Javascript object bracket notation ({ Navigation } =) on left side of assign

I haven't seen this syntax before and am wondering what it's all about.

var { Navigation } = require('react-router');

The brackets on the left are throwing a syntax error:

unexpected token {

I'm not sure what part of the webpack config is transforming or what the purpose of the syntax is. Is it a Harmony thing? Can someone enlighten me?

Solution 1:

It's called destructuring assignment and it's part of the ES2015 standard.

The destructuring assignment syntax is a JavaScript expression that makes it possible to extract data from arrays or objects using a syntax that mirrors the construction of array and object literals.

Source: Destructuring assignment reference on MDN

Object destructuring

 var o = {p: 42, q: true};
 var {p, q} = o;

 console.log(p); // 42
 console.log(q); // true 

 // Assign new variable names
 var {p: foo, q: bar} = o;

 console.log(foo); // 42
 console.log(bar); // true

Array destructuring

var foo = ["one", "two", "three"];

// without destructuring
var one   = foo[0];
var two   = foo[1];
var three = foo[2];

// with destructuring
var [one, two, three] = foo;

Solution 2:

This is destructuring assignment. It's a new feature of ECMAScript 2015.

var {
  AppRegistry,
  StyleSheet,
  Text,
  View,
} = React;

Is the equivalent to:

var AppRegistry = React.AppRegistry;
var StyleSheet = React.StyleSheet;
var Text = React.Text;
var View = React.View;

Solution 3:

var { Navigation } = require('react-router');

... uses destructuring to achieve the same thing as ...

var Navigation = require('react-router').Navigation;

... but is far more readable.