ES6 is right around the corner. But thanks to a few of libraries and tools, developing and testing ES6 apps today on ES5 is trivial. Lets see how we can do it.

The code for this post can be found here.


Developing ES6 apps on ES5 requires three things:

  1. Polyfill ES6 polyfills add new functionality to ES5 browsers and Node.js.
  2. Module Loader Module loaders do just what their name implies, load modules.
  3. Transpiler Converts ES6 code to ES5 code so it can be run by ES5 browsers and Node.js.


In this post we’ll be working with the following layout:


I’ve gone the route of naming ES6 files with the .es6 extension (which will be transpiled to .js ES5 files). Linguist, the library GitHub uses for syntax highlighting, recognizes the .es6 extension, so they will be properly highlighted in GitHub.


Polyfills add new ES6 functionality to ES5 browsers and Node.js. Polyfills can only do so much though, for example they cannot add support for new language constructs (But we can work around this with transpilation). There are a number of polyfill libraries out there but we’ll use core-js in this example.

You will need to add core-js to your html file. It’s available through Bower (bower install core.js) or directly here.

<script src="core.js"></script>

Next install the core-js package (npm install core-js --save) and require it in your gulpfile.js.


Module Loader

Since ES5 browsers do not have the ability load modules we will need to use a module loader. There are many module loaders out there but we will be using SystemJS in this example. SystemJS has the ability to load a number of different module formats. In this example we will be transpiling to CommonJS modules as these can be handled by SystemJS in the browser and natively by Node.js.

You will need to add SystemJS and the ES6 module loader polyfill (Which SystemJS depends on) to your html file. It’s available through Bower (bower install system.js) or directly here and here respectively.

<script src="es6-module-loader.js"></script>
<script src="system.js"></script>

NOTE: you do not need to include a transpiler as noted here as we will not be transpiling on the fly in the browser.


As mentioned earlier, ES6 support cannot be achieved with polyfills alone as there are new language constructs. The way we deal this is through transpilation, where ES6 is converted to ES5. There are many transpilers out there but we will be using Babel (Formerly 6to5) in this example. Transpilation can be done dynamically in the browser or statically as part of a build. The former is not appropriate for production so we will do the latter and setup a watcher for dev.

Install the following node modules:

 npm install gulp-babel --save
 npm install gulp-sourcemaps --save
 npm install gulp-rename --save

… and add the following task to your gulpfile.js:

var sourcemaps = require("gulp-sourcemaps");
var babel = require("gulp-babel");
var rename = require('gulp-rename');

gulp.task('babel', function () {
    return gulp.src(['**/*.es6'])
        .pipe(rename({ extname: '.js' }))

Here we are transpiling ES6 files (with a .es6 extension) to ES5. The default module format is CommonJS. We are transpiling to this module format as it can be understood on the browser by SystemJS and natively by Node.js for our tests. We are also generating source map files so that transpiled code can be mapped to the original ES6 code when debugging. Finally the output is renamed to *.js. Since we don’t want to commit generated files to our repository we can selectively exclude them in our .gitignore:

# Any exclusions


Bootstrapping our app in the browser is done via the module loader:

    System.import('app').then(function(app) {; });


We will use the Mocha test framework and Chai assertion library. There is not really anything special we have to do on the Node side except make our test task depend on the babel task to transpile before running the tests.

Install the following node modules:

 npm install gulp-mocha --save
 npm install chai --save

Then add the following task to your gulpfile.js:

var mocha = require('gulp-mocha');
var process = require('child_process');

function spawnTests() {
    process.spawn('gulp', ['test'], { stdio: 'inherit' });

gulp.task('watch', function () {
    spawnTests();'**/*.es6', spawnTests);

gulp.task('test', ['babel'], function () {
    return gulp.src(['tests/**/*.js'])
        .pipe(mocha({ reporter: 'spec', ui: 'bdd' }));

Spawning the test runner prevents certain failures from stopping the watch loop. Also, since the watch is fired only on changes, we run it right off the bat so we don’t have to wait for changes.

Now we can write a test referencing a module:

import { expect } from 'chai';
import * as module from '../module';
describe('Module', () =>
    it('should return something.', () => 

And run the tests:

gulp watch


So writing ES6 apps today is a pretty easy task. With the finalization of the spec so close it seems like a no-brainer at this point to do all new development on ES6. So long ES5, and thanks for all the fish!