Appendix B: ES6

This appendix is a non-exhaustive list of new syntactic features and methods that were added to JavaScript in ES6. These features are the most commonly used and most helpful.

While this appendix doesn't cover ES6 classes, we go over the basics while learning about components in the book. In addition, this appendix doesn't include descriptions of some larger new features like promises and generators. If you'd like more info on those or on any topic below, we encourage you to reference the Mozilla Developer Network's website (MDN).

Prefer const and let over var

If you've worked with ES5 JavaScript before, you're likely used to seeing variables declared with var:

ar myVariable = 5;

Both the const and let statements also declare variables. They were introduced in ES6.

Use const in cases where a variable is never re-assigned. Using const makes this clear to whoever is reading your code. It refers to the "constant" state of the variable in the context it is defined within.

If the variable will be re-assigned, use let.

We encourage the use of const and let instead of var. In addition to the restriction introduced by const, both const and let are block scoped as opposed to function scoped. This scoping can help avoid unexpected bugs.

Arrow functions

There are three ways to write arrow function bodies. For the examples below, let's say we have an array of city objects:

onst cities = [
  { name: 'Cairo', pop: 7764700 },
  { name: 'Lagos', pop: 8029200 },

If we write an arrow function that spans multiple lines, we must use braces to delimit the function body like this:

const formattedPopulations = => {
  const popMM = (city.pop / 1000000).toFixed(2);
  return popMM + ' million';
// -> [ "7.76 million", "8.03 million" ]

Note that we must also explicitly specify a return for the function.

However, if we write a function body that is only a single line (or single expression) we can use parentheses to delimit it:

const formattedPopulations2 = => (
  (city.pop / 1000000).toFixed(2) + ' million'

Notably, we don't use return as it's implied.

Furthermore, if your function body is terse you can write it like so:

const pops = => city.pop);
// [ 7764700, 8029200 ]

The terseness of arrow functions is one of two reasons that we use them. Compare the one-liner above to this:

const popsNoArrow = { return city.pop });

Of greater benefit, though, is how arrow functions bind the this object.

The traditional JavaScript function declaration syntax (function () {}) will bind this in anonymous functions to the global object. To illustrate the confusion this causes, consider the following example:

unction printSong() {
  console.log("Oops - The Global Object");

const jukebox = {
  songs: [
      title: "Wanna Be Startin' Somethin'",
      artist: "Michael Jackson",
      title: "Superstar",
      artist: "Madonna",
  printSong: function (song) {
    console.log(song.title + " - " + song.artist);
  printSongs: function () {
    // `this` bound to the object (OK)
    this.songs.forEach(function(song) {
      // `this` bound to global object (bad)

// > "Oops - The Global Object"
// > "Oops - The Global Object"

The method printSongs() iterates over this.songs with forEach(). In this context, this is bound to the object (jukebox) as expected. However, the anonymous function passed to forEach() binds its internal this to the global object. As such, this.printSong(song) calls the function declared at the top of the example, not the method on jukebox.

JavaScript developers have traditionally used workarounds for this behavior, but arrow functions solve the problem by capturing the this value of the enclosing context. Using an arrow function for printSongs() has the expected result:

  printSongs: function () {
    this.songs.forEach((song) => {
      // `this` bound to same `this` as `printSongs()` (`jukebox`)

// > "Wanna Be Startin' Somethin' - Michael Jackson"
// > "Superstar - Madonna"

For this reason, throughout the book we use arrow functions for all anonymous functions.


ES6 formally supports modules using the import/export syntax.

Named exports

Inside any file, you can use export to specify a variable the module should expose. Here's an example of a file that exports two functions:

// greetings.js

export const sayHi = () => (console.log('Hi!'));
export const sayBye = () => (console.log('Bye!'));

const saySomething = () => (console.log('Something!'));

Now, anywhere we wanted to use these functions we could use import. We need to specify which functions we want to import. A common way of doing this is using ES6's destructuring assignment syntax to list them out like this:

// app.js

import { sayHi, sayBye } from './greetings';

sayHi(); // -> Hi!
sayBye(); // => Bye!

Importantly, the function that was not exported (saySomething) is unavailable outside of the module.

Also note that we supply a relative path to from, indicating that the ES6 module is a local file as opposed to an npm package.

Instead of inserting an export before each variable you'd like to export, you can use this syntax to list off all the exposed variables in one area:

// greetings.js

const sayHi = () => (console.log('Hi!'));
const sayBye = () => (console.log('Bye!'));

const saySomething = () => (console.log('Something!'));

export { sayHi, sayBye };

We can also specify that we'd like to import all of a module's functionality underneath a given namespace with the import * as <Namespace> syntax:

// app.js

import * as Greetings from './greetings';

  // -> Hi!
  // => Bye!
  // => TypeError: Greetings.saySomething is not a function

Default export

The other type of export is a default export. A module can only contain one default export:

// greetings.js

const sayHi = () => (console.log('Hi!'));
const sayBye = () => (console.log('Bye!'));

const saySomething = () => (console.log('Something!'));

const Greetings = { sayHi, sayBye };

export default Greetings;

This is a common pattern for libraries. It means you can easily import the library wholesale without specifying what individual functions you want:

// app.js

import Greetings from './greetings';

Greetings.sayHi(); // -> Hi!
Greetings.sayBye(); // => Bye!

It's not uncommon for a module to use a mix of both named exports and default exports. For instance, with react-dom, you can import ReactDOM (a default export) like this:

import ReactDOM from 'react-dom';

  // ...

Or, if you're only going to use the render() function, you can import the named render() function like this:

import { render } from 'react-dom';

  // ...

To achieve this flexibility, the export implementation for react-dom looks something like this:

// a fake react-dom.js

export const render = (component, target) => {
  // ...

const ReactDOM = {
  // ... other functions

export default ReactDOM;

If you want to play around with the module syntax, check out the folder code/webpack/es6-modules.

For more reading on ES6 modules, see this article from Mozilla: "ES6 in Depth: Modules".


We use Object.assign() often throughout the book. We use it in areas where we want to create a modified version of an existing object.

Object.assign() accepts any number of objects as arguments. When the function receives two arguments, it copies the properties of the second object onto the first, like so:

onst coffee = { };
const noCream = { cream: false };
const noMilk = { milk: false };
Object.assign(coffee, noCream);
// coffee is now: `{ cream: false }`

It is idiomatic to pass in three arguments to Object.assign(). The first argument is a new JavaScript object, the one that Object.assign() will ultimately return. The second is the object whose properties we'd like to build off of. The last is the changes we'd like to apply:

const coffeeWithMilk = Object.assign({}, coffee, { milk: true });
// coffeeWithMilk is: `{ cream: false, milk: true }`
// coffee was not modified: `{ cream: false, milk: false }`

Object.assign() is a handy method for working with "immutable" JavaScript objects.

Template literals

In ES5 JavaScript, you'd interpolate variables into strings like this:

var greeting = 'Hello, ' + user + '! It is ' + degF + ' degrees outside.';

With ES6 template literals, we can create the same string like this:

const greeting = `Hello, ${user}! It is ${degF} degrees outside.`;

The spread operator (...)

In arrays, the ellipsis ... operator will expand the array that follows into the parent array. The spread operator enables us to succinctly construct new arrays as a composite of existing arrays.

Here is an example:

onst a = [ 1, 2, 3 ];
const b = [ 4, 5, 6 ];
const c = [ ...a, ...b, 7, 8, 9 ];

console.log(c);  // -> [ 1, 2, 3, 4, 5, 6, 7, 8, 9 ]

Notice how this is different than if we wrote:

const d = [ a, b, 7, 8, 9 ];
console.log(d); // -> [ [ 1, 2, 3 ], [ 4, 5, 6 ], 7, 8, 9 ]

Enhanced object literals

In ES5, all objects were required to have explicit key and value declarations:

const explicit = {
  getState: getState,
  dispatch: dispatch,

In ES6, you can use this terser syntax whenever the property name and variable name are the same:

const implicit = {

Lots of open source libraries use this syntax, so it's good to be familiar with it. But whether you use it in your own code is a matter of stylistic preference.

Default arguments

With ES6, you can specify a default value for an argument in the case that it is undefined when the function is called.


unction divide(a, b) {
  // Default divisor to `1`
  const divisor = typeof b === 'undefined' ? 1 : b;

  return a / divisor;

Can be written as this:

function divide(a, b = 1) {
  return a / b;

In both cases, using the function looks like this:

divide(14, 2);
// => 7
divide(14, undefined);
// => 14
// => 14

Whenever the argument b in the example above is undefined, the default argument is used. Note that null will not use the default argument:

divide(14, null); // `null` is used as divisor
// => Infinity    // 14 / null

Destructuring assignments

For arrays

In ES5, extracting and assigning multiple elements from an array looked like this:

ar fruits = [ 'apples', 'bananas', 'oranges' ];
var fruit1 = fruits[0];
var fruit2 = fruits[1];

In ES6, we can use the destructuring syntax to accomplish the same task like this:

const [ veg1, veg2 ] = [ 'asparagus', 'broccoli', 'onion' ];
console.log(veg1); // -> 'asparagus'
console.log(veg2); // -> 'broccoli'

The variables in the array on the left are "matched" and assigned to the corresponding elements in the array on the right. Note that 'onion' is ignored and has no variable bound to it.

For objects

We can do something similar for extracting object properties into variables:

const smoothie = {
  fats: [ 'avocado', 'peanut butter', 'greek yogurt' ],
  liquids: [ 'almond milk' ],
  greens: [ 'spinach' ],
  fruits: [ 'blueberry', 'banana' ],

const { liquids, fruits } = smoothie;

console.log(liquids); // -> [ 'almond milk' ]
console.log(fruits); // -> [ 'blueberry', 'banana' ]

Parameter context matching

We can use these same principles to bind arguments inside a function to properties of an object supplied as an argument:

const containsSpinach = ({ greens }) => {
  if (greens.find(g => g === 'spinach')) {
    return true;
  } else {
    return false;

containsSpinach(smoothie); // -> true

We do this often with functional React components:

const IngredientList = ({ ingredients, onClick }) => (
  <ul className='IngredientList'>
    { => (
          onClick={() => onClick(}

Here, we use destructuring to extract the props into variables (ingredients and onClick) that we then use inside the component's function body.