JavaScript Environment

JavaScript Runtime

When using React Native, you're going to be running your JavaScript code in up to three environments:

• In most cases, React Native will use Hermes, an open-source JavaScript engine optimized for React Native.
• If Hermes is disabled, React Native will use JavaScriptCore, 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.
• When using Chrome debugging, all JavaScript code runs within Chrome itself, communicating with native code via WebSockets. Chrome uses V8 as its JavaScript engine.

While these environments are very similar, you may end up hitting some inconsistencies. It is best to avoid relying on specifics of any runtime.

JavaScript Syntax Transformers

Syntax transformers make writing code more enjoyable by allowing you to use new JavaScript syntax without having to wait for support on all interpreters.

React Native ships with the Babel JavaScript compiler. Check Babel documentation on its supported transformations for more details.

A full list of React Native's enabled transformations can be found in metro-react-native-babel-preset.

Transformation	Code
ECMAScript 5
Reserved Words	promise.catch(function() {...});
ECMAScript 2015 (ES6)
Arrow functions	<C onPress={() => this.setState({pressed: true})} />
Block scoping	let greeting = 'hi';
Call spread	Math.max(...array);
Classes	class C extends React.Component {render() { return <View />; }}
Computed Properties	const key = 'abc'; const obj = {[key]: 10};
Constants	const answer = 42;
Destructuring	const {isActive, style} = this.props;
for…of	for (var num of [1, 2, 3]) {...};
Function Name	let number = x => x;
Literals	const b = 0b11; const o = 0o7; const u = 'Hello\u{000A}\u{0009}!';
Modules	import React, {Component} from 'react';
Object Concise Method	const obj = {method() { return 10; }};
Object Short Notation	const name = 'vjeux'; const obj = {name};
Parameters	function test(x = 'hello', {a, b}, ...args) {}
Rest Params	function(type, ...args) {};
Shorthand Properties	const o = {a, b, c};
Sticky Regex	const a = /o+/y;
Template Literals	const who = 'world'; const str = `Hello ${who}`;
Unicode Regex	const string = 'foo💩bar'; const match = string.match(/foo(.)bar/u);
ECMAScript 2016 (ES7)
Exponentiation Operator	let x = 10 ** 2;
ECMAScript 2017 (ES8)
Async Functions	async function doStuffAsync() {const foo = await doOtherStuffAsync();};
Function Trailing Comma	function f(a, b, c,) {};
ECMAScript 2018 (ES9)
Object Spread	const extended = {...obj, a: 10};
ECMAScript 2019 (ES10)
Optional Catch Binding	try {throw 0; } catch { doSomethingWhichDoesNotCareAboutTheValueThrown();}
ECMAScript 2020 (ES11)
Dynamic Imports	const package = await import('package'); package.function()
Nullish Coalescing Operator	const foo = object.foo ?? 'default';
Optional Chaining	const name = obj.user?.name;
ECMAScript 2022 (ES13)
Class Fields	class Bork {static a = 'foo'; static b; x = 'bar'; y;}
Stage 1 Proposal
Export Default From	export v from 'mod';
Miscellaneous
Babel Template	template(`const %%importName%% = require(%%source%%);`);
Flow	function foo(x: ?number): string {};
ESM to CJS	export default 42;
JSX	<View style={{color: 'red'}} />
Object Assign	Object.assign(a, b);
React Display Name	const bar = createReactClass({});
TypeScript	function foo(x: {hello: true, target: 'react native!'}): string {};

Polyfills

Many standard functions are also available on all the supported JavaScript runtimes.

Browser

• CommonJS require
• console.{log, warn, error, info, trace, table, group, groupEnd}
• XMLHttpRequest, fetch
• {set, clear}{Timeout, Interval, Immediate}, {request, cancel}AnimationFrame

ECMAScript 2015 (ES6)

• Array.from
• Array.prototype.{find, findIndex}
• Object.assign
• String.prototype.{startsWith, endsWith, repeat, includes}

ECMAScript 2016 (ES7)

• Array.prototype.includes

ECMAScript 2017 (ES8)

• Object.{entries, values}

Specific

• __DEV__