ES6 In Depth: Using ES6 today with Babel and Broccoli

ES6 In Depth is a series on new features being added to the JavaScript programming language in the 6th Edition of the ECMAScript standard, ES6 for short.

ES6 is here, and people are already talking about ES7, what the future holds, and what shiny features a new standard can offer. As web developers, we wonder how we can make use of it all. More than once, in previous ES6 In Depth posts, we’ve encouraged you to start coding in ES6, with a little help from some interesting tools. We’ve teased you with the possibility:

If you’d like to use this new syntax on the Web, you can use Babel or Google’s Traceur to translate your ES6 code to web-friendly ES5.

Today we’re going to show you step-by-step how it is done. The above-mentioned tools are called transpilers. A transpiler is also known as a source-to-source compiler—a compiler that translates between programming languages operating at comparable levels of abstraction. Transpilers let us write code using ES6 while also guaranteeing that we’ll be able to execute the code in every browser.

Transpilation our salvation

A transpiler is very easy to use. You can describe what it does in only two steps:

1. We write code with ES6 syntax.

let q = 99;
let myVariable = `${q} bottles of beer on the wall, ${q} bottles of beer.`;

2. We use the code above as input for the transpiler, which will process it and produce the following output:

"use strict";

var q = 99;
var myVariable = "" + q + " bottles of beer on the wall, " + q + " bottles of beer."

This is the good old JavaScript we know. It can be used in any browser.

The internals of how a transpiler goes from input to output are highly complex and fall out of scope for this article. Just as we can drive a car without knowing all the internal engine mechanics, today we’ll leave the transpiler as a black box that is able to process our code.

Babel in action

There are a couple of different ways to use Babel in a project. There is a command line tool, which you can use with commands of the form:

babel script.js --out-file script-compiled.js

A browser-ready version is also available. You can include Babel as a regular JS library and then you can place your ES6 code in script tags with the type "text/babel".

<script src="node_modules/babel-core/browser.js"></script>
<script type="text/babel">
// Your ES6 code

These methods do not scale when your code base starts to grow and you start splitting everything into multiple files and folders. At that moment, you’ll need a build tool and a way to integrate Babel with a build pipeline.

In the following sections, we’ll integrate Babel into a build tool, Broccoli.js, and we’ll write and execute our first lines of ES6 through a couple of examples. In case you run into trouble, you can review the complete source code here: broccoli-babel-examples. Inside the repository you’ll find three sample projects:

  1. es6-fruits
  2. es6-website
  3. es6-modules

Each one builds on the previous example. We start with the bare minimum and progress to a general solution, which can be used as the starting point of an ambitious project. In this post, we’ll cover the first two examples in detail. After we are done, you’ll be able to read and understand the code in the third example on your own.

If you are thinking —I’ll just wait for browsers to support the new features— you’ll be left behind. Full compliance, if it ever happens, will take a long time. Transpilers are here to stay; new ECMAScript standards are planned to be released yearly. So, we’ll continue to see new standards released more often than uniform browser platforms. Hop in now and take advantage of the new features.

Our first Broccoli & Babel project

Broccoli is a tool designed to build projects as quickly as possible. You can uglify and minify files, among many other things, through the use of Broccoli plugins. It saves us the burden of handling files, directories, and executing commands each time we introduce changes to a project. Think of it as:

Comparable to the Rails asset pipeline in scope, though it runs on Node and is backend-agnostic.

Project setup


As you might have guessed, you’ll have to install Node 0.11 or later.

If you are in a unix system, avoid installing from the package manager (apt, yum). That is to avoid using root privileges during installation. It’s best to manually install the binaries, provided at the previous link, with your current user. You can read why using root is not recommended in Do not sudo npm. In there you’ll find other installation alternatives.


We’ll set up our Broccoli project first with:

mkdir es6-fruits
cd es6-fruits
npm init
# Create an empty file called Brocfile.js
touch Brocfile.js

Now we install broccoli and broccoli-cli

# the broccoli library
npm install --save-dev broccoli
# command line tool
npm install -g broccoli-cli

Write some ES6

We’ll create a src folder and inside we’ll put a fruits.js file.

mkdir src
vim src/fruits.js

In our new file we’ll write a small script using ES6 syntax.

let fruits = [
{id: 100, name: 'strawberry'},
{id: 101, name: 'grapefruit'},
{id: 102, name: 'plum'}

for (let fruit of fruits) {
let message = `ID: ${} Name: ${}`;


console.log(`List total: ${fruits.length}`);

The code sample above makes use of three ES6 features:

  1. let for local scope declarations (to be discussed in an upcoming blog post)
  2. for-of loops
  3. template strings

Save the file and try to execute it.

node src/fruits.js

It won’t work yet, but we are about to make it executable by Node and any browser.

let fruits = [
SyntaxError: Unexpected identifier

Transpilation time

Now we’ll use Broccoli to load our code and push it through Babel. We’ll edit the file Brocfile.js and add this code to it:

// import the babel plugin
var babel = require('broccoli-babel-transpiler');

// grab the source and transpile it in 1 step
fruits = babel('src'); // src/*.js

module.exports = fruits;

Notice that we require broccoli-babel-transpiler, a Broccoli plugin that wraps around the Babel library, so we must install it with:

npm install --save-dev broccoli-babel-transpiler

Now we can build our project and execute our script with:

broccoli build dist # compile
node dist/fruits.js # execute ES5

The output should look like this:

ID: 100 Name: strawberry
ID: 101 Name: grapefruit
ID: 102 Name: plum
List total: 3

That was easy! You can open dist/fruits.js to see what the transpiled code looks like. A nice feature of the Babel transpiler is that it produces readable code.

Writing ES6 code for a website

For our second example we’ll take it up a notch. First, exit the es6-fruits folder and create a new directory es6-website using the steps listed under Project setup above.

In the src folder we’ll create three files:


<!DOCTYPE html>
<title>ES6 Today</title>
body {
border: 2px solid #9a9a9a;
border-radius: 10px;
padding: 6px;
font-family: monospace;
text-align: center;
.color {
padding: 1rem;
color: #fff;
<h1>ES6 Today</h1>
<div id="info"></div>
<div id="content"></div>

<script src="//"></script>
<script src="js/my-app.js"></script>


function printInfo() {
.append('<p>minimal website example with' +
'Broccoli and Babel</p>');



// ES6 Generator
function* hexRange(start, stop, step) {
for (var i = start; i < stop; i += step) {
yield i;

function printColors() {
var content$ = $('#content');

// contrived example
for ( var hex of hexRange(900, 999, 10) ) {
var newDiv = $('<div>')
.attr('class', 'color')
.css({ 'background-color': `#${hex}` })
.append(`hex code: #${hex}`);


You might have noticed this bit: function* hexRange — yes, that’s an ES6 generator. This feature is not currently supported in all browsers. To be able to use it, we’ll need a polyfill. Babel provides this and we’ll put it to use very soon.

The next step is to merge all the JS files and use them within a website. The hardest part is writing our Brocfile. This time we install 4 plugins:

npm install --save-dev broccoli-babel-transpiler
npm install --save-dev broccoli-funnel
npm install --save-dev broccoli-concat
npm install --save-dev broccoli-merge-trees

Let’s put them to use:

// Babel transpiler
var babel = require('broccoli-babel-transpiler');
// filter trees (subsets of files)
var funnel = require('broccoli-funnel');
// concatenate trees
var concat = require('broccoli-concat');
// merge trees
var mergeTrees = require('broccoli-merge-trees');

// Transpile the source files
var appJs = babel('src');

// Grab the polyfill file provided by the Babel library
var babelPath = require.resolve('broccoli-babel-transpiler');
babelPath = babelPath.replace(/\/index.js$/, '');
babelPath += '/node_modules/babel-core';
var browserPolyfill = funnel(babelPath, {
files: ['browser-polyfill.js']

// Add the Babel polyfill to the tree of transpiled files
appJs = mergeTrees([browserPolyfill, appJs]);

// Concatenate all the JS files into a single file
appJs = concat(appJs, {
// we specify a concatenation order
inputFiles: ['browser-polyfill.js', '**/*.js'],
outputFile: '/js/my-app.js'

// Grab the index file
var index = funnel('src', {files: ['index.html']});

// Grab all our trees and
// export them as a single and final tree
module.exports = mergeTrees([index, appJs]);

Time to build and execute our code.

broccoli build dist

This time you should see the following structure in the dist folder:

$> tree dist/
├── index.html
└── js
└── my-app.js

That is a static website you can serve with any server to verify that the code is working. For instance:

cd dist/
python -m SimpleHTTPServer
# visit http://localhost:8000/

You should see this:

simple ES6 website

More fun with Babel and Broccoli

The second example above gives an idea of how much we can accomplish with Babel. It might be enough to keep you going for a while. If you want to do more with ES6, Babel, and Broccoli, you should check out this repository: broccoli-babel-boilerplate. It is also a Broccoli+Babel setup, that takes it up at least two notches. This boilerplate handles modules, imports, and unit testing.

You can try an example of that configuration in action here: es6-modules. All the magic is in the Brocfile and it’s very similar to what we have done already.

As you can see, Babel and Broccoli really do make it quite practical to use ES6 features in web sites right now. Thanks to Gastón I. Silva for contributing this week’s post!

Next week, ES6 In Depth starts a two-week summer break. This series has covered a lot of ground, but some of ES6’s most powerful features are yet to come. So please join us when we return with new content on July 9.

Jason Orendorff

ES6 In Depth Editor

About Gastón I. Silva

Software engineer dedicated to web technologies and open source software.

More articles by Gastón I. Silva…


  1. Grahame Bowland

    great article, thanks!

    I think there’s one minor bug – in the second example, the line:

    appJs = mergeTrees([browserPolyfill, appJs]);

    is duplicated, which causes the example to fail if copy & pasted.

    June 18th, 2015 at 07:20

    1. Gastón I. Silva

      You are right. I just fixed it, thanks!

      June 18th, 2015 at 08:31

  2. Chris Deely

    Thanks for the nice post.

    One thing I just don’t understand is why wouldn’t Babel automatically take care of including the polyfills?

    More than half of the second example is dedicated to locating & merging in the polyfills, but that seems like a job the transpiler should be responsible for, no?

    June 19th, 2015 at 11:54

    1. Gastón I. Silva

      Yes, the code that adds the polyfill will always be the same. The Broccoli plugin should save us from that boiler plate. I already submitted a PR: The idea is to be able to simply turn on a flag, like “browserPolyfill: true”. In the mean time, copy the recipe.

      Babel is polyfill agnostic. Core-js is provided for simplicity, but there are others.

      June 19th, 2015 at 12:17

  3. Owen Densmore

    No JSPM workflow? It is so smooth once you get it. Actually, there’s little workflow at all, now that I think about it.

    And while we’re at it, start adding your favorite “legacy” libraries to the jspm Registry. Pretty nice.

    June 19th, 2015 at 20:20

  4. Alexander Ivantsov

    Thanks for the good post.
    But I prefer Webpack instead Broccoli because hot-loading and it’s awesome for React components.

    June 23rd, 2015 at 01:35

  5. Luis G.

    Great to see clear and smooth like you said, lest keep up, you guys are good creatores..

    June 23rd, 2015 at 18:07

  6. Bergi

    Is there a transpiler that combines multiple ES6 modules in a single file (exposing them as a single module, not just concatenation)?

    June 23rd, 2015 at 19:58

Comments are closed for this article.