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react-native/docs/javascript-environment.html
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<!DOCTYPE html><html><head><meta charset="utf-8"><meta http-equiv="X-UA-Compatible" content="IE=edge,chrome=1"><title>React Native | A framework for building native apps using React</title><meta name="viewport" content="width=device-width"><meta property="og:title" content="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></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/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/nativemodulesios.html#content">Native Modules (iOS)</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/nativecomponentsios.html#content">Native UI Components (iOS)</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/linking-libraries.html#content">Linking Libraries</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="" href="/react-native/docs/runningondevice.html#content">Running On Device</a></li><li><a style="margin-left:0;" class="" href="/react-native/docs/embedded-app.html#content">Integration with Existing App</a></li><li><a style="margin-left:0;" class="active" href="/react-native/docs/javascript-environment.html#content">JavaScript Environment</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/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/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/scrollview.html#content">ScrollView</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/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/touchablehighlight.html#content">TouchableHighlight</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/alertios.html#content">AlertIOS</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/cameraroll.html#content">CameraRoll</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/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/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&#x27;re going to be running your JavaScript code in two environments:</p><ul><li>In the simulator and on the phone: <a href="http://trac.webkit.org/wiki/JavaScriptCore" target="_blank">JavaScriptCore</a> which is the JavaScript engine that powers Safari and web views. Due to the absence of writable executable memory in iOS apps, it doesn&#x27;t run with JIT.</li><li>When using Chrome debugging, it runs all the JavaScript code within Chrome itself and communicates with Objective-C 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&#x27;re likely going to experiment with other JS engines in the future, so it&#x27;s best to avoid relying on specifics of any runtime.</p><h2><a class="anchor" name="javascript-transforms"></a>JavaScript Transforms <a class="hash-link" href="#javascript-transforms">#</a></h2><p>React Native ships with many JavaScript transforms to make writing code more enjoyable. If you are curious, you can see the <a href="https://github.com/facebook/jstransform/tree/master/visitors" target="_blank">implementation of all those transformations</a>. Here&#x27;s the full list:</p><p>ES5</p><ul><li>Reserved Words: <code>promise.catch(function() { });</code></li></ul><p>ES6</p><ul><li>Arrow function: <code>&lt;C onPress={() =&gt; this.setState({pressed: true})}</code></li><li>Call spread: <code>Math.max(...array);</code></li><li>Class: <code>class C extends React.Component { render() { return &lt;View /&gt;; } }</code></li><li>Destructuring: <code>var {isActive, style} = this.props;</code></li><li>Iteration: <code>for (var element of array) { }</code></li><li>Computed Properties: <code>var key = &#x27;abc&#x27;; var obj = {[key]: 10};</code></li><li>Object Consise Method: <code>var obj = { method() { return 10; } };</code></li><li>Object Short Notation: <code>var name = &#x27;vjeux&#x27;; var obj = { name };</code></li><li>Rest Params: <code>function(type, ...args) { }</code></li><li>Template: <code>var who = &#x27;world&#x27;; var str = `Hello ${who}`;</code></li></ul><p>ES7</p><ul><li>Object Spread: <code>var extended = { ...obj, a: 10 };</code></li><li>Function Trailing Comma: <code>function f(a, b, c,) { }</code></li></ul></div><div class="docs-prevnext"><a class="docs-next" href="activityindicatorios.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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