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<!DOCTYPE html><html lang="en"><head><meta charSet="utf-8"/><meta http-equiv="X-UA-Compatible" content="IE=edge"/><title>JavaScript Environment · React Native</title><meta name="viewport" content="width=device-width"/><meta name="generator" content="Docusaurus"/><meta name="description" content="## JavaScript Runtime"/><meta name="docsearch:version" content="next"/><meta name="docsearch:language" content="en"/><meta property="og:title" content="JavaScript Environment · React Native"/><meta property="og:type" content="website"/><meta property="og:url" content="https://facebook.github.io/react-native/"/><meta property="og:description" content="## JavaScript Runtime"/><meta name="twitter:card" content="summary"/><link rel="shortcut icon" href="/react-native/img/favicon.ico"/><link rel="stylesheet" href="https://cdn.jsdelivr.net/docsearch.js/1/docsearch.min.css"/><link rel="stylesheet" href="//cdnjs.cloudflare.com/ajax/libs/highlight.js/9.12.0/styles/solarized-dark.min.css"/><link rel="alternate" type="application/atom+xml" href="https://facebook.github.io/react-native/blog/atom.xml" title="React Native Blog ATOM Feed"/><link rel="alternate" type="application/rss+xml" href="https://facebook.github.io/react-native/blog/feed.xml" title="React Native Blog RSS Feed"/><script>
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10.83l4.59 4.58L18 14l-6-6-6 6z"></path><path d="M0 0h24v24H0z" fill="none"></path></svg></span></h3><ul class="hide"><li class="navListItem"><a class="navItem" href="/react-native/docs/next/getting-started">Getting Started</a></li><li class="navListItem"><a class="navItem" href="/react-native/docs/next/tutorial">Learn the Basics</a></li><li class="navListItem"><a class="navItem" href="/react-native/docs/next/props">Props</a></li><li class="navListItem"><a class="navItem" href="/react-native/docs/next/state">State</a></li><li class="navListItem"><a class="navItem" href="/react-native/docs/next/style">Style</a></li><li class="navListItem"><a class="navItem" href="/react-native/docs/next/handling-text-input">Handling Text Input</a></li><li class="navListItem"><a class="navItem" href="/react-native/docs/next/using-a-scrollview">Using a ScrollView</a></li><li class="navListItem"><a class="navItem" href="/react-native/docs/next/using-a-listview">Using List Views</a></li><li 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href="/react-native/docs/next/out-of-tree-platforms">Out-of-Tree Platforms</a></li></ul></div><div class="navGroup"><h3 class="navGroupCategoryTitle collapsible">Workflow<span class="arrow"><svg width="24" height="24" viewBox="0 0 24 24"><path fill="#565656" d="M7.41 15.41L12 10.83l4.59 4.58L18 14l-6-6-6 6z"></path><path d="M0 0h24v24H0z" fill="none"></path></svg></span></h3><ul class="hide"><li class="navListItem"><a class="navItem" href="/react-native/docs/next/running-on-device">Running On Device</a></li><li class="navListItem"><a class="navItem" href="/react-native/docs/next/fast-refresh">Fast Refresh</a></li><li class="navListItem"><a class="navItem" href="/react-native/docs/next/debugging">Debugging</a></li><li class="navListItem"><a class="navItem" href="/react-native/docs/next/typescript">Using TypeScript with React Native</a></li><li class="navListItem"><a class="navItem" href="/react-native/docs/next/upgrading">Upgrading to new React Native versions</a></li></ul></div><div class="navGroup"><h3 class="navGroupCategoryTitle collapsible">Design<span class="arrow"><svg width="24" height="24" viewBox="0 0 24 24"><path fill="#565656" d="M7.41 15.41L12 10.83l4.59 4.58L18 14l-6-6-6 6z"></path><path d="M0 0h24v24H0z" fill="none"></path></svg></span></h3><ul class="hide"><li class="navListItem"><a class="navItem" href="/react-native/docs/next/colors">Color Reference</a></li><li class="navListItem"><a class="navItem" href="/react-native/docs/next/images">Images</a></li><li class="navListItem"><a class="navItem" href="/react-native/docs/next/stylesheet">StyleSheet</a></li><li class="navListItem"><a class="navItem" href="/react-native/docs/next/flexbox">Layout with Flexbox</a></li><li class="navListItem"><a class="navItem" href="/react-native/docs/next/height-and-width">Height and Width</a></li></ul></div><div class="navGroup"><h3 class="navGroupCategoryTitle collapsible">Interaction<span class="arrow"><svg width="24" height="24" viewBox="0 0 24 24"><path 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href="/react-native/docs/next/accessibility">Accessibility</a></li></ul></div><div class="navGroup"><h3 class="navGroupCategoryTitle collapsible">Performance<span class="arrow"><svg width="24" height="24" viewBox="0 0 24 24"><path fill="#565656" d="M7.41 15.41L12 10.83l4.59 4.58L18 14l-6-6-6 6z"></path><path d="M0 0h24v24H0z" fill="none"></path></svg></span></h3><ul class="hide"><li class="navListItem"><a class="navItem" href="/react-native/docs/next/performance">Performance</a></li><li class="navListItem"><a class="navItem" href="/react-native/docs/next/optimizing-flatlist-configuration">Optimizing Flatlist Configuration</a></li></ul></div><div class="navGroup"><h3 class="navGroupCategoryTitle collapsible">JavaScript Runtime<span class="arrow"><svg width="24" height="24" viewBox="0 0 24 24"><path fill="#565656" d="M7.41 15.41L12 10.83l4.59 4.58L18 14l-6-6-6 6z"></path><path d="M0 0h24v24H0z" fill="none"></path></svg></span></h3><ul class="hide"><li class="navListItem navListItemActive"><a class="navItem" href="/react-native/docs/next/javascript-environment">JavaScript Environment</a></li><li class="navListItem"><a class="navItem" href="/react-native/docs/next/timers">Timers</a></li><li class="navListItem"><a class="navItem" href="/react-native/docs/next/hermes">Using Hermes</a></li></ul></div><div class="navGroup"><h3 class="navGroupCategoryTitle collapsible">Connectivity<span class="arrow"><svg width="24" height="24" viewBox="0 0 24 24"><path fill="#565656" d="M7.41 15.41L12 10.83l4.59 4.58L18 14l-6-6-6 6z"></path><path d="M0 0h24v24H0z" fill="none"></path></svg></span></h3><ul class="hide"><li class="navListItem"><a class="navItem" href="/react-native/docs/next/network">Networking</a></li></ul></div><div class="navGroup"><h3 class="navGroupCategoryTitle collapsible">Native Components and Modules<span class="arrow"><svg width="24" height="24" viewBox="0 0 24 24"><path fill="#565656" d="M7.41 15.41L12 10.83l4.59 4.58L18 14l-6-6-6 6z"></path><path 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class="navListItem"><a class="navItem" href="/react-native/docs/next/running-on-simulator-ios">Running On Simulator</a></li><li class="navListItem"><a class="navItem" href="/react-native/docs/next/communication-ios">Communication between native and React Native</a></li><li class="navListItem"><a class="navItem" href="/react-native/docs/next/app-extensions">App Extensions</a></li></ul></div><div class="navGroup"><h3 class="navGroupCategoryTitle collapsible">Guides (Android)<span class="arrow"><svg width="24" height="24" viewBox="0 0 24 24"><path fill="#565656" d="M7.41 15.41L12 10.83l4.59 4.58L18 14l-6-6-6 6z"></path><path d="M0 0h24v24H0z" fill="none"></path></svg></span></h3><ul class="hide"><li class="navListItem"><a class="navItem" href="/react-native/docs/next/native-modules-android">Native Modules</a></li><li class="navListItem"><a class="navItem" href="/react-native/docs/next/native-components-android">Native UI Components</a></li><li class="navListItem"><a class="navItem" href="/react-native/docs/next/headless-js-android">Headless JS</a></li><li class="navListItem"><a class="navItem" href="/react-native/docs/next/signed-apk-android">Publishing to Google Play Store</a></li><li class="navListItem"><a class="navItem" href="/react-native/docs/next/removing-default-permissions">Removing Default Permissions</a></li></ul></div></div></section></div><script>
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</script></nav></div><div class="container mainContainer docsContainer"><div class="wrapper"><div class="post"><header class="postHeader"><a class="edit-page-link button" href="https://github.com/facebook/react-native-website/blob/master/docs/javascript-environment.md" target="_blank" rel="noreferrer noopener">Edit</a><h1 id="__docusaurus" class="postHeaderTitle">JavaScript Environment</h1></header><article><div><span><h2><a class="anchor" aria-hidden="true" id="javascript-runtime"></a><a href="#javascript-runtime" aria-hidden="true" class="hash-link"><svg class="hash-link-icon" aria-hidden="true" height="16" version="1.1" viewBox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"></path></svg></a>JavaScript Runtime</h2>
<p>When using React Native, you're going to be running your JavaScript code in two environments:</p>
<ul>
<li>In most cases, React Native will use <a href="http://trac.webkit.org/wiki/JavaScriptCore">JavaScriptCore</a>, the JavaScript engine that powers Safari. Note that on iOS, JavaScriptCore does not use JIT due to the absence of writable executable memory in iOS apps.</li>
<li>When using Chrome debugging, all JavaScript code runs within Chrome itself, communicating with native code via WebSockets. Chrome uses <a href="https://v8.dev/">V8</a> as its JavaScript engine.</li>
</ul>
<p>While both environments are very similar, you may end up hitting some inconsistencies. We're likely going to experiment with other JavaScript engines in the future, so it's best to avoid relying on specifics of any runtime.</p>
<h2><a class="anchor" aria-hidden="true" id="javascript-syntax-transformers"></a><a href="#javascript-syntax-transformers" aria-hidden="true" class="hash-link"><svg class="hash-link-icon" aria-hidden="true" height="16" version="1.1" viewBox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"></path></svg></a>JavaScript Syntax Transformers</h2>
<p>Syntax transformers make writing code more enjoyable by allowing you to use new JavaScript syntax without having to wait for support on all interpreters.</p>
<p>React Native ships with the <a href="https://babeljs.io">Babel JavaScript compiler</a>. Check <a href="https://babeljs.io/docs/plugins/#transform-plugins">Babel documentation</a> on its supported transformations for more details.</p>
<p>A full list of React Native's enabled transformations can be found in <a href="https://github.com/facebook/metro/tree/master/packages/metro-react-native-babel-preset">metro-react-native-babel-preset</a>.</p>
<p>ES5</p>
<ul>
<li>Reserved Words: <code>promise.catch(function() { });</code></li>
</ul>
<p>ES6</p>
<ul>
<li><a href="http://babeljs.io/docs/learn-es2015/#arrows">Arrow functions</a>: <code>&lt;C onPress={() =&gt; this.setState({pressed: true})} /&gt;</code></li>
<li><a href="https://babeljs.io/docs/learn-es2015/#let-const">Block scoping</a>: <code>let greeting = 'hi';</code></li>
<li><a href="http://babeljs.io/docs/learn-es2015/#default-rest-spread">Call spread</a>: <code>Math.max(...array);</code></li>
<li><a href="http://babeljs.io/docs/learn-es2015/#classes">Classes</a>: <code>class C extends React.Component { render() { return &lt;View /&gt;; } }</code></li>
<li><a href="https://babeljs.io/docs/learn-es2015/#let-const">Constants</a>: <code>const answer = 42;</code></li>
<li><a href="http://babeljs.io/docs/learn-es2015/#destructuring">Destructuring</a>: <code>var {isActive, style} = this.props;</code></li>
<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/for...of">for...of</a>: <code>for (var num of [1, 2, 3]) {};</code></li>
<li><a href="http://babeljs.io/docs/learn-es2015/#modules">Modules</a>: <code>import React, { Component } from 'react';</code></li>
<li><a href="http://babeljs.io/docs/learn-es2015/#enhanced-object-literals">Computed Properties</a>: <code>var key = 'abc'; var obj = {[key]: 10};</code></li>
<li><a href="http://babeljs.io/docs/learn-es2015/#enhanced-object-literals">Object Concise Method</a>: <code>var obj = { method() { return 10; } };</code></li>
<li><a href="http://babeljs.io/docs/learn-es2015/#enhanced-object-literals">Object Short Notation</a>: <code>var name = 'vjeux'; var obj = { name };</code></li>
<li><a href="https://github.com/sebmarkbage/ecmascript-rest-spread">Rest Params</a>: <code>function(type, ...args) {};</code></li>
<li><a href="http://babeljs.io/docs/learn-es2015/#template-strings">Template Literals</a>: <code>var who = 'world'; var str = `Hello ${who}`;</code></li>
</ul>
<p>ES8</p>
<ul>
<li><a href="https://github.com/jeffmo/es-trailing-function-commas">Function Trailing Comma</a>: <code>function f(a, b, c,) {};</code></li>
<li><a href="https://github.com/tc39/ecmascript-asyncawait">Async Functions</a>: <code>async function doStuffAsync() { const foo = await doOtherStuffAsync(); };</code></li>
</ul>
<p>Stage 3</p>
<ul>
<li><a href="https://github.com/tc39/proposal-object-rest-spread">Object Spread</a>: <code>var extended = { ...obj, a: 10 };</code></li>
<li><a href="https://github.com/tc39/proposal-static-class-features">Static class fields</a>: <code>class CustomDate { static epoch = new CustomDate(0); }</code></li>
<li><a href="https://github.com/tc39/proposal-optional-chaining">Optional Chaining</a>: <code>var name = obj.user?.name;</code></li>
</ul>
<p>Specific</p>
<ul>
<li><a href="https://reactjs.org/docs/jsx-in-depth.html">JSX</a>: <code>&lt;View style={{color: 'red'}} /&gt;</code></li>
<li><a href="https://flowtype.org/">Flow</a>: <code>function foo(x: ?number): string {};</code></li>
<li><a href="https://flowtype.org/">TypeScript</a>: <code>function foo(x: number | undefined): string {};</code></li>
<li><a href="https://babeljs.io/docs/en/babel-template">Babel Template</a>: allows AST templating</li>
</ul>
<h2><a class="anchor" aria-hidden="true" id="polyfills"></a><a href="#polyfills" aria-hidden="true" class="hash-link"><svg class="hash-link-icon" aria-hidden="true" height="16" version="1.1" viewBox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"></path></svg></a>Polyfills</h2>
<p>Many standards functions are also available on all the supported JavaScript runtimes.</p>
<p>Browser</p>
<ul>
<li><a href="https://developer.chrome.com/devtools/docs/console-api">console.{log, warn, error, info, trace, table, group, groupEnd}</a></li>
<li><a href="https://nodejs.org/docs/latest/api/modules.html">CommonJS require</a></li>
<li><a href="/react-native/docs/next/network#content">XMLHttpRequest, fetch</a></li>
<li><a href="/react-native/docs/next/timers#content">{set, clear}{Timeout, Interval, Immediate}, {request, cancel}AnimationFrame</a></li>
</ul>
<p>ES6</p>
<ul>
<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/assign">Object.assign</a></li>
<li>String.prototype.{<a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/startsWith">startsWith</a>, <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/endsWith">endsWith</a>, <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/repeat">repeat</a>, <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/includes">includes</a>}</li>
<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/from">Array.from</a></li>
<li>Array.prototype.{<a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/find">find</a>, <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/findIndex">findIndex</a>}</li>
</ul>
<p>ES7</p>
<ul>
<li>Array.prototype.{<a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/includes">includes</a>}</li>
</ul>
<p>ES8</p>
<ul>
<li>Object.{<a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/entries">entries</a>, <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/values">values</a>}</li>
</ul>
<p>Specific</p>
<ul>
<li><code>__DEV__</code></li>
</ul>
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