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45 lines
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HTML
45 lines
32 KiB
HTML
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<script src="https://snack.expo.io/embed.js" defer="defer"></script><title data-react-helmet="true">JavaScript Environment · 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="JavaScript Environment · React Native"><meta data-react-helmet="true" name="description" content="JavaScript Runtime"><meta data-react-helmet="true" property="og:description" content="JavaScript Runtime"><meta data-react-helmet="true" property="og:url" content="https://reactnative.dev/docs/javascript-environment"><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/javascript-environment"><link rel="stylesheet" href="/styles.fccefeba.css">
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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">JavaScript Environment</h1></header><div class="markdown"><h2><a aria-hidden="true" tabindex="-1" class="anchor enhancedAnchor_3pqN" id="javascript-runtime"></a>JavaScript Runtime<a aria-hidden="true" tabindex="-1" class="hash-link" href="#javascript-runtime" title="Direct link to heading">#</a></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" target="_blank" rel="noopener noreferrer">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/" target="_blank" rel="noopener noreferrer">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 aria-hidden="true" tabindex="-1" class="anchor enhancedAnchor_3pqN" id="javascript-syntax-transformers"></a>JavaScript Syntax Transformers<a aria-hidden="true" tabindex="-1" class="hash-link" href="#javascript-syntax-transformers" title="Direct link to heading">#</a></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" target="_blank" rel="noopener noreferrer">Babel JavaScript compiler</a>. Check <a href="https://babeljs.io/docs/plugins/#transform-plugins" target="_blank" rel="noopener noreferrer">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" target="_blank" rel="noopener noreferrer">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" target="_blank" rel="noopener noreferrer">Arrow functions</a>: <code><C onPress={() => this.setState({pressed: true})} /></code></li><li><a href="https://babeljs.io/docs/learn-es2015/#let-const" target="_blank" rel="noopener noreferrer">Block scoping</a>: <code>let greeting = 'hi';</code></li><li><a href="http://babeljs.io/docs/learn-es2015/#default-rest-spread" target="_blank" rel="noopener noreferrer">Call spread</a>: <code>Math.max(...array);</code></li><li><a href="http://babeljs.io/docs/learn-es2015/#classes" target="_blank" rel="noopener noreferrer">Classes</a>: <code>class C extends React.Component { render() { return <View />; } }</code></li><li><a href="https://babeljs.io/docs/learn-es2015/#let-const" target="_blank" rel="noopener noreferrer">Constants</a>: <code>const answer = 42;</code></li><li><a href="http://babeljs.io/docs/learn-es2015/#destructuring" target="_blank" rel="noopener noreferrer">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" target="_blank" rel="noopener noreferrer">for...of</a>: <code>for (var num of [1, 2, 3]) {};</code></li><li><a href="http://babeljs.io/docs/learn-es2015/#modules" target="_blank" rel="noopener noreferrer">Modules</a>: <code>import React, { Component } from 'react';</code></li><li><a href="http://babeljs.io/docs/learn-es2015/#enhanced-object-literals" target="_blank" rel="noopener noreferrer">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" target="_blank" rel="noopener noreferrer">Object Concise Method</a>: <code>var obj = { method() { return 10; } };</code></li><li><a href="http://babeljs.io/docs/learn-es2015/#enhanced-object-literals" target="_blank" rel="noopener noreferrer">Object Short Notation</a>: <code>var name = 'vjeux'; var obj = { name };</code></li><li><a href="https://github.com/sebmarkbage/ecmascript-rest-spread" target="_blank" rel="noopener noreferrer">Rest Params</a>: <code>function(type, ...args) {};</code></li><li><a href="http://babeljs.io/docs/learn-es2015/#template-strings" target="_blank" rel="noopener noreferrer">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" target="_blank" rel="noopener noreferrer">Function Trailing Comma</a>: <code>function f(a, b, c,) {};</code></li><li><a href="https://github.com/tc39/ecmascript-asyncawait" target="_blank" rel="noopener noreferrer">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" target="_blank" rel="noopener noreferrer">Object Spread</a>: <code>var extended = { ...obj, a: 10 };</code></li><li><a href="https://github.com/tc39/proposal-static-class-features" target="_blank" rel="noopener noreferrer">Static class fields</a>: <code>class CustomDate { static epoch = new CustomDate(0); }</code></li><li><a href="https://github.com/tc39/proposal-optional-chaining" target="_blank" rel="noopener noreferrer">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" target="_blank" rel="noopener noreferrer">JSX</a>: <code><View style={{color: 'red'}} /></code></li><li><a href="https://flowtype.org/" target="_blank" rel="noopener noreferrer">Flow</a>: <code>function foo(x: ?number): string {};</code></li><li><a href="https://www.typescriptlang.org/" target="_blank" rel="noopener noreferrer">TypeScript</a>: <code>function foo(x: number | undefined): string {};</code></li><li><a href="https://babeljs.io/docs/en/babel-template" target="_blank" rel="noopener noreferrer">Babel Template</a>: allows AST templating</li></ul><h2><a aria-hidden="true" tabindex="-1" class="anchor enhancedAnchor_3pqN" id="polyfills"></a>Polyfills<a aria-hidden="true" tabindex="-1" class="hash-link" href="#polyfills" title="Direct link to heading">#</a></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" target="_blank" rel="noopener noreferrer">console.{log, warn, error, info, trace, table, group, groupEnd}</a></li><li><a href="https://nodejs.org/docs/latest/api/modules.html" target="_blank" rel="noopener noreferrer">CommonJS require</a></li><li><a href="/docs/network#content">XMLHttpRequest, fetch</a></li><li><a href="/docs/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" target="_blank" rel="noopener noreferrer">Object.assign</a></li><li>String.prototype.{<a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/startsWith" target="_blank" rel="noopener noreferrer">startsWith</a>, <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/endsWith" target="_blank" rel="noopener noreferrer">endsWith</a>, <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/repeat" target="_blank" rel="noopener noreferrer">repeat</a>, <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/includes" target="_blank" rel="noopener noreferrer">includes</a>}</li><li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/from" target="_blank" rel="noopener noreferrer">Array.from</a></li><li>Array.prototype.{<a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/find" target="_blank" rel="noopener noreferrer">find</a>, <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/findIndex" target="_blank" rel="noopener noreferrer">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" target="_blank" rel="noopener noreferrer">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" target="_blank" rel="noopener noreferrer">entries</a>, <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/values" target="_blank" rel="noopener noreferrer">values</a>}</li></ul><p>Specific</p><ul><li><code>__DEV__</code></li></ul></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/javascript-environment.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="2020-10-29T04:19:29.000Z" class="docLastUpdatedAt_1gIo">10/29/2020</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/profiling"><div class="pagination-nav__sublabel">Previous</div><div class="pagination-nav__label">« Profiling</div></a></div><div class="pagination-nav__item pagination-nav__item--next"><a class="pagination-nav__link" href="/docs/timers"><div class="pagination-nav__sublabel">Next</div><div class="pagination-nav__label">Timers »</div></a></div></nav></div></div></div><div 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