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<script src="https://snack.expo.io/embed.js" defer="defer"></script><title data-react-helmet="true">Using Hermes · React Native</title><meta data-react-helmet="true" property="twitter:image" content="https://reactnative.dev/img/logo-og.png"><meta data-react-helmet="true" name="twitter:image:alt" content="Image for React Native"><meta data-react-helmet="true" name="docsearch:language" content="en"><meta data-react-helmet="true" name="docsearch:version" content="0.63"><meta data-react-helmet="true" name="docsearch:docusaurus_tag" content="docs-default-0.63"><meta data-react-helmet="true" property="og:image" content="https://reactnative.dev/img/logo-og.png"><meta data-react-helmet="true" name="twitter:card" content="summary"><meta data-react-helmet="true" name="twitter:image" content="https://reactnative.dev/img/logo-og.png"><meta data-react-helmet="true" property="og:title" content="Using Hermes · React Native"><meta data-react-helmet="true" name="description" content="Hermes is an open-source JavaScript engine optimized for running React Native apps on Android. For many apps, enabling Hermes will result in improved start-up time, decreased memory usage, and smaller app size. At this time Hermes is an opt-in React Native feature, and this guide explains how to enable it."><meta data-react-helmet="true" property="og:description" content="Hermes is an open-source JavaScript engine optimized for running React Native apps on Android. For many apps, enabling Hermes will result in improved start-up time, decreased memory usage, and smaller app size. At this time Hermes is an opt-in React Native feature, and this guide explains how to enable it."><meta data-react-helmet="true" property="og:url" content="https://reactnative.dev/docs/hermes"><link data-react-helmet="true" rel="shortcut icon" href="/img/favicon.ico"><link data-react-helmet="true" rel="preconnect" href="https://BH4D9OD16A-dsn.algolia.net" crossorigin="anonymous"><link data-react-helmet="true" rel="canonical" href="https://reactnative.dev/docs/hermes"><link rel="stylesheet" href="/styles.654d26b0.css">
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class="menu__link menu__link--sublist" href="#!">Performance</a><ul class="menu__list"><li class="menu__list-item"><a class="menu__link" tabindex="-1" href="/docs/performance">Performance Overview</a></li><li class="menu__list-item"><a class="menu__link" tabindex="-1" href="/docs/optimizing-flatlist-configuration">Optimizing Flatlist Configuration</a></li><li class="menu__list-item"><a class="menu__link" tabindex="-1" href="/docs/ram-bundles-inline-requires">RAM Bundles and Inline Requires</a></li><li class="menu__list-item"><a class="menu__link" tabindex="-1" href="/docs/profiling">Profiling</a></li></ul></li><li class="menu__list-item"><a class="menu__link menu__link--sublist menu__link--active" href="#!">JavaScript Runtime</a><ul class="menu__list"><li class="menu__list-item"><a class="menu__link" tabindex="0" href="/docs/javascript-environment">JavaScript Environment</a></li><li class="menu__list-item"><a class="menu__link" tabindex="0" href="/docs/timers">Timers</a></li><li class="menu__list-item"><a aria-current="page" class="menu__link menu__link--active active" tabindex="0" href="/docs/hermes">Using Hermes</a></li></ul></li><li class="menu__list-item menu__list-item--collapsed"><a class="menu__link menu__link--sublist" href="#!">Connectivity</a><ul class="menu__list"><li class="menu__list-item"><a class="menu__link" tabindex="-1" href="/docs/network">Networking</a></li><li class="menu__list-item"><a class="menu__link" tabindex="-1" href="/docs/security">Security</a></li></ul></li><li class="menu__list-item menu__list-item--collapsed"><a class="menu__link menu__link--sublist" href="#!">Native Modules</a><ul class="menu__list"><li class="menu__list-item"><a class="menu__link" tabindex="-1" href="/docs/native-modules-intro">Native Modules Intro</a></li><li class="menu__list-item"><a class="menu__link" tabindex="-1" href="/docs/native-modules-android">Android Native Modules</a></li><li class="menu__list-item"><a class="menu__link" tabindex="-1" href="/docs/native-modules-ios">iOS Native Modules</a></li><li class="menu__list-item"><a class="menu__link" tabindex="-1" href="/docs/native-modules-setup">Native Modules NPM Package Setup</a></li></ul></li><li class="menu__list-item menu__list-item--collapsed"><a class="menu__link menu__link--sublist" href="#!">Native Components</a><ul class="menu__list"><li class="menu__list-item"><a class="menu__link" tabindex="-1" href="/docs/native-components-android">Android Native UI Components</a></li><li class="menu__list-item"><a class="menu__link" tabindex="-1" href="/docs/native-components-ios">iOS Native UI Components</a></li><li class="menu__list-item"><a class="menu__link" tabindex="-1" href="/docs/direct-manipulation">Direct Manipulation</a></li></ul></li><li class="menu__list-item menu__list-item--collapsed"><a class="menu__link menu__link--sublist" href="#!">Guides (Android)</a><ul class="menu__list"><li class="menu__list-item"><a class="menu__link" tabindex="-1" href="/docs/headless-js-android">Headless JS</a></li><li class="menu__list-item"><a class="menu__link" tabindex="-1" href="/docs/signed-apk-android">Publishing to Google Play Store</a></li><li class="menu__list-item"><a class="menu__link" tabindex="-1" href="/docs/removing-default-permissions">Removing Default Permissions</a></li></ul></li><li class="menu__list-item menu__list-item--collapsed"><a class="menu__link menu__link--sublist" href="#!">Guides (iOS)</a><ul class="menu__list"><li class="menu__list-item"><a class="menu__link" tabindex="-1" href="/docs/linking-libraries-ios">Linking Libraries</a></li><li class="menu__list-item"><a class="menu__link" tabindex="-1" href="/docs/running-on-simulator-ios">Running On Simulator</a></li><li class="menu__list-item"><a class="menu__link" tabindex="-1" href="/docs/communication-ios">Communication between native and React Native</a></li><li class="menu__list-item"><a class="menu__link" tabindex="-1" href="/docs/app-extensions">App Extensions</a></li></ul></li></ul></div></div></div><main class="docMainContainer_1rYT"><div class="container padding-vert--lg docItemWrapper_1Hme"><div class="row"><div class="col docItemCol_2AGf"><div class="docItemContainer_1tAC"><article><header><h1 class="docTitle_cWlf">Using Hermes</h1></header><div class="markdown"><a href="https://hermesengine.dev" target="_blank" rel="noopener noreferrer"><img width="300" height="300" style="float:right;margin:-30px 4px 0" src="/docs/assets/HermesLogo.svg"></a><p><a href="https://hermesengine.dev" target="_blank" rel="noopener noreferrer">Hermes</a> is an open-source JavaScript engine optimized for running React Native apps on Android. For many apps, enabling Hermes will result in improved start-up time, decreased memory usage, and smaller app size. At this time Hermes is an <strong>opt-in</strong> React Native feature, and this guide explains how to enable it.</p><p>First, ensure you're using at least version 0.60.4 of React Native.</p><p>If you have an existing app based on an earlier version of React Native, you will have to upgrade it first. See <a href="/docs/upgrading">Upgrading to new React Native Versions</a> for how to do this. Make especially sure that all changes to <code>android/app/build.gradle</code> have been applied, as detailed by the <a href="https://react-native-community.github.io/upgrade-helper/?from=0.59.0" target="_blank" rel="noopener noreferrer">React Native upgrade helper</a>. After upgrading the app, make sure everything works before trying to switch to Hermes.</p><blockquote><h2><a aria-hidden="true" tabindex="-1" class="anchor enhancedAnchor_3pqN" id="note-for-rn-compatibility"></a>Note for RN compatibility.<a aria-hidden="true" tabindex="-1" class="hash-link" href="#note-for-rn-compatibility" title="Direct link to heading">#</a></h2><p>Each Hermes release is aimed at a specific RN version. The rule of thumb is to always follow <a href="https://github.com/facebook/hermes/releases" target="_blank" rel="noopener noreferrer">Hermes releases</a> strictly. Version mismatch can result in instant crash of your apps in the worst case scenario.</p></blockquote><blockquote><h2><a aria-hidden="true" tabindex="-1" class="anchor enhancedAnchor_3pqN" id="note-for-windows-users"></a>Note for Windows users.<a aria-hidden="true" tabindex="-1" class="hash-link" href="#note-for-windows-users" title="Direct link to heading">#</a></h2><p>Hermes requires <a href="https://www.microsoft.com/en-us/download/details.aspx?id=48145" target="_blank" rel="noopener noreferrer">Microsoft Visual C++ 2015 Redistributable</a></p></blockquote><p>Edit your <code>android/app/build.gradle</code> file and make the change illustrated below:</p><div class="mdxCodeBlock_xYIz"><div class="codeBlockContent_rlEb"><button tabindex="0" type="button" aria-label="Copy code to clipboard" class="copyButton_kOW4">Copy</button><div class="prism-code language-diff codeBlock_1AP7"><div class="codeBlockLines_1boI" style="color:#FFFFFF;background:#282C34"><div class="token-line" style="color:#FFFFFF;background:#282C34"><span class="token unchanged"> project.ext.react = [</span></div><div class="token-line" style="color:#FFFFFF;background:#282C34"><span class="token unchanged"> entryFile: "index.js",</span></div><div class="token-line" style="color:#FFFFFF;background:#282C34"><span class="token unchanged"></span><span class="token deleted-sign deleted" style="color:#cb4b16">- enableHermes: false // clean and rebuild if changing</span></div><div class="token-line" style="color:#FFFFFF;background:#282C34"><span class="token deleted-sign deleted" style="color:#cb4b16"></span><span class="token inserted-sign inserted" style="color:#859900">+ enableHermes: true // clean and rebuild if changing</span></div><div class="token-line" style="color:#FFFFFF;background:#282C34"><span class="token inserted-sign inserted" style="color:#859900"></span><span class="token unchanged"> ]</span></div></div></div></div></div><p>Also, if you're using ProGuard, you will need to add these rules in <code>proguard-rules.pro</code> :</p><div class="mdxCodeBlock_xYIz"><div class="codeBlockContent_rlEb"><button tabindex="0" type="button" aria-label="Copy code to clipboard" class="copyButton_kOW4">Copy</button><div class="prism-code language-jsx codeBlock_1AP7"><div class="codeBlockLines_1boI" style="color:#FFFFFF;background:#282C34"><div class="token-line" style="color:#FFFFFF;background:#282C34"><span class="token operator" style="color:#fc929e">-</span><span class="token plain">keep </span><span class="token keyword" style="color:#c5a5c5">class</span><span class="token plain"> </span><span class="token class-name" style="color:#fac863">com</span><span class="token class-name punctuation" style="color:#657b83">.</span><span class="token class-name" style="color:#fac863">facebook</span><span class="token class-name punctuation" style="color:#657b83">.</span><span class="token class-name" style="color:#fac863">hermes</span><span class="token class-name punctuation" style="color:#657b83">.</span><span class="token class-name" style="color:#fac863">unicode</span><span class="token class-name punctuation" style="color:#657b83">.</span><span class="token operator" style="color:#fc929e">**</span><span class="token plain"> </span><span class="token punctuation" style="color:#657b83">{</span><span class="token plain"> </span><span class="token operator" style="color:#fc929e">*</span><span class="token punctuation" style="color:#657b83">;</span><span class="token plain"> </span><span class="token punctuation" style="color:#657b83">}</span><span class="token plain"></span></div><div class="token-line" style="color:#FFFFFF;background:#282C34"><span class="token plain"></span><span class="token operator" style="color:#fc929e">-</span><span class="token plain">keep </span><span class="token keyword" style="color:#c5a5c5">class</span><span class="token plain"> </span><span class="token class-name" style="color:#fac863">com</span><span class="token class-name punctuation" style="color:#657b83">.</span><span class="token class-name" style="color:#fac863">facebook</span><span class="token class-name punctuation" style="color:#657b83">.</span><span class="token class-name" style="color:#fac863">jni</span><span class="token class-name punctuation" style="color:#657b83">.</span><span class="token operator" style="color:#fc929e">**</span><span class="token plain"> </span><span class="token punctuation" style="color:#657b83">{</span><span class="token plain"> </span><span class="token operator" style="color:#fc929e">*</span><span class="token punctuation" style="color:#657b83">;</span><span class="token plain"> </span><span class="token punctuation" style="color:#657b83">}</span></div></div></div></div></div><p>Next, if you've already built your app at least once, clean the build:</p><div class="mdxCodeBlock_xYIz"><div class="codeBlockContent_rlEb"><button tabindex="0" type="button" aria-label="Copy code to clipboard" class="copyButton_kOW4">Copy</button><div class="prism-code language-shell codeBlock_1AP7"><div class="codeBlockLines_1boI" style="color:#FFFFFF;background:#282C34"><div class="token-line" style="color:#FFFFFF;background:#282C34"><span class="token plain">$ </span><span class="token builtin class-name" style="color:#fac863">cd</span><span class="token plain"> android </span><span class="token operator" style="color:#fc929e">&&</span><span class="token plain"> ./gradlew clean</span></div></div></div></div></div><p>That's it! You should now be able to develop and deploy your app as normal:</p><div class="mdxCodeBlock_xYIz"><div class="codeBlockContent_rlEb"><button tabindex="0" type="button" aria-label="Copy code to clipboard" class="copyButton_kOW4">Copy</button><div class="prism-code language-shell codeBlock_1AP7"><div class="codeBlockLines_1boI" style="color:#FFFFFF;background:#282C34"><div class="token-line" style="color:#FFFFFF;background:#282C34"><span class="token plain">$ npx react-native run-android</span></div></div></div></div></div><blockquote><h2><a aria-hidden="true" tabindex="-1" class="anchor enhancedAnchor_3pqN" id="note-about-android-app-bundles"></a>Note about Android App Bundles<a aria-hidden="true" tabindex="-1" class="hash-link" href="#note-about-android-app-bundles" title="Direct link to heading">#</a></h2><p>Android app bundles are supported from react-native 0.62.0 and up.</p></blockquote><h2><a aria-hidden="true" tabindex="-1" class="anchor enhancedAnchor_3pqN" id="confirming-hermes-is-in-use"></a>Confirming Hermes is in use<a aria-hidden="true" tabindex="-1" class="hash-link" href="#confirming-hermes-is-in-use" title="Direct link to heading">#</a></h2><p>If you've recently created a new app from scratch, you should see if Hermes is enabled in the welcome view:</p><p><img alt="Where to find JS engine status in AwesomeProject" src="/assets/images/HermesApp-ae778d80caa321ba00b558b025dc9805.jpg"></p><p>A <code>HermesInternal</code> global variable will be available in JavaScript that can be used to verify that Hermes is in use:</p><div class="mdxCodeBlock_xYIz"><div class="codeBlockContent_rlEb"><button tabindex="0" type="button" aria-label="Copy code to clipboard" class="copyButton_kOW4">Copy</button><div class="prism-code language-jsx codeBlock_1AP7"><div class="codeBlockLines_1boI" style="color:#FFFFFF;background:#282C34"><div class="token-line" style="color:#FFFFFF;background:#282C34"><span class="token keyword" style="color:#c5a5c5">const</span><span class="token plain"> </span><span class="token function-variable function" style="color:#79b6f2">isHermes</span><span class="token plain"> </span><span class="token operator" style="color:#fc929e">=</span><span class="token plain"> </span><span class="token punctuation" style="color:#657b83">(</span><span class="token punctuation" style="color:#657b83">)</span><span class="token plain"> </span><span class="token operator" style="color:#fc929e">=></span><span class="token plain"> </span><span class="token operator" style="color:#fc929e">!</span><span class="token operator" style="color:#fc929e">!</span><span class="token plain">global</span><span class="token punctuation" style="color:#657b83">.</span><span class="token plain">HermesInternal</span><span class="token punctuation" style="color:#657b83">;</span></div></div></div></div></div><p>To see the benefits of Hermes, try making a release build/deployment of your app to compare. For example:</p><div class="mdxCodeBlock_xYIz"><div class="codeBlockContent_rlEb"><button tabindex="0" type="button" aria-label="Copy code to clipboard" class="copyButton_kOW4">Copy</button><div class="prism-code language-shell codeBlock_1AP7"><div class="codeBlockLines_1boI" style="color:#FFFFFF;background:#282C34"><div class="token-line" style="color:#FFFFFF;background:#282C34"><span class="token plain">$ npx react-native run-android --variant release</span></div></div></div></div></div><p>This will compile JavaScript to bytecode during build time which will improve your app's startup speed on device.</p><h2><a aria-hidden="true" tabindex="-1" class="anchor enhancedAnchor_3pqN" id="debugging-js-on-hermes-using-google-chromes-devtools"></a>Debugging JS on Hermes using Google Chrome's DevTools<a aria-hidden="true" tabindex="-1" class="hash-link" href="#debugging-js-on-hermes-using-google-chromes-devtools" title="Direct link to heading">#</a></h2><p>Hermes supports the Chrome debugger by implementing the Chrome inspector protocol. This means Chrome's tools can be used to directly debug JavaScript running on Hermes, on an emulator or on a real, physical, device.</p><blockquote><p>Note that this is very different with the "Remote JS Debugging" from the In-App Developer Menu documented in the <a href="/docs/debugging#debugging-using-a-custom-javascript-debugger">Debugging</a> section, which actually runs the JS code on Chrome's V8 on your development machine (laptop or desktop).</p></blockquote><p>Chrome connects to Hermes running on device via Metro, so you'll need to know where Metro is listening. Typically this will be on <code>localhost:8081</code>, but this is <a href="https://facebook.github.io/metro/docs/configuration" target="_blank" rel="noopener noreferrer">configurable</a>. When running <code>yarn start</code> the address is written to stdout on startup.</p><p>Once you know where the Metro server is listening, you can connect with Chrome using the following steps:</p><ol><li><p>Navigate to <code>chrome://inspect</code> in a Chrome browser instance.</p></li><li><p>Use the <code>Configure...</code> button to add the Metro server address (typically <code>localhost:8081</code> as described above).</p></li></ol><p><img alt="Configure button in Chrome DevTools devices page" src="/assets/images/HermesDebugChromeConfig-31cb28d5b642a616aa547edd3095253b.png"></p><p><img alt="Dialog for adding Chrome DevTools network targets" src="/assets/images/HermesDebugChromeMetroAddress-d21dc83b9eee0545a154301e1ce0be8b.png"></p><ol start="3"><li><p>You should now see a "Hermes React Native" target with an "inspect" link which can be used to bring up debugger. If you don't see the "inspect" link, make sure the Metro server is running. <img alt="Target inspect link" src="/assets/images/HermesDebugChromeInspect-8aa08afba4c7ce76a85d47d31200dd55.png"></p></li><li><p>You can now use the Chrome debug tools. For example, to breakpoint the next time some JavaScript is run, click on the pause button and trigger an action in your app which would cause JavaScript to execute. <img alt="Pause button in debug tools" src="/assets/images/HermesDebugChromePause-5bac724c8b705ba3e7dc9676dedd6c4f.png"></p></li></ol></div></article><div class="docMetadata margin-vert--xl"><div class="row"><div class="col"><a href="https://github.com/facebook/react-native-website/blob/master/website/versioned_docs/version-0.63/hermes.md" target="_blank" rel="noreferrer noopener"><svg fill="currentColor" height="1.2em" width="1.2em" preserveAspectRatio="xMidYMid meet" viewBox="0 0 40 40" style="margin-right:0.3em;vertical-align:sub"><g><path d="m34.5 11.7l-3 3.1-6.3-6.3 3.1-3q0.5-0.5 1.2-0.5t1.1 0.5l3.9 3.9q0.5 0.4 0.5 1.1t-0.5 1.2z m-29.5 17.1l18.4-18.5 6.3 6.3-18.4 18.4h-6.3v-6.2z"></path></g></svg>Edit this page</a></div><div class="col text--right"><em><small class="docMetadata-updated">Last updated on <time datetime="2020-12-04T17:12:36.000Z" class="docLastUpdatedAt_1gIo">12/4/2020</time></small></em></div></div></div><div class="margin-vert--lg"><nav class="pagination-nav" aria-label="Blog list page navigation"><div class="pagination-nav__item"><a class="pagination-nav__link" href="/docs/timers"><div class="pagination-nav__sublabel">Previous</div><div class="pagination-nav__label">« Timers</div></a></div><div class="pagination-nav__item pagination-nav__item--next"><a class="pagination-nav__link" href="/docs/network"><div class="pagination-nav__sublabel">Next</div><div class="pagination-nav__label">Networking »</div></a></div></nav></div></div></div><div class="col col--3"><div class="tableOfContents_3iuQ"><ul class="table-of-contents table-of-contents__left-border"><li><a href="#confirming-hermes-is-in-use" class="table-of-contents__link">Confirming Hermes is in use</a></li><li><a href="#debugging-js-on-hermes-using-google-chromes-devtools" class="table-of-contents__link">Debugging JS on Hermes using Google Chrome's DevTools</a></li></ul></div></div></div></div></main></div></div><footer class="footer footer--dark"><div class="container"><div class="row footer__links"><div class="col footer__col"><h4 class="footer__title">Docs</h4><ul class="footer__items"><li class="footer__item"><a class="footer__link-item" href="/docs/getting-started">Getting Started</a></li><li class="footer__item"><a class="footer__link-item" href="/docs/tutorial">Tutorial</a></li><li class="footer__item"><a class="footer__link-item" href="/docs/components-and-apis">Components and APIs</a></li><li class="footer__item"><a class="footer__link-item" href="/docs/more-resources">More Resources</a></li></ul></div><div class="col footer__col"><h4 class="footer__title">Community</h4><ul class="footer__items"><li class="footer__item"><a class="footer__link-item" href="/help">The React Native Community</a></li><li class="footer__item"><a class="footer__link-item" href="/showcase">Who's using React Native?</a></li><li class="footer__item"><a href="https://stackoverflow.com/questions/tagged/react-native" target="_blank" rel="noopener noreferrer" class="footer__link-item">Ask Questions on Stack Overflow</a></li><li class="footer__item"><a href="https://github.com/facebook/react-native/blob/master/CONTRIBUTING.md" target="_blank" rel="noopener noreferrer" class="footer__link-item">Contributor Guide</a></li><li class="footer__item"><a href="https://dev.to/t/reactnative" target="_blank" rel="noopener noreferrer" class="footer__link-item">DEV Community</a></li></ul></div><div class="col footer__col"><h4 class="footer__title">Find us</h4><ul class="footer__items"><li class="footer__item"><a class="footer__link-item" href="/blog">Blog</a></li><li class="footer__item"><a href="https://twitter.com/reactnative" target="_blank" rel="noopener noreferrer" class="footer__link-item">Twitter</a></li><li class="footer__item"><a href="https://github.com/facebook/react-native" target="_blank" rel="noopener noreferrer" class="footer__link-item">GitHub</a></li></ul></div><div class="col footer__col"><h4 class="footer__title">More</h4><ul class="footer__items"><li class="footer__item"><a href="https://reactjs.org/" target="_blank" rel="noopener noreferrer" class="footer__link-item">React</a></li><li class="footer__item"><a href="https://opensource.facebook.com/legal/privacy" target="_blank" rel="noopener noreferrer" class="footer__link-item">Privacy Policy</a></li><li class="footer__item"><a href="https://opensource.facebook.com/legal/terms" target="_blank" rel="noopener noreferrer" class="footer__link-item">Terms of Service</a></li></ul></div></div><div class="text--center"><div class="margin-bottom--sm"><a href="https://opensource.facebook.com" target="_blank" rel="noopener noreferrer" class="footerLogoLink_19Ac"><img class="footer__logo" alt="Facebook Open Source Logo" src="/img/oss_logo.png"></a></div><div>Copyright © 2021 Facebook, Inc.</div></div></div></footer></div>
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