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?

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;

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;

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

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

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

... but is far more readable.


It's a new feature in ES6 to destructure objects.

As we all know that there is an assignment operation taking place here, Which means right side value is getting assigned to left side variable.

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

In this case require('react-router') method returns an object with key value pair some thing like

{ Navigation: function a(){}, Example1: function b(){}, Example2: function c(){} }.

And if we would like to take one key in that returned object say Navigation to a variable we can use Object destructing for that.

This will only be possible only if we have the key inhand.

So, after the assignment statement, local variable Navigation will contain function a(){}

Another example looks like this.

var { p, q } = { p: 1, q:2, r:3, s:4 };
console.log(p) //1;
console.log(q) //2;