How to bundle and compile ES2015 into JavaScript

Recently I worked on my first project in ES2015. Because not all browsers support all features of ES2015, you need to compile it into "normal" JavaScript. Also, because the code is divided into modules, these need to be bundled. This blog post describes the tools and configuration I used.

Automating your workflow with Gulp

Gulp is my task runner of choice. It can run different tasks (concat and minimize Javascript, compile SASS into CSS, etc) and watch for changes in your source files. Gulp also has plugins for a variety of tools, like Rollup and Babel.

Bundle modules with Rollup

Modules allow you to separate your logic into multiple files. In your scripts, you can import classes and functions from other files. Browsers don't support this new way of importing modules. Therefore we need rollup to bundle all necessary files into one.

From the rollup website:

Rollup is a JavaScript module bundler. It allows you to write your application or library as a set of modules – using modern ES2015 import/export syntax – and bundle them up into a single file that you can easily include on an HTML page as a <script> tag, or distribute via package managers like npm.

With the rollup-stream plugin, we can incorporate this tool into our gulp tasks.

Compile ES2015 with Babel

Babel is a JavaScript compiler. It can transform ES2015 code into ordinary JavaScript without loss of functionality. This ensures that we can use the newest features and still support all major browsers.

With the gulp-babel plugin, we can automate this step within our gulp workflow.


The GameOfLife plugin has the following npm dependencies:

"devDependencies": {
    "babel-preset-es2015": "^6.9.0",
    "gulp": "3.9.*",
    "gulp-babel": "^6.1.2",
    "gulp-rename": "1.2.*",
    "gulp-sourcemaps": "1.6.*",
    "rollup-stream": "^1.8.0",
    "vinyl-buffer": "^1.0.0",
    "vinyl-source-stream": "^1.1.0"


And the gulpfile looks like this:

var babel = require("gulp-babel");
var gulp = require('gulp');
var rename = require("gulp-rename");
var rollup = require('rollup-stream');
var sourcemaps = require('gulp-sourcemaps');
var source = require('vinyl-source-stream');
var buffer = require('vinyl-buffer');

gulp.task('build-js', function () {
            entry: './src/gameoflife.js',
            sourceMap: true,
            format: 'iife',
            moduleName: 'GameOfLife',
            banner: `//  GameOfLife JavaScript Plugin v1.0.3
//  Released under the MIT license

        // print errors to console, this makes sure Gulp can keep watching and running
        .on('error', e => {

        // point to the entry file
        .pipe(source('gameoflife.js', './src'))

        // buffer the output, needed for gulp-sourcemaps

        // tell gulp-sourcemaps to load the inline sourcemap produced by rollup-stream
        .pipe(sourcemaps.init({loadMaps: true}))

        // compile es2015 into plain javascript
        .pipe(babel({presets: ['es2015']}))

        // rename output file

        // write source map

        // output to /dist

gulp.task('watch-js', function(){'src/**/*.js', ['build-js']);

gulp.task('default', ['build-js', 'watch-js']);

Some points of interest:

  1. Rollup supports multiple output formats. I've chosen for the IIFE (Immediately-Invoked Function Expression) format. It allows anyone to instantiate the plugin using new GameOfLife() without forcing the use of a RequireJS/CommonJS system.
  2. The moduleName option decides the name of the IIFE function, in this case, "GameOfLife".
  3. The Babel es2015 preset is all the configuration you probably need. Babel also provides a preset for react and a polyfill for more advanced features.

Further reading

Related articles

Comments (2)

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Check out (the comment mentioning "big picture") for how to set up proper error handling, otherwise static problems in .js files will interrupt the watch task.
Rollup API has since changes, requiring update to this post.