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<!DOCTYPE html><html><head><meta charset="utf-8"><meta http-equiv="X-UA-Compatible" content="IE=edge,chrome=1"><title>JavaScript Environment – React Native | A framework for building native apps using React</title><meta name="viewport" content="width=device-width"><meta property="og:title" content="JavaScript Environment – React Native | A framework for building native apps using React"><meta property="og:type" content="website"><meta property="og:url" content="http://facebook.github.io/react-native/index.html"><meta property="og:image" content="http://facebook.github.io/react-native/img/opengraph.png?2"><meta property="og:description" content="A framework for building native apps using React"><link rel="shortcut icon" href="/react-native/img/favicon.png?2"><link rel="stylesheet" href="/react-native/css/react-native.css"><script type="text/javascript" src="//use.typekit.net/vqa1hcx.js"></script><script type="text/javascript">try{Typekit.load();}catch(e){}</script></head><body><div class="container"><div class="nav-main"><div class="wrap"><a class="nav-home" href="/react-native/"><img src="/react-native/img/header_logo.png">React Native</a><div class="nav-site-wrapper"><ul class="nav-site nav-site-internal"><li><a href="/react-native/docs/getting-started.html" class="active">Docs</a></li><li><a href="/react-native/support.html" class="">Support</a></li><li><a href="https://github.com/facebook/react-native/releases" class="">Releases</a></li><li><a href="http://reactnative.cc" class="">Newsletter</a></li><li><a href="/react-native/showcase.html" class="">Showcase</a></li></ul><ul class="nav-site nav-site-external"><li><a href="https://github.com/facebook/react-native" class="">GitHub</a></li><li><a href="http://facebook.github.io/react" class="">React</a></li></ul></div></div></div><section class="content wrap documentationContent"><div class="nav-docs"><div class="nav-docs-section"><h3>Quick Start</h3><ul><li><a style="margin-left:0;" class="" href="/react-native/docs/getting-started.html#content">Getting Started</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/android-setup.html#content">Android Setup</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/tutorial.html#content">Tutorial</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/videos.html#content">Videos</a></li></ul></div><div class="nav-docs-section"><h3>Guides</h3><ul><li><a style="margin-left:0;" class="" href="/react-native/docs/style.html#content">Style</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/gesture-responder-system.html#content">Gesture Responder System</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/animations.html#content">Animations</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/accessibility.html#content">Accessibility</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/direct-manipulation.html#content">Direct Manipulation</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/debugging.html#content">Debugging</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/testing.html#content">Testing</a></li><li><a style="margin-left:0;" class="active" href="/react-native/docs/javascript-environment.html#content">JavaScript Environment</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/navigator-comparison.html#content">Navigator Comparison</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/known-issues.html#content">Known Issues</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/performance.html#content">Performance</a></li></ul></div><div class="nav-docs-section"><h3>Guides (iOS)</h3><ul><li><a style="margin-left:0;" class="" href="/react-native/docs/native-modules-ios.html#content">Native Modules</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/native-components-ios.html#content">Native UI Components</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/linking-libraries-ios.html#content">Linking Libraries</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/running-on-device-ios.html#content">Running On Device</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/embedded-app-ios.html#content">Integrating with Existing Apps</a></li></ul></div><div class="nav-docs-section"><h3>Guides (Android)</h3><ul><li><a style="margin-left:0;" class="" href="/react-native/docs/native-modules-android.html#content">Native Modules</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/native-components-android.html#content">Native UI Components</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/running-on-device-android.html#content">Running On Device</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/embedded-app-android.html#content">Integrating with Existing Apps</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/signed-apk-android.html#content">Generating Signed APK</a></li></ul></div><div class="nav-docs-section"><h3>components</h3><ul><li><a style="margin-left:0;" class="" href="/react-native/docs/activityindicatorios.html#content">ActivityIndicatorIOS</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/datepickerios.html#content">DatePickerIOS</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/drawerlayoutandroid.html#content">DrawerLayoutAndroid</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/image.html#content">Image</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/listview.html#content">ListView</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/mapview.html#content">MapView</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/navigator.html#content">Navigator</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/modal.html#content">Modal</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/navigatorios.html#content">NavigatorIOS</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/pickerios.html#content">PickerIOS</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/progressbarandroid.html#content">ProgressBarAndroid</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/progressviewios.html#content">ProgressViewIOS</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/scrollview.html#content">ScrollView</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/segmentedcontrolios.html#content">SegmentedControlIOS</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/sliderios.html#content">SliderIOS</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/switchandroid.html#content">SwitchAndroid</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/switchios.html#content">SwitchIOS</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/tabbarios.html#content">TabBarIOS</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/tabbarios-item.html#content">TabBarIOS.Item</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/text.html#content">Text</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/textinput.html#content">TextInput</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/toolbarandroid.html#content">ToolbarAndroid</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/touchablehighlight.html#content">TouchableHighlight</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/touchablenativefeedback.html#content">TouchableNativeFeedback</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/touchableopacity.html#content">TouchableOpacity</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/touchablewithoutfeedback.html#content">TouchableWithoutFeedback</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/view.html#content">View</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/webview.html#content">WebView</a></li></ul></div><div class="nav-docs-section"><h3>apis</h3><ul><li><a style="margin-left:0;" class="" href="/react-native/docs/actionsheetios.html#content">ActionSheetIOS</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/alertios.html#content">AlertIOS</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/animated.html#content">Animated</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/appregistry.html#content">AppRegistry</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/appstateios.html#content">AppStateIOS</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/asyncstorage.html#content">AsyncStorage</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/backandroid.html#content">BackAndroid</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/cameraroll.html#content">CameraRoll</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/dimensions.html#content">Dimensions</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/interactionmanager.html#content">InteractionManager</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/layoutanimation.html#content">LayoutAnimation</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/linkingios.html#content">LinkingIOS</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/nativemethodsmixin.html#content">NativeMethodsMixin</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/netinfo.html#content">NetInfo</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/panresponder.html#content">PanResponder</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/pixelratio.html#content">PixelRatio</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/pushnotificationios.html#content">PushNotificationIOS</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/statusbarios.html#content">StatusBarIOS</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/stylesheet.html#content">StyleSheet</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/toastandroid.html#content">ToastAndroid</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/vibrationios.html#content">VibrationIOS</a></li></ul></div><div class="nav-docs-section"><h3>Polyfills</h3><ul><li><a style="margin-left:0;" class="" href="/react-native/docs/flexbox.html#content">Flexbox</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/geolocation.html#content">Geolocation</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/network.html#content">Network</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/timers.html#content">Timers</a></li></ul></div></div><div class="inner-content"><a id="content"></a><h1>JavaScript Environment</h1><div><h2><a class="anchor" name="javascript-runtime"></a>JavaScript Runtime <a class="hash-link" href="#javascript-runtime">#</a></h2><p>When using React Native, you're going to be running your JavaScript code in two environments:</p><ul><li>On iOS simulators and devices, Android emulators and devices React Native uses <a href="http://trac.webkit.org/wiki/JavaScriptCore" target="_blank">JavaScriptCore</a> which is the JavaScript engine that powers Safari. On iOS JSC doesn't use JIT due to the absence of writable executable memory in iOS apps.</li><li>When using Chrome debugging, it runs all the JavaScript code within Chrome itself and communicates with native code via WebSocket. So you are using <a href="https://code.google.com/p/v8/" target="_blank">V8</a>.</li></ul><p>While both environments are very similar, you may end up hitting some inconsistencies. We're likely going to experiment with other JS engines in the future, so it's best to avoid relying on specifics of any runtime.</p><h2><a class="anchor" name="javascript-syntax-transformers"></a>JavaScript Syntax Transformers <a class="hash-link" href="#javascript-syntax-transformers">#</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>As of version 0.5.0, React Native ships with the <a href="https://babeljs.io" target="_blank">Babel JavaScript compiler</a>. Check <a href="http://babeljs.io/docs/advanced/transformers/" target="_blank">Babel documentation</a> on its supported transformations for more details.</p><p>Here's a full list of React Native's <a href="https://github.com/facebook/react-native/blob/master/packager/transformer.js#L21" target="_blank">enabled transformations</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">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">Block scoping</a>: <code>let greeting = 'hi';</code></li><li><a href="http://babeljs.io/docs/learn-es2015/#default-rest-spread" target="_blank">Call spread</a>: <code>Math.max(...array);</code></li><li><a href="http://babeljs.io/docs/learn-es2015/#classes" target="_blank">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">Constants</a>: <code>const answer = 42;</code></li><li><a href="http://babeljs.io/docs/learn-es2015/#destructuring" target="_blank">Destructuring</a>: <code>var {isActive, style} = this.props;</code></li><li><a href="http://babeljs.io/docs/learn-es2015/#modules" target="_blank">Modules</a>: <code>import React, { Component } from 'react-native';</code></li><li><a href="http://babeljs.io/docs/learn-es2015/#enhanced-object-literals" target="_blank">Computed Properties</a>: <code>var key = 'abc'; var obj = {[key]: 10};</code></li><li>Object Consise Method: <code>var obj = { method() { return 10; } };</code></li><li><a href="http://babeljs.io/docs/learn-es2015/#enhanced-object-literals" target="_blank">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">Rest Params</a>: <code>function(type, ...args) { }</code></li><li><a href="http://babeljs.io/docs/learn-es2015/#template-strings" target="_blank">Template Literals</a>: <code>var who = 'world'; var str = `Hello ${who}`;</code></li></ul><p>ES7</p><ul><li><a href="https://github.com/sebmarkbage/ecmascript-rest-spread" target="_blank">Object Spread</a>: <code>var extended = { ...obj, a: 10 };</code></li><li><a href="https://github.com/jeffmo/es-trailing-function-commas" target="_blank">Function Trailing Comma</a>: <code>function f(a, b, c,) { }</code></li><li><a href="https://github.com/tc39/ecmascript-asyncawait" target="_blank">Async Functions</a>: <code>async function doStuffAsync() { const foo = await doOtherStuffAsync(); }</code>;</li></ul></div><div class="docs-prevnext"><a class="docs-next" href="navigator-comparison.html#content">Next →</a></div></div></section><footer class="wrap"><div class="right">© 2015 Facebook Inc.</div></footer></div><div id="fb-root"></div><script>
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