url(#clip0_40_48064)"><path d="M24 27C24 25.3431 25.3431 24 27 24H45C46.6569 24 48 25.3431 48 27C48 28.6569 46.6569 30 45 30H27C25.3431 30 24 28.6569 24 27Z" fill="currentColor"></path><path d="M24 39C24 37.3431 25.3431 36 27 36H39C40.6569 36 42 37.3431 42 39C42 40.6569 40.6569 42 39 42H27C25.3431 42 24 40.6569 24 39Z" fill="currentColor"></path><path fill-rule="evenodd" clip-rule="evenodd" d="M12 18C12 13.0294 16.0294 9 21 9H51C55.9706 9 60 13.0294 60 18V54C60 58.9706 55.9706 63 51 63H21C16.0294 63 12 58.9706 12 54V18ZM21 15H51C52.6569 15 54 16.3431 54 18V54C54 55.6569 52.6569 57 51 57H21C19.3431 57 18 55.6569 18 54V18C18 16.3431 19.3431 15 21 15Z" fill="currentColor"></path></g><defs><clipPath id="clip0_40_48064"><rect width="72" height="72" fill="white"></rect></clipPath></defs></svg>Note</h3><div class="relative"><div class="py-2"><p class="whitespace-pre-wrap my-4">In some cases, React <em>knows</em> that a value never changes even though it’s declared inside the component. For example, the <a class="inline text-link dark:text-link-dark border-b border-link border-opacity-0 hover:border-opacity-100 duration-100 ease-in transition leading-normal" href="/reference/react/useState#setstate"><code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">set</code> function</a> returned from <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">useState</code> and the ref object returned by <a class="inline text-link dark:text-link-dark border-b border-link border-opacity-0 hover:border-opacity-100 duration-100 ease-in transition leading-normal" href="/reference/react/useRef"><code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">useRef</code></a> are <em>stable</em>—they are guaranteed to not change on a re-render. Stable values aren’t reactive, so you may omit them from the list. Including them is allowed: they won’t change, so it doesn’t matter.</p></div></div></div>
<h3 id="what-to-do-when-you-dont-want-to-re-synchronize" class="mdx-heading text-2xl font-display leading-9 text-primary dark:text-primary-dark font-bold my-6">What to do when you don’t want to re-synchronize <a href="#what-to-do-when-you-dont-want-to-re-synchronize" aria-label="Link for What to do when you don’t want to re-synchronize " title="Link for What to do when you don’t want to re-synchronize " class="mdx-header-anchor inline-block"><svg width="1em" height="1em" viewBox="0 0 13 13" xmlns="http://www.w3.org/2000/svg" class="text-gray-70 ms-2 h-5 w-5"><g fill="currentColor" fill-rule="evenodd"><path d="M7.778 7.975a2.5 2.5 0 0 0 .347-3.837L6.017 2.03a2.498 2.498 0 0 0-3.542-.007 2.5 2.5 0 0 0 .006 3.543l1.153 1.15c.07-.29.154-.563.25-.773.036-.077.084-.16.14-.25L3.18 4.85a1.496 1.496 0 0 1 .002-2.12 1.496 1.496 0 0 1 2.12 0l2.124 2.123a1.496 1.496 0 0 1-.333 2.37c.16.246.42.504.685.752z"></path><path d="M5.657 4.557a2.5 2.5 0 0 0-.347 3.837l2.108 2.108a2.498 2.498 0 0 0 3.542.007 2.5 2.5 0 0 0-.006-3.543L9.802 5.815c-.07.29-.154.565-.25.774-.036.076-.084.16-.14.25l.842.84c.585.587.59 1.532 0 2.122-.587.585-1.532.59-2.12 0L6.008 7.68a1.496 1.496 0 0 1 .332-2.372c-.16-.245-.42-.503-.685-.75z"></path></g></svg></a></h3>
<p class="whitespace-pre-wrap my-4">In the previous example, you’ve fixed the lint error by listing <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">roomId</code> and <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">serverUrl</code> as dependencies.</p>
<p class="whitespace-pre-wrap my-4"><strong class="font-bold">However, you could instead “prove” to the linter that these values aren’t reactive values,</strong> i.e. that they <em>can’t</em> change as a result of a re-render. For example, if <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">serverUrl</code> and <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">roomId</code> don’t depend on rendering and always have the same values, you can move them outside the component. Now they don’t need to be dependencies:</p>
<!--$--><div dir="ltr" class="sandpack sandpack--codeblock rounded-2xl h-full w-full overflow-x-auto flex items-center bg-wash dark:bg-gray-95 shadow-lg my-8" style="contain:content"><div class="sp-wrapper"><div class="sp-stack"><div class="sp-code-editor"><pre class="sp-cm sp-pristine sp-javascript flex align-start"><code class="sp-pre-placeholder grow-[2]"><div class="cm-line bg-github-highlight dark:bg-opacity-10"><span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">serverUrl</span> = <span class="sp-syntax-string">'https://localhost:1234'</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-comment">// serverUrl is not reactive</span><br></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"><span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">roomId</span> = <span class="sp-syntax-string">'general'</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-comment">// roomId is not reactive</span><br></div><div class="cm-line "><br></div><div class="cm-line "><span class="sp-syntax-keyword">function</span> <span class="sp-syntax-definition">ChatRoom</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span><br></div><div class="cm-line "> <span class="sp-syntax-definition">useEffect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br></div><div class="cm-line "> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">connection</span> = <span class="sp-syntax-definition">createConnection</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">serverUrl</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br></div><div class="cm-line "> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">connect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br></div><div class="cm-line "> <span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br></div><div class="cm-line "> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">disconnect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br></div><div class="cm-line "> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">;</span><br></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-punctuation">]</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-comment">// ✅ All dependencies declared</span><br></div><div class="cm-line "> <span class="sp-syntax-comment">// ...</span><br></div><div class="cm-line "><span class="sp-syntax-punctuation">}</span></div></code></pre></div></div></div></div><!--/$-->
<p class="whitespace-pre-wrap my-4">You can also move them <em>inside the Effect.</em> They aren’t calculated during rendering, so they’re not reactive:</p>
<!--$--><div dir="ltr" class="sandpack sandpack--codeblock rounded-2xl h-full w-full overflow-x-auto flex items-center bg-wash dark:bg-gray-95 shadow-lg my-8" style="contain:content"><div class="sp-wrapper"><div class="sp-stack"><div class="sp-code-editor"><pre class="sp-cm sp-pristine sp-javascript flex align-start"><code class="sp-pre-placeholder grow-[2]"><div class="cm-line "><span class="sp-syntax-keyword">function</span> <span class="sp-syntax-definition">ChatRoom</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span><br></div><div class="cm-line "> <span class="sp-syntax-definition">useEffect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">serverUrl</span> = <span class="sp-syntax-string">'https://localhost:1234'</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-comment">// serverUrl is not reactive</span><br></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">roomId</span> = <span class="sp-syntax-string">'general'</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-comment">// roomId is not reactive</span><br></div><div class="cm-line "> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">connection</span> = <span class="sp-syntax-definition">createConnection</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">serverUrl</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br></div><div class="cm-line "> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">connect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br></div><div class="cm-line "> <span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br></div><div class="cm-line "> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">disconnect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br></div><div class="cm-line "> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">;</span><br></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-punctuation">]</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-comment">// ✅ All dependencies declared</span><br></div><div class="cm-line "> <span class="sp-syntax-comment">// ...</span><br></div><div class="cm-line "><span class="sp-syntax-punctuation">}</span></div></code></pre></div></div></div></div><!--/$-->
<p class="whitespace-pre-wrap my-4"><strong class="font-bold">Effects are reactive blocks of code.</strong> They re-synchronize when the values you read inside of them change. Unlike event handlers, which only run once per interaction, Effects run whenever synchronization is necessary.</p>
<p class="whitespace-pre-wrap my-4"><strong class="font-bold">You can’t “choose” your dependencies.</strong> Your dependencies must include every <a class="inline text-link dark:text-link-dark border-b border-link border-opacity-0 hover:border-opacity-100 duration-100 ease-in transition leading-normal" href="#all-variables-declared-in-the-component-body-are-reactive">reactive value</a> you read in the Effect. The linter enforces this. Sometimes this may lead to problems like infinite loops and to your Effect re-synchronizing too often. Don’t fix these problems by suppressing the linter! Here’s what to try instead:</p>
<ul class="ms-6 my-3 list-disc">
<li class="leading-relaxed mb-1">
<p class="whitespace-pre-wrap my-4"><strong class="font-bold">Check that your Effect represents an independent synchronization process.</strong> If your Effect doesn’t synchronize anything, <a class="inline text-link dark:text-link-dark border-b border-link border-opacity-0 hover:border-opacity-100 duration-100 ease-in transition leading-normal" href="/learn/you-might-not-need-an-effect">it might be unnecessary.</a> If it synchronizes several independent things, <a class="inline text-link dark:text-link-dark border-b border-link border-opacity-0 hover:border-opacity-100 duration-100 ease-in transition leading-normal" href="#each-effect-represents-a-separate-synchronization-process">split it up.</a></p>
</li>
<li class="leading-relaxed mb-1">
<p class="whitespace-pre-wrap my-4"><strong class="font-bold">If you want to read the latest value of props or state without “reacting” to it and re-synchronizing the Effect,</strong> you can split your Effect into a reactive part (which you’ll keep in the Effect) and a non-reactive part (which you’ll extract into something called an <em>Effect Event</em>). <a class="inline text-link dark:text-link-dark border-b border-link border-opacity-0 hover:border-opacity-100 duration-100 ease-in transition leading-normal" href="/learn/separating-events-from-effects">Read about separating Events from Effects.</a></p>
</li>
<li class="leading-relaxed mb-1">
<p class="whitespace-pre-wrap my-4"><strong class="font-bold">Avoid relying on objects and functions as dependencies.</strong> If you create objects and functions during rendering and then read them from an Effect, they will be different on every render. This will cause your Effect to re-synchronize every time. <a class="inline text-link dark:text-link-dark border-b border-link border-opacity-0 hover:border-opacity-100 duration-100 ease-in transition leading-normal" href="/learn/removing-effect-dependencies">Read more about removing unnecessary dependencies from Effects.</a></p>
</li>
</ul>
<div class="expandable-callout pt-8 pb-4 px-5 sm:px-8 my-8 relative rounded-none shadow-inner-border -mx-5 sm:mx-auto sm:rounded-2xl bg-yellow-5 dark:bg-yellow-60 dark:bg-opacity-20"><h3 class="text-2xl font-display font-bold text-yellow-50 dark:text-yellow-40"><svg class="inline me-3 mb-1 text-lg text-yellow-50 dark:text-yellow-40" width="2em" height="2em" viewBox="0 0 72 72" fill="none" xmlns="http://www.w3.org/2000/svg"><g clip-path="url(#clip0_738_836)"><path fill-rule="evenodd" clip-rule="evenodd" d="M27 48L27 57.3409L40.0772 48L55.6975 48C57.1595 48 58.1986 47.0112 58.3851 45.8604C59.1824 40.9398 60 34.619 60 29.625C60 24.7282 59.2125 18.7546 58.4302 14.0813C58.2445 12.9721 57.2326 12 55.7805 12L16.2195 12C14.7674 12 13.7555 12.9721 13.5698 14.0813C12.7875 18.7546 12 24.7282 12 29.625C12 34.619 12.8176 40.9398 13.6149 45.8604C13.8014 47.0112 14.8404 48 16.3025 48H27ZM42 54H55.6975C59.9534 54 63.6271 51.0213 64.3078 46.8201C65.1161 41.8322 66 35.1209 66 29.625C66 24.2196 65.1449 17.8522 64.3478 13.0906C63.6513 8.93026 59.9987 6 55.7805 6H16.2195C12.0013 6 8.34867 8.93026 7.65218 13.0906C6.85505 17.8522 6 24.2196 6 29.625C6 35.1209 6.88391 41.8322 7.69215 46.8201C8.37291 51.0213 12.0466 54 16.3025 54H21L21 63.1704C21 65.6106 23.7581 67.0299 25.7437 65.6116L42 54ZM39 39.3686C39 40.9422 38 41.9912 36 41.9912C34 41.9912 33 40.9422 33 39.3686C33 37.7951 34 36.746 36 36.746C38 36.746 39 37.7951 39 39.3686ZM38.1771 20.2412C38.1771 18.9986 37.1697 17.9912 35.9271 17.9912C34.6845 17.9912 33.6771 18.9986 33.6771 20.2412V31.5956C33.6771 32.8382 34.6845 33.8456 35.9271 33.8456C37.1697 33.8456 38.1771 32.8382 38.1771 31.5956V20.2412Z" fill="currentColor"></path></g><defs><clipPath id="clip0_738_836"><rect width="72" height="72" fill="white"></rect></clipPath></defs></svg>Pitfall</h3><div class="relative"><div class="py-2"><p class="whitespace-pre-wrap my-4">The linter is your friend, but its powers are limited. The linter only knows when the dependencies are <em>wrong</em>. It doesn’t know <em>the best</em> way to solve each case. If the linter suggests a dependency, but adding it causes a loop, it doesn’t mean the linter should be ignored. You need to change the code inside (or outside) the Effect so that that value isn’t reactive and doesn’t <em>need</em> to be a dependency.</p><p class="whitespace-pre-wrap my-4">If you have an existing codebase, you might have some Effects that suppress the linter like this:</p><!--$--><div dir="ltr" class="sandpack sandpack--codeblock rounded-2xl h-full w-full overflow-x-auto flex items-center bg-wash dark:bg-gray-95 shadow-lg my-8" style="contain:content"><div class="sp-wrapper"><div class="sp-stack"><div class="sp-code-editor"><pre class="sp-cm sp-pristine sp-javascript flex align-start"><code class="sp-pre-placeholder grow-[2]"><div class="cm-line "><span class="sp-syntax-definition">useEffect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br></div><div class="cm-line "> <span class="sp-syntax-comment">// ...</span><br></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-comment">// 🔴 Avoid suppressing the linter like this:</span><br></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-comment">// eslint-ignore-next-line react-hooks/exhaustive-deps</span><br></div><div class="cm-line "><span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-punctuation">]</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span></div></code></pre></div></div></div></div><!--/$--><p class="whitespace-pre-wrap my-4">On the <a class="inline text-link dark:text-link-dark border-b border-link border-opacity-0 hover:border-opacity-100 duration-100 ease-in transition leading-normal" href="/learn/separating-events-from-effects">next</a> <a class="inline text-link dark:text-link-dark border-b border-link border-opacity-0 hover:border-opacity-100 duration-100 ease-in transition leading-normal" href="/learn/removing-effect-dependencies">pages</a>, you’ll learn how to fix this code without breaking the rules. It’s always worth fixing!</p></div></div></div>
<section><h2 id="recap" class="mdx-heading text-3xl font-display leading-10 text-primary dark:text-primary-dark font-bold my-6">Recap<a href="#recap" aria-label="Link for Recap" title="Link for Recap" class="mdx-header-anchor inline-block"><svg width="1em" height="1em" viewBox="0 0 13 13" xmlns="http://www.w3.org/2000/svg" class="text-gray-70 ms-2 h-5 w-5"><g fill="currentColor" fill-rule="evenodd"><path d="M7.778 7.975a2.5 2.5 0 0 0 .347-3.837L6.017 2.03a2.498 2.498 0 0 0-3.542-.007 2.5 2.5 0 0 0 .006 3.543l1.153 1.15c.07-.29.154-.563.25-.773.036-.077.084-.16.14-.25L3.18 4.85a1.496 1.496 0 0 1 .002-2.12 1.496 1.496 0 0 1 2.12 0l2.124 2.123a1.496 1.496 0 0 1-.333 2.37c.16.246.42.504.685.752z"></path><path d="M5.657 4.557a2.5 2.5 0 0 0-.347 3.837l2.108 2.108a2.498 2.498 0 0 0 3.542.007 2.5 2.5 0 0 0-.006-3.543L9.802 5.815c-.07.29-.154.565-.25.774-.036.076-.084.16-.14.25l.842.84c.585.587.59 1.532 0 2.122-.587.585-1.532.59-2.12 0L6.008 7.68a1.496 1.496 0 0 1 .332-2.372c-.16-.245-.42-.503-.685-.75z"></path></g></svg></a></h2><ul class="ms-6 my-3 list-disc">
<li class="leading-relaxed mb-1">Components can mount, update, and unmount.</li>
<li class="leading-relaxed mb-1">Each Effect has a separate lifecycle from the surrounding component.</li>
<li class="leading-relaxed mb-1">Each Effect describes a separate synchronization process that can <em>start</em> and <em>stop</em>.</li>
<li class="leading-relaxed mb-1">When you write and read Effects, think from each individual Effect’s perspective (how to start and stop synchronization) rather than from the component’s perspective (how it mounts, updates, or unmounts).</li>
<li class="leading-relaxed mb-1">Values declared inside the component body are “reactive”.</li>
<li class="leading-relaxed mb-1">Reactive values should re-synchronize the Effect because they can change over time.</li>
<li class="leading-relaxed mb-1">The linter verifies that all reactive values used inside the Effect are specified as dependencies.</li>
<li class="leading-relaxed mb-1">All errors flagged by the linter are legitimate. There’s always a way to fix the code to not break the rules.</li>
</ul></section>
</div><div class="max-w-7xl mx-auto py-4 w-full"><div class="border-gray-10 bg-card dark:bg-card-dark shadow-inner rounded-none -mx-5 sm:mx-auto sm:rounded-2xl"><div class="py-2 px-5 sm:px-8 pb-0 md:pb-0"><h2 id="challenges" class="mdx-heading text-3xl font-display leading-10 text-primary dark:text-primary-dark font-bold my-6 mb-2 leading-10 relative text-3xl text-link">Try out some challenges<a href="#challenges" aria-label="Link for Try out some challenges" title="Link for Try out some challenges" class="mdx-header-anchor inline-block"><svg width="1em" height="1em" viewBox="0 0 13 13" xmlns="http://www.w3.org/2000/svg" class="text-gray-70 ms-2 h-5 w-5"><g fill="currentColor" fill-rule="evenodd"><path d="M7.778 7.975a2.5 2.5 0 0 0 .347-3.837L6.017 2.03a2.498 2.498 0 0 0-3.542-.007 2.5 2.5 0 0 0 .006 3.543l1.153 1.15c.07-.29.154-.563.25-.773.036-.077.084-.16.14-.25L3.18 4.85a1.496 1.496 0 0 1 .002-2.12 1.496 1.496 0 0 1 2.12 0l2.124 2.123a1.496 1.496 0 0 1-.333 2.37c.16.246.42.504.685.752z"></path><path d="M5.657 4.557a2.5 2.5 0 0 0-.347 3.837l2.108 2.108a2.498 2.498 0 0 0 3.542.007 2.5 2.5 0 0 0-.006-3.543L9.802 5.815c-.07.29-.154.565-.25.774-.036.076-.084.16-.14.25l.842.84c.585.587.59 1.532 0 2.122-.587.585-1.532.59-2.12 0L6.008 7.68a1.496 1.496 0 0 1 .332-2.372c-.16-.245-.42-.503-.685-.75z"></path></g></svg></a></h2><div class="flex items-center justify-between"><div class="overflow-hidden"><div class="flex relative transition-transform content-box overflow-x-auto"><button class="py-2 me-4 text-base border-b-4 duration-100 ease-in transition whitespace-nowrap text-ellipsis text-link border-link hover:text-link dark:text-link-dark dark:border-link-dark dark:hover:text-link-dark">1<!-- -->. <!-- -->Fix reconnecting on every keystroke </button><button class="py-2 me-4 text-base border-b-4 duration-100 ease-in transition whitespace-nowrap text-ellipsis">2<!-- -->. <!-- -->Switch synchronization on and off </button><button class="py-2 me-4 text-base border-b-4 duration-100 ease-in transition whitespace-nowrap text-ellipsis">3<!-- -->. <!-- -->Investigate a stale value bug </button><button class="py-2 me-4 text-base border-b-4 duration-100 ease-in transition whitespace-nowrap text-ellipsis">4<!-- -->. <!-- -->Fix a connection switch </button><button class="py-2 me-4 text-base border-b-4 duration-100 ease-in transition whitespace-nowrap text-ellipsis">5<!-- -->. <!-- -->Populate a chain of select boxes </button></div></div><div class="flex z-10 pb-2 ps-2"><button aria-label="Scroll left" class="bg-secondary-button dark:bg-secondary-button-dark h-8 px-2 rounded-l border-gray-20 border-r rtl:rotate-180 text-gray-30"><svg class="rotate-90 rtl:-rotate-90" xmlns="http://www.w3.org/2000/svg" width="20" height="20" viewBox="0 0 20 20"><g fill="none" fill-rule="evenodd" transform="translate(-446 -398)"><path fill="currentColor" fill-rule="nonzero" d="M95.8838835,240.366117 C95.3957281,239.877961 94.6042719,239.877961 94.1161165,240.366117 C93.6279612,240.854272 93.6279612,241.645728 94.1161165,242.133883 L98.6161165,246.633883 C99.1042719,247.122039 99.8957281,247.122039 100.383883,246.633883 L104.883883,242.133883 C105.372039,241.645728 105.372039,240.854272 104.883883,240.366117 C104.395728,239.877961 103.604272,239.877961 103.116117,240.366117 L99.5,243.982233 L95.8838835,240.366117 Z" transform="translate(356.5 164.5)"></path><polygon points="446 418 466 418 466 398 446 398"></polygon></g></svg></button><button aria-label="Scroll right" class="bg-secondary-button dark:bg-secondary-button-dark h-8 px-2 rounded-e rtl:rotate-180 text-primary dark:text-primary-dark"><svg class="-rotate-90 rtl:rotate-90" xmlns="http://www.w3.org/2000/svg" width="20" height="20" viewBox="0 0 20 20"><g fill="none" fill-rule="evenodd" transform="translate(-446 -398)"><path fill="currentColor" fill-rule="nonzero" d="M95.8838835,240.366117 C95.3957281,239.877961 94.6042719,239.877961 94.1161165,240.366117 C93.6279612,240.854272 93.6279612,241.645728 94.1161165,242.133883 L98.6161165,246.633883 C99.1042719,247.122039 99.8957281,247.122039 100.383883,246.633883 L104.883883,242.133883 C105.372039,241.645728 105.372039,240.854272 104.883883,240.366117 C104.395728,239.877961 103.604272,239.877961 103.116117,240.366117 L99.5,243.982233 L95.8838835,240.366117 Z" transform="translate(356.5 164.5)"></path><polygon points="446 418 466 418 466 398 446 398"></polygon></g></svg></button></div></div></div><div class="p-5 sm:py-8 sm:px-8"><div><h4 id="fix-reconnecting-on-every-keystroke" class="mdx-heading text-xl text-primary dark:text-primary-dark mb-2 mt-0 font-medium text-xl font-display font-bold leading-9 my-4"><div class="font-bold block md:inline">Challenge<!-- --> <!-- -->1<!-- --> of<!-- --> <!-- -->5<span class="text-primary dark:text-primary-dark">: </span></div>Fix reconnecting on every keystroke <a href="#fix-reconnecting-on-every-keystroke" aria-label="Link for this heading" title="Link for this heading" class="mdx-header-anchor inline-block"><svg width="1em" height="1em" viewBox="0 0 13 13" xmlns="http://www.w3.org/2000/svg" class="text-gray-70 ms-2 h-5 w-5"><g fill="currentColor" fill-rule="evenodd"><path d="M7.778 7.975a2.5 2.5 0 0 0 .347-3.837L6.017 2.03a2.498 2.498 0 0 0-3.542-.007 2.5 2.5 0 0 0 .006 3.543l1.153 1.15c.07-.29.154-.563.25-.773.036-.077.084-.16.14-.25L3.18 4.85a1.496 1.496 0 0 1 .002-2.12 1.496 1.496 0 0 1 2.12 0l2.124 2.123a1.496 1.496 0 0 1-.333 2.37c.16.246.42.504.685.752z"></path><path d="M5.657 4.557a2.5 2.5 0 0 0-.347 3.837l2.108 2.108a2.498 2.498 0 0 0 3.542.007 2.5 2.5 0 0 0-.006-3.543L9.802 5.815c-.07.29-.154.565-.25.774-.036.076-.084.16-.14.25l.842.84c.585.587.59 1.532 0 2.122-.587.585-1.532.59-2.12 0L6.008 7.68a1.496 1.496 0 0 1 .332-2.372c-.16-.245-.42-.503-.685-.75z"></path></g></svg></a></h4><p class="whitespace-pre-wrap my-4">In this example, the <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">ChatRoom</code> component connects to the chat room when the component mounts, disconnects when it unmounts, and reconnects when you select a different chat room. This behavior is correct, so you need to keep it working.</p><p class="whitespace-pre-wrap my-4">However, there is a problem. Whenever you type into the message box input at the bottom, <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">ChatRoom</code> <em>also</em> reconnects to the chat. (You can notice this by clearing the console and typing into the input.) Fix the issue so that this doesn’t happen.</p><!--$--><div class="sandpack sandpack--playground w-full my-8" dir="ltr"><div class="sp-wrapper"><div class="shadow-lg dark:shadow-lg-dark rounded-lg" style="contain:content"><div class="bg-wash dark:bg-card-dark flex justify-between items-center relative z-10 border-b border-border dark:border-border-dark rounded-t-lg text-lg"><div class="flex-1 grow min-w-0 px-4 lg:px-6"><div><div class="relative overflow-hidden"><div class="w-[fit-content]"><div class=" sp-tabs" translate="no"><div aria-label="Select active file" class=" sp-tabs-scrollable-container" role="tablist"><button aria-selected="true" class=" sp-tab-button" data-active="true" role="tab" title="/src/App.js" type="button">App.js</button><button aria-selected="false" class=" sp-tab-button" data-active="false" role="tab" title="/src/chat.js" type="button">chat.js</button></div></div></div><button class="absolute top-0 start-[2px] invisible" id="headlessui-listbox-button-:R4pf3acq6:" aria-haspopup="true" aria-expanded="false" data-headlessui-state="" type="button"><span class="h-full py-2 px-1 mt-px -mb-px flex border-b text-link dark:text-link-dark border-link dark:border-link-dark items-center text-md leading-tight truncate" style="max-width:160px">App.js<span class="ms-2"><svg class="rotate-0" xmlns="http://www.w3.org/2000/svg" width="20" height="20" viewBox="0 0 20 20"><g fill="none" fill-rule="evenodd" transform="translate(-446 -398)"><path fill="currentColor" fill-rule="nonzero" d="M95.8838835,240.366117 C95.3957281,239.877961 94.6042719,239.877961 94.1161165,240.366117 C93.6279612,240.854272 93.6279612,241.645728 94.1161165,242.133883 L98.6161165,246.633883 C99.1042719,247.122039 99.8957281,247.122039 100.383883,246.633883 L104.883883,242.133883 C105.372039,241.645728 105.372039,240.854272 104.883883,240.366117 C104.395728,239.877961 103.604272,239.877961 103.116117,240.366117 L99.5,243.982233 L95.8838835,240.366117 Z" transform="translate(356.5 164.5)"></path><polygon points="446 418 466 418 466 398 446 398"></polygon></g></svg></span></span></button></div></div></div><div class="px-3 flex items-center justify-end text-start" translate="yes"><button class="text-sm text-primary dark:text-primary-dark inline-flex items-center hover:text-link duration-100 ease-in transition mx-1" title="Reset Sandbox" type="button"><svg width="1em" height="1em" viewBox="0 0 24 24" fill="none" xmlns="http://www.w3.org/2000/svg" class="inline mx-1 relative"><path d="M13.8982 5.20844C12.4626 4.88688 10.9686 4.93769 9.55821 5.35604L11.8524 3.06184C11.8989 3.0154 11.9357 2.96028 11.9608 2.89961C11.986 2.83894 11.9989 2.77391 11.9989 2.70824C11.9989 2.64256 11.986 2.57754 11.9608 2.51686C11.9357 2.45619 11.8989 2.40107 11.8524 2.35464L11.1456 1.64784C11.0992 1.60139 11.0441 1.56455 10.9834 1.53942C10.9227 1.51428 10.8577 1.50134 10.792 1.50134C10.7263 1.50134 10.6613 1.51428 10.6006 1.53942C10.54 1.56455 10.4848 1.60139 10.4384 1.64784L6.14571 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for App.js" aria-multiline="true" class="sp-pristine sp-javascript sp-cm" role="textbox" tabindex="0" translate="no"><pre class=" sp-pre-placeholder" style="margin-left:var(--sp-space-11)"><span class="sp-syntax-keyword">import</span> <span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-plain">useState</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">useEffect</span> <span class="sp-syntax-punctuation">}</span> <span class="sp-syntax-keyword">from</span> <span class="sp-syntax-string">'react'</span><span class="sp-syntax-punctuation">;</span>
<span class="sp-syntax-keyword">import</span> <span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-plain">createConnection</span> <span class="sp-syntax-punctuation">}</span> <span class="sp-syntax-keyword">from</span> <span class="sp-syntax-string">'./chat.js'</span><span class="sp-syntax-punctuation">;</span>
<span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">serverUrl</span> = <span class="sp-syntax-string">'https://localhost:1234'</span><span class="sp-syntax-punctuation">;</span>
<span class="sp-syntax-keyword">function</span> <span class="sp-syntax-definition">ChatRoom</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-property">roomId</span> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span>
<span class="sp-syntax-keyword">const</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">message</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">setMessage</span><span class="sp-syntax-punctuation">]</span> = <span class="sp-syntax-definition">useState</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-string">''</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span>
<span class="sp-syntax-definition">useEffect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span>
<span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">connection</span> = <span class="sp-syntax-definition">createConnection</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">serverUrl</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span>
<span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">connect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span>
<span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">disconnect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span>
<span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span>
<span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation">(</span>
<span class="sp-syntax-punctuation"><</span><span class="sp-syntax-punctuation">></span>
<span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">h1</span><span class="sp-syntax-punctuation">></span>Welcome to the <span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">}</span> room!<span class="sp-syntax-punctuation"></</span><span class="sp-syntax-tag">h1</span><span class="sp-syntax-punctuation">></span>
<span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">input</span>
<span class="sp-syntax-property">value</span>=<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">message</span><span class="sp-syntax-punctuation">}</span>
<span class="sp-syntax-property">onChange</span>=<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">e</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-definition">setMessage</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">e</span>.<span class="sp-syntax-property">target</span>.<span class="sp-syntax-property">value</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">}</span>
<span class="sp-syntax-punctuation">/></span>
<span class="sp-syntax-punctuation"></</span><span class="sp-syntax-punctuation">></span>
<span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span>
<span class="sp-syntax-punctuation">}</span>
<span class="sp-syntax-keyword">export</span> <span class="sp-syntax-keyword">default</span> <span class="sp-syntax-keyword">function</span> <span class="sp-syntax-definition">App</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span>
<span class="sp-syntax-keyword">const</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">setRoomId</span><span class="sp-syntax-punctuation">]</span> = <span class="sp-syntax-definition">useState</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-string">'general'</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span>
<span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation">(</span>
<span class="sp-syntax-punctuation"><</span><span class="sp-syntax-punctuation">></span>
<span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">label</span><span class="sp-syntax-punctuation">></span>
Choose the chat room:<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-string">' '</span><span class="sp-syntax-punctuation">}</span>
<span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">select</span>
<span class="sp-syntax-property">value</span>=<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">}</span>
<span class="sp-syntax-property">onChange</span>=<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">e</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-definition">setRoomId</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">e</span>.<span class="sp-syntax-property">target</span>.<span class="sp-syntax-property">value</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">}</span>
<span class="sp-syntax-punctuation">></span>
<span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">option</span> <span class="sp-syntax-property">value</span>=<span class="sp-syntax-string">"general"</span><span class="sp-syntax-punctuation">></span>general<span class="sp-syntax-punctuation"></</span><span class="sp-syntax-tag">option</span><span class="sp-syntax-punctuation">></span>
<span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">option</span> <span class="sp-syntax-property">value</span>=<span class="sp-syntax-string">"travel"</span><span class="sp-syntax-punctuation">></span>travel<span class="sp-syntax-punctuation"></</span><span class="sp-syntax-tag">option</span><span class="sp-syntax-punctuation">></span>
<span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">option</span> <span class="sp-syntax-property">value</span>=<span class="sp-syntax-string">"music"</span><span class="sp-syntax-punctuation">></span>music<span class="sp-syntax-punctuation"></</span><span class="sp-syntax-tag">option</span><span class="sp-syntax-punctuation">></span>
<span class="sp-syntax-punctuation"></</span><span class="sp-syntax-tag">select</span><span class="sp-syntax-punctuation">></span>
<span class="sp-syntax-punctuation"></</span><span class="sp-syntax-tag">label</span><span class="sp-syntax-punctuation">></span>
<span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">hr</span> <span class="sp-syntax-punctuation">/></span>
<span class="sp-syntax-punctuation"><</span><span class="sp-syntax-definition">ChatRoom</span> <span class="sp-syntax-property">roomId</span>=<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">}</span> <span class="sp-syntax-punctuation">/></span>
<span class="sp-syntax-punctuation"></</span><span class="sp-syntax-punctuation">></span>
<span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span>
<span class="sp-syntax-punctuation">}</span>
</pre></div></div></div><div class=" order-last xl:order-2 sp-stack"><div class="p-0 sm:p-2 md:p-4 lg:p-8 bg-card dark:bg-wash-dark h-full relative md:rounded-b-lg lg:rounded-b-none"><div style="position:relative"><iframe class="rounded-t-none bg-white md:shadow-md sm:rounded-lg w-full max-w-full transition-opacity absolute opacity-0 pointer-events-none duration-75" title="Sandbox Preview" style="height:15px;z-index:-1"></iframe></div></div></div><button translate="yes" class="sandpack-expand flex text-base justify-between dark:border-card-dark bg-wash dark:bg-card-dark items-center z-10 p-1 w-full order-2 xl:order-last border-b-1 relative top-0"><span class="flex p-2 focus:outline-none text-primary dark:text-primary-dark leading-[20px]"><svg class="rotate-0 inline me-1.5 text-xl" xmlns="http://www.w3.org/2000/svg" width="20" height="20" viewBox="0 0 20 20"><g fill="none" fill-rule="evenodd" transform="translate(-446 -398)"><path fill="currentColor" fill-rule="nonzero" 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L104.883883,242.133883 C105.372039,241.645728 105.372039,240.854272 104.883883,240.366117 C104.395728,239.877961 103.604272,239.877961 103.116117,240.366117 L99.5,243.982233 L95.8838835,240.366117 Z" transform="translate(356.5 164.5)"></path><polygon points="446 418 466 418 466 398 446 398"></polygon></g></svg><div class="flex flex-col overflow-hidden"><span class="text-sm font-bold tracking-wide no-underline uppercase text-secondary dark:text-secondary-dark group-focus:text-link dark:group-focus:text-link-dark group-focus:text-opacity-100">Previous</span><span class="text-lg break-words group-hover:underline">You Might Not Need an Effect</span></div></a><a class="flex gap-x-4 md:gap-x-6 items-center w-full md:min-w-80 md:w-fit md:max-w-md px-4 md:px-5 py-6 border-2 border-transparent text-base leading-base text-link dark:text-link-dark rounded-lg group focus:text-link dark:focus:text-link-dark focus:bg-highlight focus:border-link dark:focus:bg-highlight-dark 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2.91.83.1-.65.35-1.09.63-1.34-2.22-.25-4.55-1.11-4.55-4.94 0-1.1.39-1.99 1.03-2.69a3.6 3.6 0 0 1 .1-2.64s.84-.27 2.75 1.02a9.58 9.58 0 0 1 5 0c1.91-1.3 2.75-1.02 2.75-1.02.55 1.37.2 2.4.1 2.64.64.7 1.03 1.6 1.03 2.69 0 3.84-2.34 4.68-4.57 4.93.36.31.68.92.68 1.85l-.01 2.75c0 .26.18.58.69.48A10 10 0 0 0 10 0"></path></svg></a></div></div></div></footer></div></div></main><!--/$--><div class="hidden -mt-16 lg:max-w-custom-xs 2xl:block"><nav role="navigation" class="pt-20 sticky top-0 end-0"><h2 class="mb-3 lg:mb-3 uppercase tracking-wide font-bold text-sm text-secondary dark:text-secondary-dark px-4 w-full">On this page</h2><div class="h-full overflow-y-auto ps-4 max-h-[calc(100vh-7.5rem)]" style="overscroll-behavior:contain"><ul class="space-y-2 pb-16"><li class="text-sm px-2 rounded-s-xl bg-highlight dark:bg-highlight-dark"><a class="text-link dark:text-link-dark font-bold block hover:text-link dark:hover:text-link-dark leading-normal py-2" href="#">Overview</a></li><li class="text-sm px-2 rounded-s-xl"><a class="text-secondary dark:text-secondary-dark block hover:text-link dark:hover:text-link-dark leading-normal py-2" href="#the-lifecycle-of-an-effect">The lifecycle of an Effect </a></li><li class="text-sm px-2 rounded-s-xl ps-4"><a class="text-secondary dark:text-secondary-dark block hover:text-link dark:hover:text-link-dark leading-normal py-2" href="#why-synchronization-may-need-to-happen-more-than-once">Why synchronization may need to happen more than once </a></li><li class="text-sm px-2 rounded-s-xl ps-4"><a class="text-secondary dark:text-secondary-dark block hover:text-link dark:hover:text-link-dark leading-normal py-2" href="#how-react-re-synchronizes-your-effect">How React re-synchronizes your Effect </a></li><li class="text-sm px-2 rounded-s-xl ps-4"><a class="text-secondary dark:text-secondary-dark block hover:text-link dark:hover:text-link-dark leading-normal py-2" href="#thinking-from-the-effects-perspective">Thinking from the Effect’s perspective </a></li><li class="text-sm px-2 rounded-s-xl ps-4"><a class="text-secondary dark:text-secondary-dark block hover:text-link dark:hover:text-link-dark leading-normal py-2" href="#how-react-verifies-that-your-effect-can-re-synchronize">How React verifies that your Effect can re-synchronize </a></li><li class="text-sm px-2 rounded-s-xl ps-4"><a class="text-secondary dark:text-secondary-dark block hover:text-link dark:hover:text-link-dark leading-normal py-2" href="#how-react-knows-that-it-needs-to-re-synchronize-the-effect">How React knows that it needs to re-synchronize the Effect </a></li><li class="text-sm px-2 rounded-s-xl ps-4"><a class="text-secondary dark:text-secondary-dark block hover:text-link dark:hover:text-link-dark leading-normal py-2" href="#each-effect-represents-a-separate-synchronization-process">Each Effect represents a separate synchronization process </a></li><li class="text-sm px-2 rounded-s-xl"><a class="text-secondary dark:text-secondary-dark block hover:text-link dark:hover:text-link-dark leading-normal py-2" href="#effects-react-to-reactive-values">Effects “react” to reactive values </a></li><li class="text-sm px-2 rounded-s-xl ps-4"><a class="text-secondary dark:text-secondary-dark block hover:text-link dark:hover:text-link-dark leading-normal py-2" href="#what-an-effect-with-empty-dependencies-means">What an Effect with empty dependencies means </a></li><li class="text-sm px-2 rounded-s-xl ps-4"><a class="text-secondary dark:text-secondary-dark block hover:text-link dark:hover:text-link-dark leading-normal py-2" href="#all-variables-declared-in-the-component-body-are-reactive">All variables declared in the component body are reactive </a></li><li class="text-sm px-2 rounded-s-xl ps-4"><a class="text-secondary dark:text-secondary-dark block hover:text-link dark:hover:text-link-dark leading-normal py-2" href="#react-verifies-that-you-specified-every-reactive-value-as-a-dependency">React verifies that you specified every reactive value as a dependency </a></li><li class="text-sm px-2 rounded-s-xl ps-4"><a class="text-secondary dark:text-secondary-dark block hover:text-link dark:hover:text-link-dark leading-normal py-2" href="#what-to-do-when-you-dont-want-to-re-synchronize">What to do when you don’t want to re-synchronize </a></li><li class="text-sm px-2 rounded-s-xl"><a class="text-secondary dark:text-secondary-dark block hover:text-link dark:hover:text-link-dark leading-normal py-2" href="#recap">Recap</a></li><li class="text-sm px-2 rounded-s-xl"><a class="text-secondary dark:text-secondary-dark block hover:text-link dark:hover:text-link-dark leading-normal py-2" href="#challenges">Challenges</a></li></ul></div></nav></div></div></div><script id="__NEXT_DATA__" type="application/json">{"props":{"pageProps":{"toc":"[{\"url\":\"#\",\"text\":\"Overview\",\"depth\":2},{\"url\":\"#the-lifecycle-of-an-effect\",\"depth\":2,\"text\":\"The lifecycle of an Effect \"},{\"url\":\"#why-synchronization-may-need-to-happen-more-than-once\",\"depth\":3,\"text\":\"Why synchronization may need to happen more than once \"},{\"url\":\"#how-react-re-synchronizes-your-effect\",\"depth\":3,\"text\":\"How React re-synchronizes your Effect \"},{\"url\":\"#thinking-from-the-effects-perspective\",\"depth\":3,\"text\":\"Thinking from the Effect’s perspective \"},{\"url\":\"#how-react-verifies-that-your-effect-can-re-synchronize\",\"depth\":3,\"text\":\"How React verifies that your Effect can re-synchronize \"},{\"url\":\"#how-react-knows-that-it-needs-to-re-synchronize-the-effect\",\"depth\":3,\"text\":\"How React knows that it needs to re-synchronize the Effect \"},{\"url\":\"#each-effect-represents-a-separate-synchronization-process\",\"depth\":3,\"text\":\"Each Effect represents a separate synchronization process \"},{\"url\":\"#effects-react-to-reactive-values\",\"depth\":2,\"text\":\"Effects “react” to reactive values \"},{\"url\":\"#what-an-effect-with-empty-dependencies-means\",\"depth\":3,\"text\":\"What an Effect with empty dependencies means \"},{\"url\":\"#all-variables-declared-in-the-component-body-are-reactive\",\"depth\":3,\"text\":\"All variables declared in the component body are reactive \"},{\"url\":\"#react-verifies-that-you-specified-every-reactive-value-as-a-dependency\",\"depth\":3,\"text\":\"React verifies that you specified every reactive value as a dependency \"},{\"url\":\"#what-to-do-when-you-dont-want-to-re-synchronize\",\"depth\":3,\"text\":\"What to do when you don’t want to re-synchronize \"},{\"url\":\"#recap\",\"depth\":2,\"text\":\"Recap\"},{\"url\":\"#challenges\",\"depth\":2,\"text\":\"Challenges\"}]","content":"[[\"$r\",\"MaxWidth\",\"100\",{\"children\":[[\"$r\",\"Intro\",null,{\"children\":[\"$r\",\"p\",null,{\"children\":\"Effects have a different lifecycle from components. Components may mount, update, or unmount. An Effect can only do two things: to start synchronizing something, and later to stop synchronizing it. This cycle can happen multiple times if your Effect depends on props and state that change over time. React provides a linter rule to check that you’ve specified your Effect’s dependencies correctly. This keeps your Effect synchronized to the latest props and state.\"}]}],\"\\n\",[\"$r\",\"YouWillLearn\",null,{\"children\":[\"$r\",\"ul\",null,{\"children\":[\"\\n\",[\"$r\",\"li\",null,{\"children\":\"How an Effect’s lifecycle is different from a component’s lifecycle\"}],\"\\n\",[\"$r\",\"li\",null,{\"children\":\"How to think about each individual Effect in isolation\"}],\"\\n\",[\"$r\",\"li\",null,{\"children\":\"When your Effect needs to re-synchronize, and why\"}],\"\\n\",[\"$r\",\"li\",null,{\"children\":\"How your Effect’s dependencies are determined\"}],\"\\n\",[\"$r\",\"li\",null,{\"children\":\"What it means for a value to be reactive\"}],\"\\n\",[\"$r\",\"li\",null,{\"children\":\"What an empty dependency array means\"}],\"\\n\",[\"$r\",\"li\",null,{\"children\":\"How React verifies your dependencies are correct with a linter\"}],\"\\n\",[\"$r\",\"li\",null,{\"children\":\"What to do when you disagree with the linter\"}],\"\\n\"]}]}],\"\\n\",[\"$r\",\"h2\",null,{\"id\":\"the-lifecycle-of-an-effect\",\"children\":\"The lifecycle of an Effect \"}],\"\\n\",[\"$r\",\"p\",null,{\"children\":\"Every React component goes through the same lifecycle:\"}],\"\\n\",[\"$r\",\"ul\",null,{\"children\":[\"\\n\",[\"$r\",\"li\",null,{\"children\":[\"A component \",[\"$r\",\"em\",null,{\"children\":\"mounts\"}],\" when it’s added to the screen.\"]}],\"\\n\",[\"$r\",\"li\",null,{\"children\":[\"A component \",[\"$r\",\"em\",null,{\"children\":\"updates\"}],\" when it receives new props or state, usually in response to an interaction.\"]}],\"\\n\",[\"$r\",\"li\",null,{\"children\":[\"A component \",[\"$r\",\"em\",null,{\"children\":\"unmounts\"}],\" when it’s removed from the screen.\"]}],\"\\n\"]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[[\"$r\",\"strong\",null,{\"children\":[\"It’s a good way to think about components, but \",[\"$r\",\"em\",null,{\"children\":\"not\"}],\" about Effects.\"]}],\" Instead, try to think about each Effect independently from your component’s lifecycle. An Effect describes how to \",[\"$r\",\"a\",null,{\"href\":\"/learn/synchronizing-with-effects\",\"children\":\"synchronize an external system\"}],\" to the current props and state. As your code changes, synchronization will need to happen more or less often.\"]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":\"To illustrate this point, consider this Effect connecting your component to a chat server:\"}],\"\\n\",[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"children\":\"const serverUrl = 'https://localhost:1234';\\n\\nfunction ChatRoom({ roomId }) {\\n useEffect(() =\u003e {\\n const connection = createConnection(serverUrl, roomId);\\n connection.connect();\\n return () =\u003e {\\n connection.disconnect();\\n };\\n }, [roomId]);\\n // ...\\n}\\n\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"Your Effect’s body specifies how to \",[\"$r\",\"strong\",null,{\"children\":\"start synchronizing:\"}]]}],\"\\n\",[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"{2-3}\",\"children\":\" // ...\\n const connection = createConnection(serverUrl, roomId);\\n connection.connect();\\n return () =\u003e {\\n connection.disconnect();\\n };\\n // ...\\n\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"The cleanup function returned by your Effect specifies how to \",[\"$r\",\"strong\",null,{\"children\":\"stop synchronizing:\"}]]}],\"\\n\",[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"{5}\",\"children\":\" // ...\\n const connection = createConnection(serverUrl, roomId);\\n connection.connect();\\n return () =\u003e {\\n connection.disconnect();\\n };\\n // ...\\n\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"Intuitively, you might think that React would \",[\"$r\",\"strong\",null,{\"children\":\"start synchronizing\"}],\" when your component mounts and \",[\"$r\",\"strong\",null,{\"children\":\"stop synchronizing\"}],\" when your component unmounts. However, this is not the end of the story! Sometimes, it may also be necessary to \",[\"$r\",\"strong\",null,{\"children\":\"start and stop synchronizing multiple times\"}],\" while the component remains mounted.\"]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"Let’s look at \",[\"$r\",\"em\",null,{\"children\":\"why\"}],\" this is necessary, \",[\"$r\",\"em\",null,{\"children\":\"when\"}],\" it happens, and \",[\"$r\",\"em\",null,{\"children\":\"how\"}],\" you can control this behavior.\"]}],\"\\n\",[\"$r\",\"Note\",null,{\"children\":[\"$r\",\"p\",null,{\"children\":[\"Some Effects don’t return a cleanup function at all. \",[\"$r\",\"a\",null,{\"href\":\"/learn/synchronizing-with-effects#how-to-handle-the-effect-firing-twice-in-development\",\"children\":\"More often than not,\"}],\" you’ll want to return one—but if you don’t, React will behave as if you returned an empty cleanup function.\"]}]}],\"\\n\",[\"$r\",\"h3\",null,{\"id\":\"why-synchronization-may-need-to-happen-more-than-once\",\"children\":\"Why synchronization may need to happen more than once \"}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"Imagine this \",[\"$r\",\"code\",null,{\"children\":\"ChatRoom\"}],\" component receives a \",[\"$r\",\"code\",null,{\"children\":\"roomId\"}],\" prop that the user picks in a dropdown. Let’s say that initially the user picks the \",[\"$r\",\"code\",null,{\"children\":\"\\\"general\\\"\"}],\" room as the \",[\"$r\",\"code\",null,{\"children\":\"roomId\"}],\". Your app displays the \",[\"$r\",\"code\",null,{\"children\":\"\\\"general\\\"\"}],\" chat room:\"]}],\"\\n\",[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"{3}\",\"children\":\"const serverUrl = 'https://localhost:1234';\\n\\nfunction ChatRoom({ roomId /* \\\"general\\\" */ }) {\\n // ...\\n return \u003ch1\u003eWelcome to the {roomId} room!\u003c/h1\u003e;\\n}\\n\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"After the UI is displayed, React will run your Effect to \",[\"$r\",\"strong\",null,{\"children\":\"start synchronizing.\"}],\" It connects to the \",[\"$r\",\"code\",null,{\"children\":\"\\\"general\\\"\"}],\" room:\"]}],\"\\n\",[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"{3,4}\",\"children\":\"function ChatRoom({ roomId /* \\\"general\\\" */ }) {\\n useEffect(() =\u003e {\\n const connection = createConnection(serverUrl, roomId); // Connects to the \\\"general\\\" room\\n connection.connect();\\n return () =\u003e {\\n connection.disconnect(); // Disconnects from the \\\"general\\\" room\\n };\\n }, [roomId]);\\n // ...\\n\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":\"So far, so good.\"}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"Later, the user picks a different room in the dropdown (for example, \",[\"$r\",\"code\",null,{\"children\":\"\\\"travel\\\"\"}],\"). First, React will update the UI:\"]}],\"\\n\",[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"{1}\",\"children\":\"function ChatRoom({ roomId /* \\\"travel\\\" */ }) {\\n // ...\\n return \u003ch1\u003eWelcome to the {roomId} room!\u003c/h1\u003e;\\n}\\n\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"Think about what should happen next. The user sees that \",[\"$r\",\"code\",null,{\"children\":\"\\\"travel\\\"\"}],\" is the selected chat room in the UI. However, the Effect that ran the last time is still connected to the \",[\"$r\",\"code\",null,{\"children\":\"\\\"general\\\"\"}],\" room. \",[\"$r\",\"strong\",null,{\"children\":[\"The \",[\"$r\",\"code\",null,{\"children\":\"roomId\"}],\" prop has changed, so what your Effect did back then (connecting to the \",[\"$r\",\"code\",null,{\"children\":\"\\\"general\\\"\"}],\" room) no longer matches the UI.\"]}]]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":\"At this point, you want React to do two things:\"}],\"\\n\",[\"$r\",\"ol\",null,{\"children\":[\"\\n\",[\"$r\",\"li\",null,{\"children\":[\"Stop synchronizing with the old \",[\"$r\",\"code\",null,{\"children\":\"roomId\"}],\" (disconnect from the \",[\"$r\",\"code\",null,{\"children\":\"\\\"general\\\"\"}],\" room)\"]}],\"\\n\",[\"$r\",\"li\",null,{\"children\":[\"Start synchronizing with the new \",[\"$r\",\"code\",null,{\"children\":\"roomId\"}],\" (connect to the \",[\"$r\",\"code\",null,{\"children\":\"\\\"travel\\\"\"}],\" room)\"]}],\"\\n\"]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[[\"$r\",\"strong\",null,{\"children\":\"Luckily, you’ve already taught React how to do both of these things!\"}],\" Your Effect’s body specifies how to start synchronizing, and your cleanup function specifies how to stop synchronizing. All that React needs to do now is to call them in the correct order and with the correct props and state. Let’s see how exactly that happens.\"]}],\"\\n\",[\"$r\",\"h3\",null,{\"id\":\"how-react-re-synchronizes-your-effect\",\"children\":\"How React re-synchronizes your Effect \"}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"Recall that your \",[\"$r\",\"code\",null,{\"children\":\"ChatRoom\"}],\" component has received a new value for its \",[\"$r\",\"code\",null,{\"children\":\"roomId\"}],\" prop. It used to be \",[\"$r\",\"code\",null,{\"children\":\"\\\"general\\\"\"}],\", and now it is \",[\"$r\",\"code\",null,{\"children\":\"\\\"travel\\\"\"}],\". React needs to re-synchronize your Effect to re-connect you to a different room.\"]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"To \",[\"$r\",\"strong\",null,{\"children\":\"stop synchronizing,\"}],\" React will call the cleanup function that your Effect returned after connecting to the \",[\"$r\",\"code\",null,{\"children\":\"\\\"general\\\"\"}],\" room. Since \",[\"$r\",\"code\",null,{\"children\":\"roomId\"}],\" was \",[\"$r\",\"code\",null,{\"children\":\"\\\"general\\\"\"}],\", the cleanup function disconnects from the \",[\"$r\",\"code\",null,{\"children\":\"\\\"general\\\"\"}],\" room:\"]}],\"\\n\",[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"{6}\",\"children\":\"function ChatRoom({ roomId /* \\\"general\\\" */ }) {\\n useEffect(() =\u003e {\\n const connection = createConnection(serverUrl, roomId); // Connects to the \\\"general\\\" room\\n connection.connect();\\n return () =\u003e {\\n connection.disconnect(); // Disconnects from the \\\"general\\\" room\\n };\\n // ...\\n\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"Then React will run the Effect that you’ve provided during this render. This time, \",[\"$r\",\"code\",null,{\"children\":\"roomId\"}],\" is \",[\"$r\",\"code\",null,{\"children\":\"\\\"travel\\\"\"}],\" so it will \",[\"$r\",\"strong\",null,{\"children\":\"start synchronizing\"}],\" to the \",[\"$r\",\"code\",null,{\"children\":\"\\\"travel\\\"\"}],\" chat room (until its cleanup function is eventually called too):\"]}],\"\\n\",[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"{3,4}\",\"children\":\"function ChatRoom({ roomId /* \\\"travel\\\" */ }) {\\n useEffect(() =\u003e {\\n const connection = createConnection(serverUrl, roomId); // Connects to the \\\"travel\\\" room\\n connection.connect();\\n // ...\\n\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":\"Thanks to this, you’re now connected to the same room that the user chose in the UI. Disaster averted!\"}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"Every time after your component re-renders with a different \",[\"$r\",\"code\",null,{\"children\":\"roomId\"}],\", your Effect will re-synchronize. For example, let’s say the user changes \",[\"$r\",\"code\",null,{\"children\":\"roomId\"}],\" from \",[\"$r\",\"code\",null,{\"children\":\"\\\"travel\\\"\"}],\" to \",[\"$r\",\"code\",null,{\"children\":\"\\\"music\\\"\"}],\". React will again \",[\"$r\",\"strong\",null,{\"children\":\"stop synchronizing\"}],\" your Effect by calling its cleanup function (disconnecting you from the \",[\"$r\",\"code\",null,{\"children\":\"\\\"travel\\\"\"}],\" room). Then it will \",[\"$r\",\"strong\",null,{\"children\":\"start synchronizing\"}],\" again by running its body with the new \",[\"$r\",\"code\",null,{\"children\":\"roomId\"}],\" prop (connecting you to the \",[\"$r\",\"code\",null,{\"children\":\"\\\"music\\\"\"}],\" room).\"]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"Finally, when the user goes to a different screen, \",[\"$r\",\"code\",null,{\"children\":\"ChatRoom\"}],\" unmounts. Now there is no need to stay connected at all. React will \",[\"$r\",\"strong\",null,{\"children\":\"stop synchronizing\"}],\" your Effect one last time and disconnect you from the \",[\"$r\",\"code\",null,{\"children\":\"\\\"music\\\"\"}],\" chat room.\"]}],\"\\n\",[\"$r\",\"h3\",null,{\"id\":\"thinking-from-the-effects-perspective\",\"children\":\"Thinking from the Effect’s perspective \"}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"Let’s recap everything that’s happened from the \",[\"$r\",\"code\",null,{\"children\":\"ChatRoom\"}],\" component’s perspective:\"]}],\"\\n\",[\"$r\",\"ol\",null,{\"children\":[\"\\n\",[\"$r\",\"li\",null,{\"children\":[[\"$r\",\"code\",null,{\"children\":\"ChatRoom\"}],\" mounted with \",[\"$r\",\"code\",null,{\"children\":\"roomId\"}],\" set to \",[\"$r\",\"code\",null,{\"children\":\"\\\"general\\\"\"}]]}],\"\\n\",[\"$r\",\"li\",null,{\"children\":[[\"$r\",\"code\",null,{\"children\":\"ChatRoom\"}],\" updated with \",[\"$r\",\"code\",null,{\"children\":\"roomId\"}],\" set to \",[\"$r\",\"code\",null,{\"children\":\"\\\"travel\\\"\"}]]}],\"\\n\",[\"$r\",\"li\",null,{\"children\":[[\"$r\",\"code\",null,{\"children\":\"ChatRoom\"}],\" updated with \",[\"$r\",\"code\",null,{\"children\":\"roomId\"}],\" set to \",[\"$r\",\"code\",null,{\"children\":\"\\\"music\\\"\"}]]}],\"\\n\",[\"$r\",\"li\",null,{\"children\":[[\"$r\",\"code\",null,{\"children\":\"ChatRoom\"}],\" unmounted\"]}],\"\\n\"]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":\"During each of these points in the component’s lifecycle, your Effect did different things:\"}],\"\\n\",[\"$r\",\"ol\",null,{\"children\":[\"\\n\",[\"$r\",\"li\",null,{\"children\":[\"Your Effect connected to the \",[\"$r\",\"code\",null,{\"children\":\"\\\"general\\\"\"}],\" room\"]}],\"\\n\",[\"$r\",\"li\",null,{\"children\":[\"Your Effect disconnected from the \",[\"$r\",\"code\",null,{\"children\":\"\\\"general\\\"\"}],\" room and connected to the \",[\"$r\",\"code\",null,{\"children\":\"\\\"travel\\\"\"}],\" room\"]}],\"\\n\",[\"$r\",\"li\",null,{\"children\":[\"Your Effect disconnected from the \",[\"$r\",\"code\",null,{\"children\":\"\\\"travel\\\"\"}],\" room and connected to the \",[\"$r\",\"code\",null,{\"children\":\"\\\"music\\\"\"}],\" room\"]}],\"\\n\",[\"$r\",\"li\",null,{\"children\":[\"Your Effect disconnected from the \",[\"$r\",\"code\",null,{\"children\":\"\\\"music\\\"\"}],\" room\"]}],\"\\n\"]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":\"Now let’s think about what happened from the perspective of the Effect itself:\"}],\"\\n\",[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"children\":\" useEffect(() =\u003e {\\n // Your Effect connected to the room specified with roomId...\\n const connection = createConnection(serverUrl, roomId);\\n connection.connect();\\n return () =\u003e {\\n // ...until it disconnected\\n connection.disconnect();\\n };\\n }, [roomId]);\\n\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":\"This code’s structure might inspire you to see what happened as a sequence of non-overlapping time periods:\"}],\"\\n\",[\"$r\",\"ol\",null,{\"children\":[\"\\n\",[\"$r\",\"li\",null,{\"children\":[\"Your Effect connected to the \",[\"$r\",\"code\",null,{\"children\":\"\\\"general\\\"\"}],\" room (until it disconnected)\"]}],\"\\n\",[\"$r\",\"li\",null,{\"children\":[\"Your Effect connected to the \",[\"$r\",\"code\",null,{\"children\":\"\\\"travel\\\"\"}],\" room (until it disconnected)\"]}],\"\\n\",[\"$r\",\"li\",null,{\"children\":[\"Your Effect connected to the \",[\"$r\",\"code\",null,{\"children\":\"\\\"music\\\"\"}],\" room (until it disconnected)\"]}],\"\\n\"]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":\"Previously, you were thinking from the component’s perspective. When you looked from the component’s perspective, it was tempting to think of Effects as “callbacks” or “lifecycle events” that fire at a specific time like “after a render” or “before unmount”. This way of thinking gets complicated very fast, so it’s best to avoid.\"}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"$r\",\"strong\",null,{\"children\":\"Instead, always focus on a single start/stop cycle at a time. It shouldn’t matter whether a component is mounting, updating, or unmounting. All you need to do is to describe how to start synchronization and how to stop it. If you do it well, your Effect will be resilient to being started and stopped as many times as it’s needed.\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"This might remind you how you don’t think whether a component is mounting or updating when you write the rendering logic that creates JSX. You describe what should be on the screen, and React \",[\"$r\",\"a\",null,{\"href\":\"/learn/reacting-to-input-with-state\",\"children\":\"figures out the rest.\"}]]}],\"\\n\",[\"$r\",\"h3\",null,{\"id\":\"how-react-verifies-that-your-effect-can-re-synchronize\",\"children\":\"How React verifies that your Effect can re-synchronize \"}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"Here is a live example that you can play with. Press “Open chat” to mount the \",[\"$r\",\"code\",null,{\"children\":\"ChatRoom\"}],\" component:\"]}],\"\\n\"]}],[\"$r\",\"Sandpack\",null,{\"children\":[[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"children\":\"import { useState, useEffect } from 'react';\\nimport { createConnection } from './chat.js';\\n\\nconst serverUrl = 'https://localhost:1234';\\n\\nfunction ChatRoom({ roomId }) {\\n useEffect(() =\u003e {\\n const connection = createConnection(serverUrl, roomId);\\n connection.connect();\\n return () =\u003e connection.disconnect();\\n }, [roomId]);\\n return \u003ch1\u003eWelcome to the {roomId} room!\u003c/h1\u003e;\\n}\\n\\nexport default function App() {\\n const [roomId, setRoomId] = useState('general');\\n const [show, setShow] = useState(false);\\n return (\\n \u003c\u003e\\n \u003clabel\u003e\\n Choose the chat room:{' '}\\n \u003cselect\\n value={roomId}\\n onChange={e =\u003e setRoomId(e.target.value)}\\n \u003e\\n \u003coption value=\\\"general\\\"\u003egeneral\u003c/option\u003e\\n \u003coption value=\\\"travel\\\"\u003etravel\u003c/option\u003e\\n \u003coption value=\\\"music\\\"\u003emusic\u003c/option\u003e\\n \u003c/select\u003e\\n \u003c/label\u003e\\n \u003cbutton onClick={() =\u003e setShow(!show)}\u003e\\n {show ? 'Close chat' : 'Open chat'}\\n \u003c/button\u003e\\n {show \u0026\u0026 \u003chr /\u003e}\\n {show \u0026\u0026 \u003cChatRoom roomId={roomId} /\u003e}\\n \u003c/\u003e\\n );\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"src/chat.js\",\"children\":\"export function createConnection(serverUrl, roomId) {\\n // A real implementation would actually connect to the server\\n return {\\n connect() {\\n console.log('✅ Connecting to \\\"' + roomId + '\\\" room at ' + serverUrl + '...');\\n },\\n disconnect() {\\n console.log('❌ Disconnected from \\\"' + roomId + '\\\" room at ' + serverUrl);\\n }\\n };\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-css\",\"children\":\"input { display: block; margin-bottom: 20px; }\\nbutton { margin-left: 10px; }\\n\"}]}]]}],[\"$r\",\"MaxWidth\",\"162\",{\"children\":[\"\\n\",[\"$r\",\"p\",null,{\"children\":\"Notice that when the component mounts for the first time, you see three logs:\"}],\"\\n\",[\"$r\",\"ol\",null,{\"children\":[\"\\n\",[\"$r\",\"li\",null,{\"children\":[[\"$r\",\"code\",null,{\"children\":\"✅ Connecting to \\\"general\\\" room at https://localhost:1234...\"}],\" \",[\"$r\",\"em\",null,{\"children\":\"(development-only)\"}]]}],\"\\n\",[\"$r\",\"li\",null,{\"children\":[[\"$r\",\"code\",null,{\"children\":\"❌ Disconnected from \\\"general\\\" room at https://localhost:1234.\"}],\" \",[\"$r\",\"em\",null,{\"children\":\"(development-only)\"}]]}],\"\\n\",[\"$r\",\"li\",null,{\"children\":[\"$r\",\"code\",null,{\"children\":\"✅ Connecting to \\\"general\\\" room at https://localhost:1234...\"}]}],\"\\n\"]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":\"The first two logs are development-only. In development, React always remounts each component once.\"}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[[\"$r\",\"strong\",null,{\"children\":\"React verifies that your Effect can re-synchronize by forcing it to do that immediately in development.\"}],\" This might remind you of opening a door and closing it an extra time to check if the door lock works. React starts and stops your Effect one extra time in development to check \",[\"$r\",\"a\",null,{\"href\":\"/learn/synchronizing-with-effects#how-to-handle-the-effect-firing-twice-in-development\",\"children\":\"you’ve implemented its cleanup well.\"}]]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"The main reason your Effect will re-synchronize in practice is if some data it uses has changed. In the sandbox above, change the selected chat room. Notice how, when the \",[\"$r\",\"code\",null,{\"children\":\"roomId\"}],\" changes, your Effect re-synchronizes.\"]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"However, there are also more unusual cases in which re-synchronization is necessary. For example, try editing the \",[\"$r\",\"code\",null,{\"children\":\"serverUrl\"}],\" in the sandbox above while the chat is open. Notice how the Effect re-synchronizes in response to your edits to the code. In the future, React may add more features that rely on re-synchronization.\"]}],\"\\n\",[\"$r\",\"h3\",null,{\"id\":\"how-react-knows-that-it-needs-to-re-synchronize-the-effect\",\"children\":\"How React knows that it needs to re-synchronize the Effect \"}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"You might be wondering how React knew that your Effect needed to re-synchronize after \",[\"$r\",\"code\",null,{\"children\":\"roomId\"}],\" changes. It’s because \",[\"$r\",\"em\",null,{\"children\":\"you told React\"}],\" that its code depends on \",[\"$r\",\"code\",null,{\"children\":\"roomId\"}],\" by including it in the \",[\"$r\",\"a\",null,{\"href\":\"/learn/synchronizing-with-effects#step-2-specify-the-effect-dependencies\",\"children\":\"list of dependencies:\"}]]}],\"\\n\",[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"{1,3,8}\",\"children\":\"function ChatRoom({ roomId }) { // The roomId prop may change over time\\n useEffect(() =\u003e {\\n const connection = createConnection(serverUrl, roomId); // This Effect reads roomId \\n connection.connect();\\n return () =\u003e {\\n connection.disconnect();\\n };\\n }, [roomId]); // So you tell React that this Effect \\\"depends on\\\" roomId\\n // ...\\n\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":\"Here’s how this works:\"}],\"\\n\",[\"$r\",\"ol\",null,{\"children\":[\"\\n\",[\"$r\",\"li\",null,{\"children\":[\"You knew \",[\"$r\",\"code\",null,{\"children\":\"roomId\"}],\" is a prop, which means it can change over time.\"]}],\"\\n\",[\"$r\",\"li\",null,{\"children\":[\"You knew that your Effect reads \",[\"$r\",\"code\",null,{\"children\":\"roomId\"}],\" (so its logic depends on a value that may change later).\"]}],\"\\n\",[\"$r\",\"li\",null,{\"children\":[\"This is why you specified it as your Effect’s dependency (so that it re-synchronizes when \",[\"$r\",\"code\",null,{\"children\":\"roomId\"}],\" changes).\"]}],\"\\n\"]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":\"Every time after your component re-renders, React will look at the array of dependencies that you have passed. If any of the values in the array is different from the value at the same spot that you passed during the previous render, React will re-synchronize your Effect.\"}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"For example, if you passed \",[\"$r\",\"code\",null,{\"children\":\"[\\\"general\\\"]\"}],\" during the initial render, and later you passed \",[\"$r\",\"code\",null,{\"children\":\"[\\\"travel\\\"]\"}],\" during the next render, React will compare \",[\"$r\",\"code\",null,{\"children\":\"\\\"general\\\"\"}],\" and \",[\"$r\",\"code\",null,{\"children\":\"\\\"travel\\\"\"}],\". These are different values (compared with \",[\"$r\",\"a\",null,{\"href\":\"https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/is\",\"target\":\"_blank\",\"rel\":\"nofollow noopener noreferrer\",\"children\":[\"$r\",\"code\",null,{\"children\":\"Object.is\"}]}],\"), so React will re-synchronize your Effect. On the other hand, if your component re-renders but \",[\"$r\",\"code\",null,{\"children\":\"roomId\"}],\" has not changed, your Effect will remain connected to the same room.\"]}],\"\\n\",[\"$r\",\"h3\",null,{\"id\":\"each-effect-represents-a-separate-synchronization-process\",\"children\":\"Each Effect represents a separate synchronization process \"}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"Resist adding unrelated logic to your Effect only because this logic needs to run at the same time as an Effect you already wrote. For example, let’s say you want to send an analytics event when the user visits the room. You already have an Effect that depends on \",[\"$r\",\"code\",null,{\"children\":\"roomId\"}],\", so you might feel tempted to add the analytics call there:\"]}],\"\\n\",[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"{3}\",\"children\":\"function ChatRoom({ roomId }) {\\n useEffect(() =\u003e {\\n logVisit(roomId);\\n const connection = createConnection(serverUrl, roomId);\\n connection.connect();\\n return () =\u003e {\\n connection.disconnect();\\n };\\n }, [roomId]);\\n // ...\\n}\\n\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"But imagine you later add another dependency to this Effect that needs to re-establish the connection. If this Effect re-synchronizes, it will also call \",[\"$r\",\"code\",null,{\"children\":\"logVisit(roomId)\"}],\" for the same room, which you did not intend. Logging the visit \",[\"$r\",\"strong\",null,{\"children\":\"is a separate process\"}],\" from connecting. Write them as two separate Effects:\"]}],\"\\n\",[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"{2-4}\",\"children\":\"function ChatRoom({ roomId }) {\\n useEffect(() =\u003e {\\n logVisit(roomId);\\n }, [roomId]);\\n\\n useEffect(() =\u003e {\\n const connection = createConnection(serverUrl, roomId);\\n // ...\\n }, [roomId]);\\n // ...\\n}\\n\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"$r\",\"strong\",null,{\"children\":\"Each Effect in your code should represent a separate and independent synchronization process.\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"In the above example, deleting one Effect wouldn’t break the other Effect’s logic. This is a good indication that they synchronize different things, and so it made sense to split them up. On the other hand, if you split up a cohesive piece of logic into separate Effects, the code may look “cleaner” but will be \",[\"$r\",\"a\",null,{\"href\":\"/learn/you-might-not-need-an-effect#chains-of-computations\",\"children\":\"more difficult to maintain.\"}],\" This is why you should think whether the processes are same or separate, not whether the code looks cleaner.\"]}],\"\\n\",[\"$r\",\"h2\",null,{\"id\":\"effects-react-to-reactive-values\",\"children\":\"Effects “react” to reactive values \"}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"Your Effect reads two variables (\",[\"$r\",\"code\",null,{\"children\":\"serverUrl\"}],\" and \",[\"$r\",\"code\",null,{\"children\":\"roomId\"}],\"), but you only specified \",[\"$r\",\"code\",null,{\"children\":\"roomId\"}],\" as a dependency:\"]}],\"\\n\",[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"{5,10}\",\"children\":\"const serverUrl = 'https://localhost:1234';\\n\\nfunction ChatRoom({ roomId }) {\\n useEffect(() =\u003e {\\n const connection = createConnection(serverUrl, roomId);\\n connection.connect();\\n return () =\u003e {\\n connection.disconnect();\\n };\\n }, [roomId]);\\n // ...\\n}\\n\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"Why doesn’t \",[\"$r\",\"code\",null,{\"children\":\"serverUrl\"}],\" need to be a dependency?\"]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"This is because the \",[\"$r\",\"code\",null,{\"children\":\"serverUrl\"}],\" never changes due to a re-render. It’s always the same no matter how many times the component re-renders and why. Since \",[\"$r\",\"code\",null,{\"children\":\"serverUrl\"}],\" never changes, it wouldn’t make sense to specify it as a dependency. After all, dependencies only do something when they change over time!\"]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"On the other hand, \",[\"$r\",\"code\",null,{\"children\":\"roomId\"}],\" may be different on a re-render. \",[\"$r\",\"strong\",null,{\"children\":[\"Props, state, and other values declared inside the component are \",[\"$r\",\"em\",null,{\"children\":\"reactive\"}],\" because they’re calculated during rendering and participate in the React data flow.\"]}]]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"If \",[\"$r\",\"code\",null,{\"children\":\"serverUrl\"}],\" was a state variable, it would be reactive. Reactive values must be included in dependencies:\"]}],\"\\n\",[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"{2,5,10}\",\"children\":\"function ChatRoom({ roomId }) { // Props change over time\\n const [serverUrl, setServerUrl] = useState('https://localhost:1234'); // State may change over time\\n\\n useEffect(() =\u003e {\\n const connection = createConnection(serverUrl, roomId); // Your Effect reads props and state\\n connection.connect();\\n return () =\u003e {\\n connection.disconnect();\\n };\\n }, [roomId, serverUrl]); // So you tell React that this Effect \\\"depends on\\\" on props and state\\n // ...\\n}\\n\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"By including \",[\"$r\",\"code\",null,{\"children\":\"serverUrl\"}],\" as a dependency, you ensure that the Effect re-synchronizes after it changes.\"]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":\"Try changing the selected chat room or edit the server URL in this sandbox:\"}],\"\\n\"]}],[\"$r\",\"Sandpack\",null,{\"children\":[[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"children\":\"import { useState, useEffect } from 'react';\\nimport { createConnection } from './chat.js';\\n\\nfunction ChatRoom({ roomId }) {\\n const [serverUrl, setServerUrl] = useState('https://localhost:1234');\\n\\n useEffect(() =\u003e {\\n const connection = createConnection(serverUrl, roomId);\\n connection.connect();\\n return () =\u003e connection.disconnect();\\n }, [roomId, serverUrl]);\\n\\n return (\\n \u003c\u003e\\n \u003clabel\u003e\\n Server URL:{' '}\\n \u003cinput\\n value={serverUrl}\\n onChange={e =\u003e setServerUrl(e.target.value)}\\n /\u003e\\n \u003c/label\u003e\\n \u003ch1\u003eWelcome to the {roomId} room!\u003c/h1\u003e\\n \u003c/\u003e\\n );\\n}\\n\\nexport default function App() {\\n const [roomId, setRoomId] = useState('general');\\n return (\\n \u003c\u003e\\n \u003clabel\u003e\\n Choose the chat room:{' '}\\n \u003cselect\\n value={roomId}\\n onChange={e =\u003e setRoomId(e.target.value)}\\n \u003e\\n \u003coption value=\\\"general\\\"\u003egeneral\u003c/option\u003e\\n \u003coption value=\\\"travel\\\"\u003etravel\u003c/option\u003e\\n \u003coption value=\\\"music\\\"\u003emusic\u003c/option\u003e\\n \u003c/select\u003e\\n \u003c/label\u003e\\n \u003chr /\u003e\\n \u003cChatRoom roomId={roomId} /\u003e\\n \u003c/\u003e\\n );\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"src/chat.js\",\"children\":\"export function createConnection(serverUrl, roomId) {\\n // A real implementation would actually connect to the server\\n return {\\n connect() {\\n console.log('✅ Connecting to \\\"' + roomId + '\\\" room at ' + serverUrl + '...');\\n },\\n disconnect() {\\n console.log('❌ Disconnected from \\\"' + roomId + '\\\" room at ' + serverUrl);\\n }\\n };\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-css\",\"children\":\"input { display: block; margin-bottom: 20px; }\\nbutton { margin-left: 10px; }\\n\"}]}]]}],[\"$r\",\"MaxWidth\",\"176\",{\"children\":[\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"Whenever you change a reactive value like \",[\"$r\",\"code\",null,{\"children\":\"roomId\"}],\" or \",[\"$r\",\"code\",null,{\"children\":\"serverUrl\"}],\", the Effect re-connects to the chat server.\"]}],\"\\n\",[\"$r\",\"h3\",null,{\"id\":\"what-an-effect-with-empty-dependencies-means\",\"children\":\"What an Effect with empty dependencies means \"}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"What happens if you move both \",[\"$r\",\"code\",null,{\"children\":\"serverUrl\"}],\" and \",[\"$r\",\"code\",null,{\"children\":\"roomId\"}],\" outside the component?\"]}],\"\\n\",[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"{1,2}\",\"children\":\"const serverUrl = 'https://localhost:1234';\\nconst roomId = 'general';\\n\\nfunction ChatRoom() {\\n useEffect(() =\u003e {\\n const connection = createConnection(serverUrl, roomId);\\n connection.connect();\\n return () =\u003e {\\n connection.disconnect();\\n };\\n }, []); // ✅ All dependencies declared\\n // ...\\n}\\n\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"Now your Effect’s code does not use \",[\"$r\",\"em\",null,{\"children\":\"any\"}],\" reactive values, so its dependencies can be empty (\",[\"$r\",\"code\",null,{\"children\":\"[]\"}],\").\"]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"Thinking from the component’s perspective, the empty \",[\"$r\",\"code\",null,{\"children\":\"[]\"}],\" dependency array means this Effect connects to the chat room only when the component mounts, and disconnects only when the component unmounts. (Keep in mind that React would still \",[\"$r\",\"a\",null,{\"href\":\"#how-react-verifies-that-your-effect-can-re-synchronize\",\"children\":\"re-synchronize it an extra time\"}],\" in development to stress-test your logic.)\"]}],\"\\n\"]}],[\"$r\",\"Sandpack\",null,{\"children\":[[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"children\":\"import { useState, useEffect } from 'react';\\nimport { createConnection } from './chat.js';\\n\\nconst serverUrl = 'https://localhost:1234';\\nconst roomId = 'general';\\n\\nfunction ChatRoom() {\\n useEffect(() =\u003e {\\n const connection = createConnection(serverUrl, roomId);\\n connection.connect();\\n return () =\u003e connection.disconnect();\\n }, []);\\n return \u003ch1\u003eWelcome to the {roomId} room!\u003c/h1\u003e;\\n}\\n\\nexport default function App() {\\n const [show, setShow] = useState(false);\\n return (\\n \u003c\u003e\\n \u003cbutton onClick={() =\u003e setShow(!show)}\u003e\\n {show ? 'Close chat' : 'Open chat'}\\n \u003c/button\u003e\\n {show \u0026\u0026 \u003chr /\u003e}\\n {show \u0026\u0026 \u003cChatRoom /\u003e}\\n \u003c/\u003e\\n );\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"src/chat.js\",\"children\":\"export function createConnection(serverUrl, roomId) {\\n // A real implementation would actually connect to the server\\n return {\\n connect() {\\n console.log('✅ Connecting to \\\"' + roomId + '\\\" room at ' + serverUrl + '...');\\n },\\n disconnect() {\\n console.log('❌ Disconnected from \\\"' + roomId + '\\\" room at ' + serverUrl);\\n }\\n };\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-css\",\"children\":\"input { display: block; margin-bottom: 20px; }\\nbutton { margin-left: 10px; }\\n\"}]}]]}],[\"$r\",\"MaxWidth\",\"200\",{\"children\":[\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"However, if you \",[\"$r\",\"a\",null,{\"href\":\"#thinking-from-the-effects-perspective\",\"children\":\"think from the Effect’s perspective,\"}],\" you don’t need to think about mounting and unmounting at all. What’s important is you’ve specified what your Effect does to start and stop synchronizing. Today, it has no reactive dependencies. But if you ever want the user to change \",[\"$r\",\"code\",null,{\"children\":\"roomId\"}],\" or \",[\"$r\",\"code\",null,{\"children\":\"serverUrl\"}],\" over time (and they would become reactive), your Effect’s code won’t change. You will only need to add them to the dependencies.\"]}],\"\\n\",[\"$r\",\"h3\",null,{\"id\":\"all-variables-declared-in-the-component-body-are-reactive\",\"children\":\"All variables declared in the component body are reactive \"}],\"\\n\",[\"$r\",\"p\",null,{\"children\":\"Props and state aren’t the only reactive values. Values that you calculate from them are also reactive. If the props or state change, your component will re-render, and the values calculated from them will also change. This is why all variables from the component body used by the Effect should be in the Effect dependency list.\"}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"Let’s say that the user can pick a chat server in the dropdown, but they can also configure a default server in settings. Suppose you’ve already put the settings state in a \",[\"$r\",\"a\",null,{\"href\":\"/learn/scaling-up-with-reducer-and-context\",\"children\":\"context\"}],\" so you read the \",[\"$r\",\"code\",null,{\"children\":\"settings\"}],\" from that context. Now you calculate the \",[\"$r\",\"code\",null,{\"children\":\"serverUrl\"}],\" based on the selected server from props and the default server:\"]}],\"\\n\",[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"{3,5,10}\",\"children\":\"function ChatRoom({ roomId, selectedServerUrl }) { // roomId is reactive\\n const settings = useContext(SettingsContext); // settings is reactive\\n const serverUrl = selectedServerUrl ?? settings.defaultServerUrl; // serverUrl is reactive\\n useEffect(() =\u003e {\\n const connection = createConnection(serverUrl, roomId); // Your Effect reads roomId and serverUrl\\n connection.connect();\\n return () =\u003e {\\n connection.disconnect();\\n };\\n }, [roomId, serverUrl]); // So it needs to re-synchronize when either of them changes!\\n // ...\\n}\\n\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"In this example, \",[\"$r\",\"code\",null,{\"children\":\"serverUrl\"}],\" is not a prop or a state variable. It’s a regular variable that you calculate during rendering. But it’s calculated during rendering, so it can change due to a re-render. This is why it’s reactive.\"]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"$r\",\"strong\",null,{\"children\":\"All values inside the component (including props, state, and variables in your component’s body) are reactive. Any reactive value can change on a re-render, so you need to include reactive values as Effect’s dependencies.\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":\"In other words, Effects “react” to all values from the component body.\"}],\"\\n\",[\"$r\",\"DeepDive\",null,{\"children\":[[\"$r\",\"h4\",null,{\"id\":\"can-global-or-mutable-values-be-dependencies\",\"children\":\"Can global or mutable values be dependencies? \"}],[\"$r\",\"p\",null,{\"children\":\"Mutable values (including global variables) aren’t reactive.\"}],[\"$r\",\"p\",null,{\"children\":[[\"$r\",\"strong\",null,{\"children\":[\"A mutable value like \",[\"$r\",\"a\",null,{\"href\":\"https://developer.mozilla.org/en-US/docs/Web/API/Location/pathname\",\"target\":\"_blank\",\"rel\":\"nofollow noopener noreferrer\",\"children\":[\"$r\",\"code\",null,{\"children\":\"location.pathname\"}]}],\" can’t be a dependency.\"]}],\" It’s mutable, so it can change at any time completely outside of the React rendering data flow. Changing it wouldn’t trigger a re-render of your component. Therefore, even if you specified it in the dependencies, React \",[\"$r\",\"em\",null,{\"children\":\"wouldn’t know\"}],\" to re-synchronize the Effect when it changes. This also breaks the rules of React because reading mutable data during rendering (which is when you calculate the dependencies) breaks \",[\"$r\",\"a\",null,{\"href\":\"/learn/keeping-components-pure\",\"children\":\"purity of rendering.\"}],\" Instead, you should read and subscribe to an external mutable value with \",[\"$r\",\"a\",null,{\"href\":\"/learn/you-might-not-need-an-effect#subscribing-to-an-external-store\",\"children\":[[\"$r\",\"code\",null,{\"children\":\"useSyncExternalStore\"}],\".\"]}]]}],[\"$r\",\"p\",null,{\"children\":[[\"$r\",\"strong\",null,{\"children\":[\"A mutable value like \",[\"$r\",\"a\",null,{\"href\":\"/reference/react/useRef#reference\",\"children\":[\"$r\",\"code\",null,{\"children\":\"ref.current\"}]}],\" or things you read from it also can’t be a dependency.\"]}],\" The ref object returned by \",[\"$r\",\"code\",null,{\"children\":\"useRef\"}],\" itself can be a dependency, but its \",[\"$r\",\"code\",null,{\"children\":\"current\"}],\" property is intentionally mutable. It lets you \",[\"$r\",\"a\",null,{\"href\":\"/learn/referencing-values-with-refs\",\"children\":\"keep track of something without triggering a re-render.\"}],\" But since changing it doesn’t trigger a re-render, it’s not a reactive value, and React won’t know to re-run your Effect when it changes.\"]}],[\"$r\",\"p\",null,{\"children\":\"As you’ll learn below on this page, a linter will check for these issues automatically.\"}]]}],\"\\n\",[\"$r\",\"h3\",null,{\"id\":\"react-verifies-that-you-specified-every-reactive-value-as-a-dependency\",\"children\":\"React verifies that you specified every reactive value as a dependency \"}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"If your linter is \",[\"$r\",\"a\",null,{\"href\":\"/learn/editor-setup#linting\",\"children\":\"configured for React,\"}],\" it will check that every reactive value used by your Effect’s code is declared as its dependency. For example, this is a lint error because both \",[\"$r\",\"code\",null,{\"children\":\"roomId\"}],\" and \",[\"$r\",\"code\",null,{\"children\":\"serverUrl\"}],\" are reactive:\"]}],\"\\n\"]}],[\"$r\",\"Sandpack\",null,{\"children\":[[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"children\":\"import { useState, useEffect } from 'react';\\nimport { createConnection } from './chat.js';\\n\\nfunction ChatRoom({ roomId }) { // roomId is reactive\\n const [serverUrl, setServerUrl] = useState('https://localhost:1234'); // serverUrl is reactive\\n\\n useEffect(() =\u003e {\\n const connection = createConnection(serverUrl, roomId);\\n connection.connect();\\n return () =\u003e connection.disconnect();\\n }, []); // \u003c-- Something's wrong here!\\n\\n return (\\n \u003c\u003e\\n \u003clabel\u003e\\n Server URL:{' '}\\n \u003cinput\\n value={serverUrl}\\n onChange={e =\u003e setServerUrl(e.target.value)}\\n /\u003e\\n \u003c/label\u003e\\n \u003ch1\u003eWelcome to the {roomId} room!\u003c/h1\u003e\\n \u003c/\u003e\\n );\\n}\\n\\nexport default function App() {\\n const [roomId, setRoomId] = useState('general');\\n return (\\n \u003c\u003e\\n \u003clabel\u003e\\n Choose the chat room:{' '}\\n \u003cselect\\n value={roomId}\\n onChange={e =\u003e setRoomId(e.target.value)}\\n \u003e\\n \u003coption value=\\\"general\\\"\u003egeneral\u003c/option\u003e\\n \u003coption value=\\\"travel\\\"\u003etravel\u003c/option\u003e\\n \u003coption value=\\\"music\\\"\u003emusic\u003c/option\u003e\\n \u003c/select\u003e\\n \u003c/label\u003e\\n \u003chr /\u003e\\n \u003cChatRoom roomId={roomId} /\u003e\\n \u003c/\u003e\\n );\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"src/chat.js\",\"children\":\"export function createConnection(serverUrl, roomId) {\\n // A real implementation would actually connect to the server\\n return {\\n connect() {\\n console.log('✅ Connecting to \\\"' + roomId + '\\\" room at ' + serverUrl + '...');\\n },\\n disconnect() {\\n console.log('❌ Disconnected from \\\"' + roomId + '\\\" room at ' + serverUrl);\\n }\\n };\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-css\",\"children\":\"input { display: block; margin-bottom: 20px; }\\nbutton { margin-left: 10px; }\\n\"}]}]]}],[\"$r\",\"MaxWidth\",\"234\",{\"children\":[\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"This may look like a React error, but really React is pointing out a bug in your code. Both \",[\"$r\",\"code\",null,{\"children\":\"roomId\"}],\" and \",[\"$r\",\"code\",null,{\"children\":\"serverUrl\"}],\" may change over time, but you’re forgetting to re-synchronize your Effect when they change. You will remain connected to the initial \",[\"$r\",\"code\",null,{\"children\":\"roomId\"}],\" and \",[\"$r\",\"code\",null,{\"children\":\"serverUrl\"}],\" even after the user picks different values in the UI.\"]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"To fix the bug, follow the linter’s suggestion to specify \",[\"$r\",\"code\",null,{\"children\":\"roomId\"}],\" and \",[\"$r\",\"code\",null,{\"children\":\"serverUrl\"}],\" as dependencies of your Effect:\"]}],\"\\n\",[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"{9}\",\"children\":\"function ChatRoom({ roomId }) { // roomId is reactive\\n const [serverUrl, setServerUrl] = useState('https://localhost:1234'); // serverUrl is reactive\\n useEffect(() =\u003e {\\n const connection = createConnection(serverUrl, roomId);\\n connection.connect();\\n return () =\u003e {\\n connection.disconnect();\\n };\\n }, [serverUrl, roomId]); // ✅ All dependencies declared\\n // ...\\n}\\n\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":\"Try this fix in the sandbox above. Verify that the linter error is gone, and the chat re-connects when needed.\"}],\"\\n\",[\"$r\",\"Note\",null,{\"children\":[\"$r\",\"p\",null,{\"children\":[\"In some cases, React \",[\"$r\",\"em\",null,{\"children\":\"knows\"}],\" that a value never changes even though it’s declared inside the component. For example, the \",[\"$r\",\"a\",null,{\"href\":\"/reference/react/useState#setstate\",\"children\":[[\"$r\",\"code\",null,{\"children\":\"set\"}],\" function\"]}],\" returned from \",[\"$r\",\"code\",null,{\"children\":\"useState\"}],\" and the ref object returned by \",[\"$r\",\"a\",null,{\"href\":\"/reference/react/useRef\",\"children\":[\"$r\",\"code\",null,{\"children\":\"useRef\"}]}],\" are \",[\"$r\",\"em\",null,{\"children\":\"stable\"}],\"—they are guaranteed to not change on a re-render. Stable values aren’t reactive, so you may omit them from the list. Including them is allowed: they won’t change, so it doesn’t matter.\"]}]}],\"\\n\",[\"$r\",\"h3\",null,{\"id\":\"what-to-do-when-you-dont-want-to-re-synchronize\",\"children\":\"What to do when you don’t want to re-synchronize \"}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"In the previous example, you’ve fixed the lint error by listing \",[\"$r\",\"code\",null,{\"children\":\"roomId\"}],\" and \",[\"$r\",\"code\",null,{\"children\":\"serverUrl\"}],\" as dependencies.\"]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[[\"$r\",\"strong\",null,{\"children\":\"However, you could instead “prove” to the linter that these values aren’t reactive values,\"}],\" i.e. that they \",[\"$r\",\"em\",null,{\"children\":\"can’t\"}],\" change as a result of a re-render. For example, if \",[\"$r\",\"code\",null,{\"children\":\"serverUrl\"}],\" and \",[\"$r\",\"code\",null,{\"children\":\"roomId\"}],\" don’t depend on rendering and always have the same values, you can move them outside the component. Now they don’t need to be dependencies:\"]}],\"\\n\",[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"{1,2,11}\",\"children\":\"const serverUrl = 'https://localhost:1234'; // serverUrl is not reactive\\nconst roomId = 'general'; // roomId is not reactive\\n\\nfunction ChatRoom() {\\n useEffect(() =\u003e {\\n const connection = createConnection(serverUrl, roomId);\\n connection.connect();\\n return () =\u003e {\\n connection.disconnect();\\n };\\n }, []); // ✅ All dependencies declared\\n // ...\\n}\\n\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"You can also move them \",[\"$r\",\"em\",null,{\"children\":\"inside the Effect.\"}],\" They aren’t calculated during rendering, so they’re not reactive:\"]}],\"\\n\",[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"{3,4,10}\",\"children\":\"function ChatRoom() {\\n useEffect(() =\u003e {\\n const serverUrl = 'https://localhost:1234'; // serverUrl is not reactive\\n const roomId = 'general'; // roomId is not reactive\\n const connection = createConnection(serverUrl, roomId);\\n connection.connect();\\n return () =\u003e {\\n connection.disconnect();\\n };\\n }, []); // ✅ All dependencies declared\\n // ...\\n}\\n\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[[\"$r\",\"strong\",null,{\"children\":\"Effects are reactive blocks of code.\"}],\" They re-synchronize when the values you read inside of them change. Unlike event handlers, which only run once per interaction, Effects run whenever synchronization is necessary.\"]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[[\"$r\",\"strong\",null,{\"children\":\"You can’t “choose” your dependencies.\"}],\" Your dependencies must include every \",[\"$r\",\"a\",null,{\"href\":\"#all-variables-declared-in-the-component-body-are-reactive\",\"children\":\"reactive value\"}],\" you read in the Effect. The linter enforces this. Sometimes this may lead to problems like infinite loops and to your Effect re-synchronizing too often. Don’t fix these problems by suppressing the linter! Here’s what to try instead:\"]}],\"\\n\",[\"$r\",\"ul\",null,{\"children\":[\"\\n\",[\"$r\",\"li\",null,{\"children\":[\"\\n\",[\"$r\",\"p\",null,{\"children\":[[\"$r\",\"strong\",null,{\"children\":\"Check that your Effect represents an independent synchronization process.\"}],\" If your Effect doesn’t synchronize anything, \",[\"$r\",\"a\",null,{\"href\":\"/learn/you-might-not-need-an-effect\",\"children\":\"it might be unnecessary.\"}],\" If it synchronizes several independent things, \",[\"$r\",\"a\",null,{\"href\":\"#each-effect-represents-a-separate-synchronization-process\",\"children\":\"split it up.\"}]]}],\"\\n\"]}],\"\\n\",[\"$r\",\"li\",null,{\"children\":[\"\\n\",[\"$r\",\"p\",null,{\"children\":[[\"$r\",\"strong\",null,{\"children\":\"If you want to read the latest value of props or state without “reacting” to it and re-synchronizing the Effect,\"}],\" you can split your Effect into a reactive part (which you’ll keep in the Effect) and a non-reactive part (which you’ll extract into something called an \",[\"$r\",\"em\",null,{\"children\":\"Effect Event\"}],\"). \",[\"$r\",\"a\",null,{\"href\":\"/learn/separating-events-from-effects\",\"children\":\"Read about separating Events from Effects.\"}]]}],\"\\n\"]}],\"\\n\",[\"$r\",\"li\",null,{\"children\":[\"\\n\",[\"$r\",\"p\",null,{\"children\":[[\"$r\",\"strong\",null,{\"children\":\"Avoid relying on objects and functions as dependencies.\"}],\" If you create objects and functions during rendering and then read them from an Effect, they will be different on every render. This will cause your Effect to re-synchronize every time. \",[\"$r\",\"a\",null,{\"href\":\"/learn/removing-effect-dependencies\",\"children\":\"Read more about removing unnecessary dependencies from Effects.\"}]]}],\"\\n\"]}],\"\\n\"]}],\"\\n\",[\"$r\",\"Pitfall\",null,{\"children\":[[\"$r\",\"p\",null,{\"children\":[\"The linter is your friend, but its powers are limited. The linter only knows when the dependencies are \",[\"$r\",\"em\",null,{\"children\":\"wrong\"}],\". It doesn’t know \",[\"$r\",\"em\",null,{\"children\":\"the best\"}],\" way to solve each case. If the linter suggests a dependency, but adding it causes a loop, it doesn’t mean the linter should be ignored. You need to change the code inside (or outside) the Effect so that that value isn’t reactive and doesn’t \",[\"$r\",\"em\",null,{\"children\":\"need\"}],\" to be a dependency.\"]}],[\"$r\",\"p\",null,{\"children\":\"If you have an existing codebase, you might have some Effects that suppress the linter like this:\"}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"{3-4}\",\"children\":\"useEffect(() =\u003e {\\n // ...\\n // 🔴 Avoid suppressing the linter like this:\\n // eslint-ignore-next-line react-hooks/exhaustive-deps\\n}, []);\\n\"}]}],[\"$r\",\"p\",null,{\"children\":[\"On the \",[\"$r\",\"a\",null,{\"href\":\"/learn/separating-events-from-effects\",\"children\":\"next\"}],\" \",[\"$r\",\"a\",null,{\"href\":\"/learn/removing-effect-dependencies\",\"children\":\"pages\"}],\", you’ll learn how to fix this code without breaking the rules. It’s always worth fixing!\"]}]]}],\"\\n\",[\"$r\",\"Recap\",null,{\"children\":[\"$r\",\"ul\",null,{\"children\":[\"\\n\",[\"$r\",\"li\",null,{\"children\":\"Components can mount, update, and unmount.\"}],\"\\n\",[\"$r\",\"li\",null,{\"children\":\"Each Effect has a separate lifecycle from the surrounding component.\"}],\"\\n\",[\"$r\",\"li\",null,{\"children\":[\"Each Effect describes a separate synchronization process that can \",[\"$r\",\"em\",null,{\"children\":\"start\"}],\" and \",[\"$r\",\"em\",null,{\"children\":\"stop\"}],\".\"]}],\"\\n\",[\"$r\",\"li\",null,{\"children\":\"When you write and read Effects, think from each individual Effect’s perspective (how to start and stop synchronization) rather than from the component’s perspective (how it mounts, updates, or unmounts).\"}],\"\\n\",[\"$r\",\"li\",null,{\"children\":\"Values declared inside the component body are “reactive”.\"}],\"\\n\",[\"$r\",\"li\",null,{\"children\":\"Reactive values should re-synchronize the Effect because they can change over time.\"}],\"\\n\",[\"$r\",\"li\",null,{\"children\":\"The linter verifies that all reactive values used inside the Effect are specified as dependencies.\"}],\"\\n\",[\"$r\",\"li\",null,{\"children\":\"All errors flagged by the linter are legitimate. There’s always a way to fix the code to not break the rules.\"}],\"\\n\"]}]}],\"\\n\"]}],[\"$r\",\"Challenges\",null,{\"children\":[[\"$r\",\"h4\",null,{\"id\":\"fix-reconnecting-on-every-keystroke\",\"children\":\"Fix reconnecting on every keystroke \"}],[\"$r\",\"p\",null,{\"children\":[\"In this example, the \",[\"$r\",\"code\",null,{\"children\":\"ChatRoom\"}],\" component connects to the chat room when the component mounts, disconnects when it unmounts, and reconnects when you select a different chat room. This behavior is correct, so you need to keep it working.\"]}],[\"$r\",\"p\",null,{\"children\":[\"However, there is a problem. Whenever you type into the message box input at the bottom, \",[\"$r\",\"code\",null,{\"children\":\"ChatRoom\"}],\" \",[\"$r\",\"em\",null,{\"children\":\"also\"}],\" reconnects to the chat. (You can notice this by clearing the console and typing into the input.) Fix the issue so that this doesn’t happen.\"]}],[\"$r\",\"Hint\",null,{\"children\":[\"$r\",\"p\",null,{\"children\":\"You might need to add a dependency array for this Effect. What dependencies should be there?\"}]}],[\"$r\",\"Sandpack\",null,{\"children\":[[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"children\":\"import { useState, useEffect } from 'react';\\nimport { createConnection } from './chat.js';\\n\\nconst serverUrl = 'https://localhost:1234';\\n\\nfunction ChatRoom({ roomId }) {\\n const [message, setMessage] = useState('');\\n\\n useEffect(() =\u003e {\\n const connection = createConnection(serverUrl, roomId);\\n connection.connect();\\n return () =\u003e connection.disconnect();\\n });\\n\\n return (\\n \u003c\u003e\\n \u003ch1\u003eWelcome to the {roomId} room!\u003c/h1\u003e\\n \u003cinput\\n value={message}\\n onChange={e =\u003e setMessage(e.target.value)}\\n /\u003e\\n \u003c/\u003e\\n );\\n}\\n\\nexport default function App() {\\n const [roomId, setRoomId] = useState('general');\\n return (\\n \u003c\u003e\\n \u003clabel\u003e\\n Choose the chat room:{' '}\\n \u003cselect\\n value={roomId}\\n onChange={e =\u003e setRoomId(e.target.value)}\\n \u003e\\n \u003coption value=\\\"general\\\"\u003egeneral\u003c/option\u003e\\n \u003coption value=\\\"travel\\\"\u003etravel\u003c/option\u003e\\n \u003coption value=\\\"music\\\"\u003emusic\u003c/option\u003e\\n \u003c/select\u003e\\n \u003c/label\u003e\\n \u003chr /\u003e\\n \u003cChatRoom roomId={roomId} /\u003e\\n \u003c/\u003e\\n );\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"src/chat.js\",\"children\":\"export function createConnection(serverUrl, roomId) {\\n // A real implementation would actually connect to the server\\n return {\\n connect() {\\n console.log('✅ Connecting to \\\"' + roomId + '\\\" room at ' + serverUrl + '...');\\n },\\n disconnect() {\\n console.log('❌ Disconnected from \\\"' + roomId + '\\\" room at ' + serverUrl);\\n }\\n };\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-css\",\"children\":\"input { display: block; margin-bottom: 20px; }\\nbutton { margin-left: 10px; }\\n\"}]}]]}],[\"$r\",\"Solution\",null,{\"children\":[[\"$r\",\"p\",null,{\"children\":[\"This Effect didn’t have a dependency array at all, so it re-synchronized after every re-render. First, add a dependency array. Then, make sure that every reactive value used by the Effect is specified in the array. For example, \",[\"$r\",\"code\",null,{\"children\":\"roomId\"}],\" is reactive (because it’s a prop), so it should be included in the array. This ensures that when the user selects a different room, the chat reconnects. On the other hand, \",[\"$r\",\"code\",null,{\"children\":\"serverUrl\"}],\" is defined outside the component. This is why it doesn’t need to be in the array.\"]}],[\"$r\",\"Sandpack\",null,{\"children\":[[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"children\":\"import { useState, useEffect } from 'react';\\nimport { createConnection } from './chat.js';\\n\\nconst serverUrl = 'https://localhost:1234';\\n\\nfunction ChatRoom({ roomId }) {\\n const [message, setMessage] = useState('');\\n\\n useEffect(() =\u003e {\\n const connection = createConnection(serverUrl, roomId);\\n connection.connect();\\n return () =\u003e connection.disconnect();\\n }, [roomId]);\\n\\n return (\\n \u003c\u003e\\n \u003ch1\u003eWelcome to the {roomId} room!\u003c/h1\u003e\\n \u003cinput\\n value={message}\\n onChange={e =\u003e setMessage(e.target.value)}\\n /\u003e\\n \u003c/\u003e\\n );\\n}\\n\\nexport default function App() {\\n const [roomId, setRoomId] = useState('general');\\n return (\\n \u003c\u003e\\n \u003clabel\u003e\\n Choose the chat room:{' '}\\n \u003cselect\\n value={roomId}\\n onChange={e =\u003e setRoomId(e.target.value)}\\n \u003e\\n \u003coption value=\\\"general\\\"\u003egeneral\u003c/option\u003e\\n \u003coption value=\\\"travel\\\"\u003etravel\u003c/option\u003e\\n \u003coption value=\\\"music\\\"\u003emusic\u003c/option\u003e\\n \u003c/select\u003e\\n \u003c/label\u003e\\n \u003chr /\u003e\\n \u003cChatRoom roomId={roomId} /\u003e\\n \u003c/\u003e\\n );\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"src/chat.js\",\"children\":\"export function createConnection(serverUrl, roomId) {\\n // A real implementation would actually connect to the server\\n return {\\n connect() {\\n console.log('✅ Connecting to \\\"' + roomId + '\\\" room at ' + serverUrl + '...');\\n },\\n disconnect() {\\n console.log('❌ Disconnected from \\\"' + roomId + '\\\" room at ' + serverUrl);\\n }\\n };\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-css\",\"children\":\"input { display: block; margin-bottom: 20px; }\\nbutton { margin-left: 10px; }\\n\"}]}]]}]]}],[\"$r\",\"h4\",null,{\"id\":\"switch-synchronization-on-and-off\",\"children\":\"Switch synchronization on and off \"}],[\"$r\",\"p\",null,{\"children\":[\"In this example, an Effect subscribes to the window \",[\"$r\",\"a\",null,{\"href\":\"https://developer.mozilla.org/en-US/docs/Web/API/Element/pointermove_event\",\"target\":\"_blank\",\"rel\":\"nofollow noopener noreferrer\",\"children\":[\"$r\",\"code\",null,{\"children\":\"pointermove\"}]}],\" event to move a pink dot on the screen. Try hovering over the preview area (or touching the screen if you’re on a mobile device), and see how the pink dot follows your movement.\"]}],[\"$r\",\"p\",null,{\"children\":[\"There is also a checkbox. Ticking the checkbox toggles the \",[\"$r\",\"code\",null,{\"children\":\"canMove\"}],\" state variable, but this state variable is not used anywhere in the code. Your task is to change the code so that when \",[\"$r\",\"code\",null,{\"children\":\"canMove\"}],\" is \",[\"$r\",\"code\",null,{\"children\":\"false\"}],\" (the checkbox is ticked off), the dot should stop moving. After you toggle the checkbox back on (and set \",[\"$r\",\"code\",null,{\"children\":\"canMove\"}],\" to \",[\"$r\",\"code\",null,{\"children\":\"true\"}],\"), the box should follow the movement again. In other words, whether the dot can move or not should stay synchronized to whether the checkbox is checked.\"]}],[\"$r\",\"Hint\",null,{\"children\":[\"$r\",\"p\",null,{\"children\":\"You can’t declare an Effect conditionally. However, the code inside the Effect can use conditions!\"}]}],[\"$r\",\"Sandpack\",null,{\"children\":[[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"children\":\"import { useState, useEffect } from 'react';\\n\\nexport default function App() {\\n const [position, setPosition] = useState({ x: 0, y: 0 });\\n const [canMove, setCanMove] = useState(true);\\n\\n useEffect(() =\u003e {\\n function handleMove(e) {\\n setPosition({ x: e.clientX, y: e.clientY });\\n }\\n window.addEventListener('pointermove', handleMove);\\n return () =\u003e window.removeEventListener('pointermove', handleMove);\\n }, []);\\n\\n return (\\n \u003c\u003e\\n \u003clabel\u003e\\n \u003cinput type=\\\"checkbox\\\"\\n checked={canMove}\\n onChange={e =\u003e setCanMove(e.target.checked)} \\n /\u003e\\n The dot is allowed to move\\n \u003c/label\u003e\\n \u003chr /\u003e\\n \u003cdiv style={{\\n position: 'absolute',\\n backgroundColor: 'pink',\\n borderRadius: '50%',\\n opacity: 0.6,\\n transform: `translate(${position.x}px, ${position.y}px)`,\\n pointerEvents: 'none',\\n left: -20,\\n top: -20,\\n width: 40,\\n height: 40,\\n }} /\u003e\\n \u003c/\u003e\\n );\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-css\",\"children\":\"body {\\n height: 200px;\\n}\\n\"}]}]]}],[\"$r\",\"Solution\",null,{\"children\":[[\"$r\",\"p\",null,{\"children\":[\"One solution is to wrap the \",[\"$r\",\"code\",null,{\"children\":\"setPosition\"}],\" call into an \",[\"$r\",\"code\",null,{\"children\":\"if (canMove) { ... }\"}],\" condition:\"]}],[\"$r\",\"Sandpack\",null,{\"children\":[[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"children\":\"import { useState, useEffect } from 'react';\\n\\nexport default function App() {\\n const [position, setPosition] = useState({ x: 0, y: 0 });\\n const [canMove, setCanMove] = useState(true);\\n\\n useEffect(() =\u003e {\\n function handleMove(e) {\\n if (canMove) {\\n setPosition({ x: e.clientX, y: e.clientY });\\n }\\n }\\n window.addEventListener('pointermove', handleMove);\\n return () =\u003e window.removeEventListener('pointermove', handleMove);\\n }, [canMove]);\\n\\n return (\\n \u003c\u003e\\n \u003clabel\u003e\\n \u003cinput type=\\\"checkbox\\\"\\n checked={canMove}\\n onChange={e =\u003e setCanMove(e.target.checked)} \\n /\u003e\\n The dot is allowed to move\\n \u003c/label\u003e\\n \u003chr /\u003e\\n \u003cdiv style={{\\n position: 'absolute',\\n backgroundColor: 'pink',\\n borderRadius: '50%',\\n opacity: 0.6,\\n transform: `translate(${position.x}px, ${position.y}px)`,\\n pointerEvents: 'none',\\n left: -20,\\n top: -20,\\n width: 40,\\n height: 40,\\n }} /\u003e\\n \u003c/\u003e\\n );\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-css\",\"children\":\"body {\\n height: 200px;\\n}\\n\"}]}]]}],[\"$r\",\"p\",null,{\"children\":[\"Alternatively, you could wrap the \",[\"$r\",\"em\",null,{\"children\":\"event subscription\"}],\" logic into an \",[\"$r\",\"code\",null,{\"children\":\"if (canMove) { ... }\"}],\" condition:\"]}],[\"$r\",\"Sandpack\",null,{\"children\":[[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"children\":\"import { useState, useEffect } from 'react';\\n\\nexport default function App() {\\n const [position, setPosition] = useState({ x: 0, y: 0 });\\n const [canMove, setCanMove] = useState(true);\\n\\n useEffect(() =\u003e {\\n function handleMove(e) {\\n setPosition({ x: e.clientX, y: e.clientY });\\n }\\n if (canMove) {\\n window.addEventListener('pointermove', handleMove);\\n return () =\u003e window.removeEventListener('pointermove', handleMove);\\n }\\n }, [canMove]);\\n\\n return (\\n \u003c\u003e\\n \u003clabel\u003e\\n \u003cinput type=\\\"checkbox\\\"\\n checked={canMove}\\n onChange={e =\u003e setCanMove(e.target.checked)} \\n /\u003e\\n The dot is allowed to move\\n \u003c/label\u003e\\n \u003chr /\u003e\\n \u003cdiv style={{\\n position: 'absolute',\\n backgroundColor: 'pink',\\n borderRadius: '50%',\\n opacity: 0.6,\\n transform: `translate(${position.x}px, ${position.y}px)`,\\n pointerEvents: 'none',\\n left: -20,\\n top: -20,\\n width: 40,\\n height: 40,\\n }} /\u003e\\n \u003c/\u003e\\n );\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-css\",\"children\":\"body {\\n height: 200px;\\n}\\n\"}]}]]}],[\"$r\",\"p\",null,{\"children\":[\"In both of these cases, \",[\"$r\",\"code\",null,{\"children\":\"canMove\"}],\" is a reactive variable that you read inside the Effect. This is why it must be specified in the list of Effect dependencies. This ensures that the Effect re-synchronizes after every change to its value.\"]}]]}],[\"$r\",\"h4\",null,{\"id\":\"investigate-a-stale-value-bug\",\"children\":\"Investigate a stale value bug \"}],[\"$r\",\"p\",null,{\"children\":[\"In this example, the pink dot should move when the checkbox is on, and should stop moving when the checkbox is off. The logic for this has already been implemented: the \",[\"$r\",\"code\",null,{\"children\":\"handleMove\"}],\" event handler checks the \",[\"$r\",\"code\",null,{\"children\":\"canMove\"}],\" state variable.\"]}],[\"$r\",\"p\",null,{\"children\":[\"However, for some reason, the \",[\"$r\",\"code\",null,{\"children\":\"canMove\"}],\" state variable inside \",[\"$r\",\"code\",null,{\"children\":\"handleMove\"}],\" appears to be “stale”: it’s always \",[\"$r\",\"code\",null,{\"children\":\"true\"}],\", even after you tick off the checkbox. How is this possible? Find the mistake in the code and fix it.\"]}],[\"$r\",\"Hint\",null,{\"children\":[\"$r\",\"p\",null,{\"children\":\"If you see a linter rule being suppressed, remove the suppression! That’s where the mistakes usually are.\"}]}],[\"$r\",\"Sandpack\",null,{\"children\":[[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"children\":\"import { useState, useEffect } from 'react';\\n\\nexport default function App() {\\n const [position, setPosition] = useState({ x: 0, y: 0 });\\n const [canMove, setCanMove] = useState(true);\\n\\n function handleMove(e) {\\n if (canMove) {\\n setPosition({ x: e.clientX, y: e.clientY });\\n }\\n }\\n\\n useEffect(() =\u003e {\\n window.addEventListener('pointermove', handleMove);\\n return () =\u003e window.removeEventListener('pointermove', handleMove);\\n // eslint-disable-next-line react-hooks/exhaustive-deps\\n }, []);\\n\\n return (\\n \u003c\u003e\\n \u003clabel\u003e\\n \u003cinput type=\\\"checkbox\\\"\\n checked={canMove}\\n onChange={e =\u003e setCanMove(e.target.checked)} \\n /\u003e\\n The dot is allowed to move\\n \u003c/label\u003e\\n \u003chr /\u003e\\n \u003cdiv style={{\\n position: 'absolute',\\n backgroundColor: 'pink',\\n borderRadius: '50%',\\n opacity: 0.6,\\n transform: `translate(${position.x}px, ${position.y}px)`,\\n pointerEvents: 'none',\\n left: -20,\\n top: -20,\\n width: 40,\\n height: 40,\\n }} /\u003e\\n \u003c/\u003e\\n );\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-css\",\"children\":\"body {\\n height: 200px;\\n}\\n\"}]}]]}],[\"$r\",\"Solution\",null,{\"children\":[[\"$r\",\"p\",null,{\"children\":[\"The problem with the original code was suppressing the dependency linter. If you remove the suppression, you’ll see that this Effect depends on the \",[\"$r\",\"code\",null,{\"children\":\"handleMove\"}],\" function. This makes sense: \",[\"$r\",\"code\",null,{\"children\":\"handleMove\"}],\" is declared inside the component body, which makes it a reactive value. Every reactive value must be specified as a dependency, or it can potentially get stale over time!\"]}],[\"$r\",\"p\",null,{\"children\":[\"The author of the original code has “lied” to React by saying that the Effect does not depend (\",[\"$r\",\"code\",null,{\"children\":\"[]\"}],\") on any reactive values. This is why React did not re-synchronize the Effect after \",[\"$r\",\"code\",null,{\"children\":\"canMove\"}],\" has changed (and \",[\"$r\",\"code\",null,{\"children\":\"handleMove\"}],\" with it). Because React did not re-synchronize the Effect, the \",[\"$r\",\"code\",null,{\"children\":\"handleMove\"}],\" attached as a listener is the \",[\"$r\",\"code\",null,{\"children\":\"handleMove\"}],\" function created during the initial render. During the initial render, \",[\"$r\",\"code\",null,{\"children\":\"canMove\"}],\" was \",[\"$r\",\"code\",null,{\"children\":\"true\"}],\", which is why \",[\"$r\",\"code\",null,{\"children\":\"handleMove\"}],\" from the initial render will forever see that value.\"]}],[\"$r\",\"p\",null,{\"children\":[[\"$r\",\"strong\",null,{\"children\":\"If you never suppress the linter, you will never see problems with stale values.\"}],\" There are a few different ways to solve this bug, but you should always start by removing the linter suppression. Then change the code to fix the lint error.\"]}],[\"$r\",\"p\",null,{\"children\":[\"You can change the Effect dependencies to \",[\"$r\",\"code\",null,{\"children\":\"[handleMove]\"}],\", but since it’s going to be a newly defined function for every render, you might as well remove dependencies array altogether. Then the Effect \",[\"$r\",\"em\",null,{\"children\":\"will\"}],\" re-synchronize after every re-render:\"]}],[\"$r\",\"Sandpack\",null,{\"children\":[[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"children\":\"import { useState, useEffect } from 'react';\\n\\nexport default function App() {\\n const [position, setPosition] = useState({ x: 0, y: 0 });\\n const [canMove, setCanMove] = useState(true);\\n\\n function handleMove(e) {\\n if (canMove) {\\n setPosition({ x: e.clientX, y: e.clientY });\\n }\\n }\\n\\n useEffect(() =\u003e {\\n window.addEventListener('pointermove', handleMove);\\n return () =\u003e window.removeEventListener('pointermove', handleMove);\\n });\\n\\n return (\\n \u003c\u003e\\n \u003clabel\u003e\\n \u003cinput type=\\\"checkbox\\\"\\n checked={canMove}\\n onChange={e =\u003e setCanMove(e.target.checked)} \\n /\u003e\\n The dot is allowed to move\\n \u003c/label\u003e\\n \u003chr /\u003e\\n \u003cdiv style={{\\n position: 'absolute',\\n backgroundColor: 'pink',\\n borderRadius: '50%',\\n opacity: 0.6,\\n transform: `translate(${position.x}px, ${position.y}px)`,\\n pointerEvents: 'none',\\n left: -20,\\n top: -20,\\n width: 40,\\n height: 40,\\n }} /\u003e\\n \u003c/\u003e\\n );\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-css\",\"children\":\"body {\\n height: 200px;\\n}\\n\"}]}]]}],[\"$r\",\"p\",null,{\"children\":[\"This solution works, but it’s not ideal. If you put \",[\"$r\",\"code\",null,{\"children\":\"console.log('Resubscribing')\"}],\" inside the Effect, you’ll notice that it resubscribes after every re-render. Resubscribing is fast, but it would still be nice to avoid doing it so often.\"]}],[\"$r\",\"p\",null,{\"children\":[\"A better fix would be to move the \",[\"$r\",\"code\",null,{\"children\":\"handleMove\"}],\" function \",[\"$r\",\"em\",null,{\"children\":\"inside\"}],\" the Effect. Then \",[\"$r\",\"code\",null,{\"children\":\"handleMove\"}],\" won’t be a reactive value, and so your Effect won’t depend on a function. Instead, it will need to depend on \",[\"$r\",\"code\",null,{\"children\":\"canMove\"}],\" which your code now reads from inside the Effect. This matches the behavior you wanted, since your Effect will now stay synchronized with the value of \",[\"$r\",\"code\",null,{\"children\":\"canMove\"}],\":\"]}],[\"$r\",\"Sandpack\",null,{\"children\":[[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"children\":\"import { useState, useEffect } from 'react';\\n\\nexport default function App() {\\n const [position, setPosition] = useState({ x: 0, y: 0 });\\n const [canMove, setCanMove] = useState(true);\\n\\n useEffect(() =\u003e {\\n function handleMove(e) {\\n if (canMove) {\\n setPosition({ x: e.clientX, y: e.clientY });\\n }\\n }\\n\\n window.addEventListener('pointermove', handleMove);\\n return () =\u003e window.removeEventListener('pointermove', handleMove);\\n }, [canMove]);\\n\\n return (\\n \u003c\u003e\\n \u003clabel\u003e\\n \u003cinput type=\\\"checkbox\\\"\\n checked={canMove}\\n onChange={e =\u003e setCanMove(e.target.checked)} \\n /\u003e\\n The dot is allowed to move\\n \u003c/label\u003e\\n \u003chr /\u003e\\n \u003cdiv style={{\\n position: 'absolute',\\n backgroundColor: 'pink',\\n borderRadius: '50%',\\n opacity: 0.6,\\n transform: `translate(${position.x}px, ${position.y}px)`,\\n pointerEvents: 'none',\\n left: -20,\\n top: -20,\\n width: 40,\\n height: 40,\\n }} /\u003e\\n \u003c/\u003e\\n );\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-css\",\"children\":\"body {\\n height: 200px;\\n}\\n\"}]}]]}],[\"$r\",\"p\",null,{\"children\":[\"Try adding \",[\"$r\",\"code\",null,{\"children\":\"console.log('Resubscribing')\"}],\" inside the Effect body and notice that now it only resubscribes when you toggle the checkbox (\",[\"$r\",\"code\",null,{\"children\":\"canMove\"}],\" changes) or edit the code. This makes it better than the previous approach that always resubscribed.\"]}],[\"$r\",\"p\",null,{\"children\":[\"You’ll learn a more general approach to this type of problem in \",[\"$r\",\"a\",null,{\"href\":\"/learn/separating-events-from-effects\",\"children\":\"Separating Events from Effects.\"}]]}]]}],[\"$r\",\"h4\",null,{\"id\":\"fix-a-connection-switch\",\"children\":\"Fix a connection switch \"}],[\"$r\",\"p\",null,{\"children\":[\"In this example, the chat service in \",[\"$r\",\"code\",null,{\"children\":\"chat.js\"}],\" exposes two different APIs: \",[\"$r\",\"code\",null,{\"children\":\"createEncryptedConnection\"}],\" and \",[\"$r\",\"code\",null,{\"children\":\"createUnencryptedConnection\"}],\". The root \",[\"$r\",\"code\",null,{\"children\":\"App\"}],\" component lets the user choose whether to use encryption or not, and then passes down the corresponding API method to the child \",[\"$r\",\"code\",null,{\"children\":\"ChatRoom\"}],\" component as the \",[\"$r\",\"code\",null,{\"children\":\"createConnection\"}],\" prop.\"]}],[\"$r\",\"p\",null,{\"children\":[\"Notice that initially, the console logs say the connection is not encrypted. Try toggling the checkbox on: nothing will happen. However, if you change the selected room after that, then the chat will reconnect \",[\"$r\",\"em\",null,{\"children\":\"and\"}],\" enable encryption (as you’ll see from the console messages). This is a bug. Fix the bug so that toggling the checkbox \",[\"$r\",\"em\",null,{\"children\":\"also\"}],\" causes the chat to reconnect.\"]}],[\"$r\",\"Hint\",null,{\"children\":[\"$r\",\"p\",null,{\"children\":\"Suppressing the linter is always suspicious. Could this be a bug?\"}]}],[\"$r\",\"Sandpack\",null,{\"children\":[[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"src/App.js\",\"children\":\"import { useState } from 'react';\\nimport ChatRoom from './ChatRoom.js';\\nimport {\\n createEncryptedConnection,\\n createUnencryptedConnection,\\n} from './chat.js';\\n\\nexport default function App() {\\n const [roomId, setRoomId] = useState('general');\\n const [isEncrypted, setIsEncrypted] = useState(false);\\n return (\\n \u003c\u003e\\n \u003clabel\u003e\\n Choose the chat room:{' '}\\n \u003cselect\\n value={roomId}\\n onChange={e =\u003e setRoomId(e.target.value)}\\n \u003e\\n \u003coption value=\\\"general\\\"\u003egeneral\u003c/option\u003e\\n \u003coption value=\\\"travel\\\"\u003etravel\u003c/option\u003e\\n \u003coption value=\\\"music\\\"\u003emusic\u003c/option\u003e\\n \u003c/select\u003e\\n \u003c/label\u003e\\n \u003clabel\u003e\\n \u003cinput\\n type=\\\"checkbox\\\"\\n checked={isEncrypted}\\n onChange={e =\u003e setIsEncrypted(e.target.checked)}\\n /\u003e\\n Enable encryption\\n \u003c/label\u003e\\n \u003chr /\u003e\\n \u003cChatRoom\\n roomId={roomId}\\n createConnection={isEncrypted ?\\n createEncryptedConnection :\\n createUnencryptedConnection\\n }\\n /\u003e\\n \u003c/\u003e\\n );\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"src/ChatRoom.js active\",\"children\":\"import { useState, useEffect } from 'react';\\n\\nexport default function ChatRoom({ roomId, createConnection }) {\\n useEffect(() =\u003e {\\n const connection = createConnection(roomId);\\n connection.connect();\\n return () =\u003e connection.disconnect();\\n // eslint-disable-next-line react-hooks/exhaustive-deps\\n }, [roomId]);\\n\\n return \u003ch1\u003eWelcome to the {roomId} room!\u003c/h1\u003e;\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"src/chat.js\",\"children\":\"export function createEncryptedConnection(roomId) {\\n // A real implementation would actually connect to the server\\n return {\\n connect() {\\n console.log('✅ 🔐 Connecting to \\\"' + roomId + '... (encrypted)');\\n },\\n disconnect() {\\n console.log('❌ 🔐 Disconnected from \\\"' + roomId + '\\\" room (encrypted)');\\n }\\n };\\n}\\n\\nexport function createUnencryptedConnection(roomId) {\\n // A real implementation would actually connect to the server\\n return {\\n connect() {\\n console.log('✅ Connecting to \\\"' + roomId + '... (unencrypted)');\\n },\\n disconnect() {\\n console.log('❌ Disconnected from \\\"' + roomId + '\\\" room (unencrypted)');\\n }\\n };\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-css\",\"children\":\"label { display: block; margin-bottom: 10px; }\\n\"}]}]]}],[\"$r\",\"Solution\",null,{\"children\":[[\"$r\",\"p\",null,{\"children\":[\"If you remove the linter suppression, you will see a lint error. The problem is that \",[\"$r\",\"code\",null,{\"children\":\"createConnection\"}],\" is a prop, so it’s a reactive value. It can change over time! (And indeed, it should—when the user ticks the checkbox, the parent component passes a different value of the \",[\"$r\",\"code\",null,{\"children\":\"createConnection\"}],\" prop.) This is why it should be a dependency. Include it in the list to fix the bug:\"]}],[\"$r\",\"Sandpack\",null,{\"children\":[[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"src/App.js\",\"children\":\"import { useState } from 'react';\\nimport ChatRoom from './ChatRoom.js';\\nimport {\\n createEncryptedConnection,\\n createUnencryptedConnection,\\n} from './chat.js';\\n\\nexport default function App() {\\n const [roomId, setRoomId] = useState('general');\\n const [isEncrypted, setIsEncrypted] = useState(false);\\n return (\\n \u003c\u003e\\n \u003clabel\u003e\\n Choose the chat room:{' '}\\n \u003cselect\\n value={roomId}\\n onChange={e =\u003e setRoomId(e.target.value)}\\n \u003e\\n \u003coption value=\\\"general\\\"\u003egeneral\u003c/option\u003e\\n \u003coption value=\\\"travel\\\"\u003etravel\u003c/option\u003e\\n \u003coption value=\\\"music\\\"\u003emusic\u003c/option\u003e\\n \u003c/select\u003e\\n \u003c/label\u003e\\n \u003clabel\u003e\\n \u003cinput\\n type=\\\"checkbox\\\"\\n checked={isEncrypted}\\n onChange={e =\u003e setIsEncrypted(e.target.checked)}\\n /\u003e\\n Enable encryption\\n \u003c/label\u003e\\n \u003chr /\u003e\\n \u003cChatRoom\\n roomId={roomId}\\n createConnection={isEncrypted ?\\n createEncryptedConnection :\\n createUnencryptedConnection\\n }\\n /\u003e\\n \u003c/\u003e\\n );\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"src/ChatRoom.js active\",\"children\":\"import { useState, useEffect } from 'react';\\n\\nexport default function ChatRoom({ roomId, createConnection }) {\\n useEffect(() =\u003e {\\n const connection = createConnection(roomId);\\n connection.connect();\\n return () =\u003e connection.disconnect();\\n }, [roomId, createConnection]);\\n\\n return \u003ch1\u003eWelcome to the {roomId} room!\u003c/h1\u003e;\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"src/chat.js\",\"children\":\"export function createEncryptedConnection(roomId) {\\n // A real implementation would actually connect to the server\\n return {\\n connect() {\\n console.log('✅ 🔐 Connecting to \\\"' + roomId + '... (encrypted)');\\n },\\n disconnect() {\\n console.log('❌ 🔐 Disconnected from \\\"' + roomId + '\\\" room (encrypted)');\\n }\\n };\\n}\\n\\nexport function createUnencryptedConnection(roomId) {\\n // A real implementation would actually connect to the server\\n return {\\n connect() {\\n console.log('✅ Connecting to \\\"' + roomId + '... (unencrypted)');\\n },\\n disconnect() {\\n console.log('❌ Disconnected from \\\"' + roomId + '\\\" room (unencrypted)');\\n }\\n };\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-css\",\"children\":\"label { display: block; margin-bottom: 10px; }\\n\"}]}]]}],[\"$r\",\"p\",null,{\"children\":[\"It is correct that \",[\"$r\",\"code\",null,{\"children\":\"createConnection\"}],\" is a dependency. However, this code is a bit fragile because someone could edit the \",[\"$r\",\"code\",null,{\"children\":\"App\"}],\" component to pass an inline function as the value of this prop. In that case, its value would be different every time the \",[\"$r\",\"code\",null,{\"children\":\"App\"}],\" component re-renders, so the Effect might re-synchronize too often. To avoid this, you can pass \",[\"$r\",\"code\",null,{\"children\":\"isEncrypted\"}],\" down instead:\"]}],[\"$r\",\"Sandpack\",null,{\"children\":[[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"src/App.js\",\"children\":\"import { useState } from 'react';\\nimport ChatRoom from './ChatRoom.js';\\n\\nexport default function App() {\\n const [roomId, setRoomId] = useState('general');\\n const [isEncrypted, setIsEncrypted] = useState(false);\\n return (\\n \u003c\u003e\\n \u003clabel\u003e\\n Choose the chat room:{' '}\\n \u003cselect\\n value={roomId}\\n onChange={e =\u003e setRoomId(e.target.value)}\\n \u003e\\n \u003coption value=\\\"general\\\"\u003egeneral\u003c/option\u003e\\n \u003coption value=\\\"travel\\\"\u003etravel\u003c/option\u003e\\n \u003coption value=\\\"music\\\"\u003emusic\u003c/option\u003e\\n \u003c/select\u003e\\n \u003c/label\u003e\\n \u003clabel\u003e\\n \u003cinput\\n type=\\\"checkbox\\\"\\n checked={isEncrypted}\\n onChange={e =\u003e setIsEncrypted(e.target.checked)}\\n /\u003e\\n Enable encryption\\n \u003c/label\u003e\\n \u003chr /\u003e\\n \u003cChatRoom\\n roomId={roomId}\\n isEncrypted={isEncrypted}\\n /\u003e\\n \u003c/\u003e\\n );\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"src/ChatRoom.js active\",\"children\":\"import { useState, useEffect } from 'react';\\nimport {\\n createEncryptedConnection,\\n createUnencryptedConnection,\\n} from './chat.js';\\n\\nexport default function ChatRoom({ roomId, isEncrypted }) {\\n useEffect(() =\u003e {\\n const createConnection = isEncrypted ?\\n createEncryptedConnection :\\n createUnencryptedConnection;\\n const connection = createConnection(roomId);\\n connection.connect();\\n return () =\u003e connection.disconnect();\\n }, [roomId, isEncrypted]);\\n\\n return \u003ch1\u003eWelcome to the {roomId} room!\u003c/h1\u003e;\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"src/chat.js\",\"children\":\"export function createEncryptedConnection(roomId) {\\n // A real implementation would actually connect to the server\\n return {\\n connect() {\\n console.log('✅ 🔐 Connecting to \\\"' + roomId + '... (encrypted)');\\n },\\n disconnect() {\\n console.log('❌ 🔐 Disconnected from \\\"' + roomId + '\\\" room (encrypted)');\\n }\\n };\\n}\\n\\nexport function createUnencryptedConnection(roomId) {\\n // A real implementation would actually connect to the server\\n return {\\n connect() {\\n console.log('✅ Connecting to \\\"' + roomId + '... (unencrypted)');\\n },\\n disconnect() {\\n console.log('❌ Disconnected from \\\"' + roomId + '\\\" room (unencrypted)');\\n }\\n };\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-css\",\"children\":\"label { display: block; margin-bottom: 10px; }\\n\"}]}]]}],[\"$r\",\"p\",null,{\"children\":[\"In this version, the \",[\"$r\",\"code\",null,{\"children\":\"App\"}],\" component passes a boolean prop instead of a function. Inside the Effect, you decide which function to use. Since both \",[\"$r\",\"code\",null,{\"children\":\"createEncryptedConnection\"}],\" and \",[\"$r\",\"code\",null,{\"children\":\"createUnencryptedConnection\"}],\" are declared outside the component, they aren’t reactive, and don’t need to be dependencies. You’ll learn more about this in \",[\"$r\",\"a\",null,{\"href\":\"/learn/removing-effect-dependencies\",\"children\":\"Removing Effect Dependencies.\"}]]}]]}],[\"$r\",\"h4\",null,{\"id\":\"populate-a-chain-of-select-boxes\",\"children\":\"Populate a chain of select boxes \"}],[\"$r\",\"p\",null,{\"children\":[\"In this example, there are two select boxes. One select box lets the user pick a planet. Another select box lets the user pick a place \",[\"$r\",\"em\",null,{\"children\":\"on that planet.\"}],\" The second box doesn’t work yet. Your task is to make it show the places on the chosen planet.\"]}],[\"$r\",\"p\",null,{\"children\":[\"Look at how the first select box works. It populates the \",[\"$r\",\"code\",null,{\"children\":\"planetList\"}],\" state with the result from the \",[\"$r\",\"code\",null,{\"children\":\"\\\"/planets\\\"\"}],\" API call. The currently selected planet’s ID is kept in the \",[\"$r\",\"code\",null,{\"children\":\"planetId\"}],\" state variable. You need to find where to add some additional code so that the \",[\"$r\",\"code\",null,{\"children\":\"placeList\"}],\" state variable is populated with the result of the \",[\"$r\",\"code\",null,{\"children\":\"\\\"/planets/\\\" + planetId + \\\"/places\\\"\"}],\" API call.\"]}],[\"$r\",\"p\",null,{\"children\":\"If you implement this right, selecting a planet should populate the place list. Changing a planet should change the place list.\"}],[\"$r\",\"Hint\",null,{\"children\":[\"$r\",\"p\",null,{\"children\":\"If you have two independent synchronization processes, you need to write two separate Effects.\"}]}],[\"$r\",\"Sandpack\",null,{\"children\":[[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"src/App.js\",\"children\":\"import { useState, useEffect } from 'react';\\nimport { fetchData } from './api.js';\\n\\nexport default function Page() {\\n const [planetList, setPlanetList] = useState([])\\n const [planetId, setPlanetId] = useState('');\\n\\n const [placeList, setPlaceList] = useState([]);\\n const [placeId, setPlaceId] = useState('');\\n\\n useEffect(() =\u003e {\\n let ignore = false;\\n fetchData('/planets').then(result =\u003e {\\n if (!ignore) {\\n console.log('Fetched a list of planets.');\\n setPlanetList(result);\\n setPlanetId(result[0].id); // Select the first planet\\n }\\n });\\n return () =\u003e {\\n ignore = true;\\n }\\n }, []);\\n\\n return (\\n \u003c\u003e\\n \u003clabel\u003e\\n Pick a planet:{' '}\\n \u003cselect value={planetId} onChange={e =\u003e {\\n setPlanetId(e.target.value);\\n }}\u003e\\n {planetList.map(planet =\u003e\\n \u003coption key={planet.id} value={planet.id}\u003e{planet.name}\u003c/option\u003e\\n )}\\n \u003c/select\u003e\\n \u003c/label\u003e\\n \u003clabel\u003e\\n Pick a place:{' '}\\n \u003cselect value={placeId} onChange={e =\u003e {\\n setPlaceId(e.target.value);\\n }}\u003e\\n {placeList.map(place =\u003e\\n \u003coption key={place.id} value={place.id}\u003e{place.name}\u003c/option\u003e\\n )}\\n \u003c/select\u003e\\n \u003c/label\u003e\\n \u003chr /\u003e\\n \u003cp\u003eYou are going to: {placeId || '???'} on {planetId || '???'} \u003c/p\u003e\\n \u003c/\u003e\\n );\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"src/api.js hidden\",\"children\":\"export function fetchData(url) {\\n if (url === '/planets') {\\n return fetchPlanets();\\n } else if (url.startsWith('/planets/')) {\\n const match = url.match(/^\\\\/planets\\\\/([\\\\w-]+)\\\\/places(\\\\/)?$/);\\n if (!match || !match[1] || !match[1].length) {\\n throw Error('Expected URL like \\\"/planets/earth/places\\\". Received: \\\"' + url + '\\\".');\\n }\\n return fetchPlaces(match[1]);\\n } else throw Error('Expected URL like \\\"/planets\\\" or \\\"/planets/earth/places\\\". Received: \\\"' + url + '\\\".');\\n}\\n\\nasync function fetchPlanets() {\\n return new Promise(resolve =\u003e {\\n setTimeout(() =\u003e {\\n resolve([{\\n id: 'earth',\\n name: 'Earth'\\n }, {\\n id: 'venus',\\n name: 'Venus'\\n }, {\\n id: 'mars',\\n name: 'Mars' \\n }]);\\n }, 1000);\\n });\\n}\\n\\nasync function fetchPlaces(planetId) {\\n if (typeof planetId !== 'string') {\\n throw Error(\\n 'fetchPlaces(planetId) expects a string argument. ' +\\n 'Instead received: ' + planetId + '.'\\n );\\n }\\n return new Promise(resolve =\u003e {\\n setTimeout(() =\u003e {\\n if (planetId === 'earth') {\\n resolve([{\\n id: 'laos',\\n name: 'Laos'\\n }, {\\n id: 'spain',\\n name: 'Spain'\\n }, {\\n id: 'vietnam',\\n name: 'Vietnam' \\n }]);\\n } else if (planetId === 'venus') {\\n resolve([{\\n id: 'aurelia',\\n name: 'Aurelia'\\n }, {\\n id: 'diana-chasma',\\n name: 'Diana Chasma'\\n }, {\\n id: 'kumsong-vallis',\\n name: 'Kŭmsŏng Vallis' \\n }]);\\n } else if (planetId === 'mars') {\\n resolve([{\\n id: 'aluminum-city',\\n name: 'Aluminum City'\\n }, {\\n id: 'new-new-york',\\n name: 'New New York'\\n }, {\\n id: 'vishniac',\\n name: 'Vishniac'\\n }]);\\n } else throw Error('Unknown planet ID: ' + planetId);\\n }, 1000);\\n });\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-css\",\"children\":\"label { display: block; margin-bottom: 10px; }\\n\"}]}]]}],[\"$r\",\"Solution\",null,{\"children\":[[\"$r\",\"p\",null,{\"children\":\"There are two independent synchronization processes:\"}],[\"$r\",\"ul\",null,{\"children\":[\"\\n\",[\"$r\",\"li\",null,{\"children\":\"The first select box is synchronized to the remote list of planets.\"}],\"\\n\",[\"$r\",\"li\",null,{\"children\":[\"The second select box is synchronized to the remote list of places for the current \",[\"$r\",\"code\",null,{\"children\":\"planetId\"}],\".\"]}],\"\\n\"]}],[\"$r\",\"p\",null,{\"children\":\"This is why it makes sense to describe them as two separate Effects. Here’s an example of how you could do this:\"}],[\"$r\",\"Sandpack\",null,{\"children\":[[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"src/App.js\",\"children\":\"import { useState, useEffect } from 'react';\\nimport { fetchData } from './api.js';\\n\\nexport default function Page() {\\n const [planetList, setPlanetList] = useState([])\\n const [planetId, setPlanetId] = useState('');\\n\\n const [placeList, setPlaceList] = useState([]);\\n const [placeId, setPlaceId] = useState('');\\n\\n useEffect(() =\u003e {\\n let ignore = false;\\n fetchData('/planets').then(result =\u003e {\\n if (!ignore) {\\n console.log('Fetched a list of planets.');\\n setPlanetList(result);\\n setPlanetId(result[0].id); // Select the first planet\\n }\\n });\\n return () =\u003e {\\n ignore = true;\\n }\\n }, []);\\n\\n useEffect(() =\u003e {\\n if (planetId === '') {\\n // Nothing is selected in the first box yet\\n return;\\n }\\n\\n let ignore = false;\\n fetchData('/planets/' + planetId + '/places').then(result =\u003e {\\n if (!ignore) {\\n console.log('Fetched a list of places on \\\"' + planetId + '\\\".');\\n setPlaceList(result);\\n setPlaceId(result[0].id); // Select the first place\\n }\\n });\\n return () =\u003e {\\n ignore = true;\\n }\\n }, [planetId]);\\n\\n return (\\n \u003c\u003e\\n \u003clabel\u003e\\n Pick a planet:{' '}\\n \u003cselect value={planetId} onChange={e =\u003e {\\n setPlanetId(e.target.value);\\n }}\u003e\\n {planetList.map(planet =\u003e\\n \u003coption key={planet.id} value={planet.id}\u003e{planet.name}\u003c/option\u003e\\n )}\\n \u003c/select\u003e\\n \u003c/label\u003e\\n \u003clabel\u003e\\n Pick a place:{' '}\\n \u003cselect value={placeId} onChange={e =\u003e {\\n setPlaceId(e.target.value);\\n }}\u003e\\n {placeList.map(place =\u003e\\n \u003coption key={place.id} value={place.id}\u003e{place.name}\u003c/option\u003e\\n )}\\n \u003c/select\u003e\\n \u003c/label\u003e\\n \u003chr /\u003e\\n \u003cp\u003eYou are going to: {placeId || '???'} on {planetId || '???'} \u003c/p\u003e\\n \u003c/\u003e\\n );\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"src/api.js hidden\",\"children\":\"export function fetchData(url) {\\n if (url === '/planets') {\\n return fetchPlanets();\\n } else if (url.startsWith('/planets/')) {\\n const match = url.match(/^\\\\/planets\\\\/([\\\\w-]+)\\\\/places(\\\\/)?$/);\\n if (!match || !match[1] || !match[1].length) {\\n throw Error('Expected URL like \\\"/planets/earth/places\\\". Received: \\\"' + url + '\\\".');\\n }\\n return fetchPlaces(match[1]);\\n } else throw Error('Expected URL like \\\"/planets\\\" or \\\"/planets/earth/places\\\". Received: \\\"' + url + '\\\".');\\n}\\n\\nasync function fetchPlanets() {\\n return new Promise(resolve =\u003e {\\n setTimeout(() =\u003e {\\n resolve([{\\n id: 'earth',\\n name: 'Earth'\\n }, {\\n id: 'venus',\\n name: 'Venus'\\n }, {\\n id: 'mars',\\n name: 'Mars' \\n }]);\\n }, 1000);\\n });\\n}\\n\\nasync function fetchPlaces(planetId) {\\n if (typeof planetId !== 'string') {\\n throw Error(\\n 'fetchPlaces(planetId) expects a string argument. ' +\\n 'Instead received: ' + planetId + '.'\\n );\\n }\\n return new Promise(resolve =\u003e {\\n setTimeout(() =\u003e {\\n if (planetId === 'earth') {\\n resolve([{\\n id: 'laos',\\n name: 'Laos'\\n }, {\\n id: 'spain',\\n name: 'Spain'\\n }, {\\n id: 'vietnam',\\n name: 'Vietnam' \\n }]);\\n } else if (planetId === 'venus') {\\n resolve([{\\n id: 'aurelia',\\n name: 'Aurelia'\\n }, {\\n id: 'diana-chasma',\\n name: 'Diana Chasma'\\n }, {\\n id: 'kumsong-vallis',\\n name: 'Kŭmsŏng Vallis' \\n }]);\\n } else if (planetId === 'mars') {\\n resolve([{\\n id: 'aluminum-city',\\n name: 'Aluminum City'\\n }, {\\n id: 'new-new-york',\\n name: 'New New York'\\n }, {\\n id: 'vishniac',\\n name: 'Vishniac'\\n }]);\\n } else throw Error('Unknown planet ID: ' + planetId);\\n }, 1000);\\n });\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-css\",\"children\":\"label { display: block; margin-bottom: 10px; }\\n\"}]}]]}],[\"$r\",\"p\",null,{\"children\":\"This code is a bit repetitive. However, that’s not a good reason to combine it into a single Effect! If you did this, you’d have to combine both Effect’s dependencies into one list, and then changing the planet would refetch the list of all planets. Effects are not a tool for code reuse.\"}],[\"$r\",\"p\",null,{\"children\":[\"Instead, to reduce repetition, you can extract some logic into a custom Hook like \",[\"$r\",\"code\",null,{\"children\":\"useSelectOptions\"}],\" below:\"]}],[\"$r\",\"Sandpack\",null,{\"children\":[[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"src/App.js\",\"children\":\"import { useState } from 'react';\\nimport { useSelectOptions } from './useSelectOptions.js';\\n\\nexport default function Page() {\\n const [\\n planetList,\\n planetId,\\n setPlanetId\\n ] = useSelectOptions('/planets');\\n\\n const [\\n placeList,\\n placeId,\\n setPlaceId\\n ] = useSelectOptions(planetId ? `/planets/${planetId}/places` : null);\\n\\n return (\\n \u003c\u003e\\n \u003clabel\u003e\\n Pick a planet:{' '}\\n \u003cselect value={planetId} onChange={e =\u003e {\\n setPlanetId(e.target.value);\\n }}\u003e\\n {planetList?.map(planet =\u003e\\n \u003coption key={planet.id} value={planet.id}\u003e{planet.name}\u003c/option\u003e\\n )}\\n \u003c/select\u003e\\n \u003c/label\u003e\\n \u003clabel\u003e\\n Pick a place:{' '}\\n \u003cselect value={placeId} onChange={e =\u003e {\\n setPlaceId(e.target.value);\\n }}\u003e\\n {placeList?.map(place =\u003e\\n \u003coption key={place.id} value={place.id}\u003e{place.name}\u003c/option\u003e\\n )}\\n \u003c/select\u003e\\n \u003c/label\u003e\\n \u003chr /\u003e\\n \u003cp\u003eYou are going to: {placeId || '...'} on {planetId || '...'} \u003c/p\u003e\\n \u003c/\u003e\\n );\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"src/useSelectOptions.js\",\"children\":\"import { useState, useEffect } from 'react';\\nimport { fetchData } from './api.js';\\n\\nexport function useSelectOptions(url) {\\n const [list, setList] = useState(null);\\n const [selectedId, setSelectedId] = useState('');\\n useEffect(() =\u003e {\\n if (url === null) {\\n return;\\n }\\n\\n let ignore = false;\\n fetchData(url).then(result =\u003e {\\n if (!ignore) {\\n setList(result);\\n setSelectedId(result[0].id);\\n }\\n });\\n return () =\u003e {\\n ignore = true;\\n }\\n }, [url]);\\n return [list, selectedId, setSelectedId];\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"src/api.js hidden\",\"children\":\"export function fetchData(url) {\\n if (url === '/planets') {\\n return fetchPlanets();\\n } else if (url.startsWith('/planets/')) {\\n const match = url.match(/^\\\\/planets\\\\/([\\\\w-]+)\\\\/places(\\\\/)?$/);\\n if (!match || !match[1] || !match[1].length) {\\n throw Error('Expected URL like \\\"/planets/earth/places\\\". Received: \\\"' + url + '\\\".');\\n }\\n return fetchPlaces(match[1]);\\n } else throw Error('Expected URL like \\\"/planets\\\" or \\\"/planets/earth/places\\\". Received: \\\"' + url + '\\\".');\\n}\\n\\nasync function fetchPlanets() {\\n return new Promise(resolve =\u003e {\\n setTimeout(() =\u003e {\\n resolve([{\\n id: 'earth',\\n name: 'Earth'\\n }, {\\n id: 'venus',\\n name: 'Venus'\\n }, {\\n id: 'mars',\\n name: 'Mars' \\n }]);\\n }, 1000);\\n });\\n}\\n\\nasync function fetchPlaces(planetId) {\\n if (typeof planetId !== 'string') {\\n throw Error(\\n 'fetchPlaces(planetId) expects a string argument. ' +\\n 'Instead received: ' + planetId + '.'\\n );\\n }\\n return new Promise(resolve =\u003e {\\n setTimeout(() =\u003e {\\n if (planetId === 'earth') {\\n resolve([{\\n id: 'laos',\\n name: 'Laos'\\n }, {\\n id: 'spain',\\n name: 'Spain'\\n }, {\\n id: 'vietnam',\\n name: 'Vietnam' \\n }]);\\n } else if (planetId === 'venus') {\\n resolve([{\\n id: 'aurelia',\\n name: 'Aurelia'\\n }, {\\n id: 'diana-chasma',\\n name: 'Diana Chasma'\\n }, {\\n id: 'kumsong-vallis',\\n name: 'Kŭmsŏng Vallis' \\n }]);\\n } else if (planetId === 'mars') {\\n resolve([{\\n id: 'aluminum-city',\\n name: 'Aluminum City'\\n }, {\\n id: 'new-new-york',\\n name: 'New New York'\\n }, {\\n id: 'vishniac',\\n name: 'Vishniac'\\n }]);\\n } else throw Error('Unknown planet ID: ' + planetId);\\n }, 1000);\\n });\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-css\",\"children\":\"label { display: block; margin-bottom: 10px; }\\n\"}]}]]}],[\"$r\",\"p\",null,{\"children\":[\"Check the \",[\"$r\",\"code\",null,{\"children\":\"useSelectOptions.js\"}],\" tab in the sandbox to see how it works. Ideally, most Effects in your application should eventually be replaced by custom Hooks, whether written by you or by the community. Custom Hooks hide the synchronization logic, so the calling component doesn’t know about the Effect. As you keep working on your app, you’ll develop a palette of Hooks to choose from, and eventually you won’t need to write Effects in your components very often.\"]}]]}]]}]]","meta":{"title":"Lifecycle of Reactive Effects"},"languages":null},"__N_SSG":true},"page":"/[[...markdownPath]]","query":{"markdownPath":["learn","lifecycle-of-reactive-effects"]},"buildId":"oavPBWYBzeYvBdXhaUWu7","isFallback":false,"gsp":true,"scriptLoader":[]}</script><next-route-announcer><p aria-live="assertive" id="__next-route-announcer__" role="alert" style="border: 0px; clip: rect(0px, 0px, 0px, 0px); height: 1px; margin: -1px; overflow: hidden; padding: 0px; position: absolute; top: 0px; width: 1px; white-space: nowrap; overflow-wrap: normal;"></p></next-route-announcer></body></html>