Speeding up webpack performance with parallel builds

How we reduced our build time from 16 to 2 minutes

When using webpack to build your assets, it’s only a matter of time until you wish for targeted builds. Whether it’s the output of the library you’re working on (CJS, UMD, AMD, Var, etc.), or the specific feature set (IE8 support, no IE8 support). parallel-webpack can run those builds in parallel, thus making full use of the multi-core processing capabilities of modern devices.

Ever since we introduced webpack in December of last year, we have been using it to create targeted builds for our applications and libraries. More specifically, we generate separate versions of our assets for users of IE8 and modern browsers as well as variants for right-to-left and left-to-right languages amongst other criteria.

We’ve also been using targeted builds for our internal JavaScript framework so that we can deliver it in different module formats, such as a global variable, CommonJS, UMD or AMD packages. This way, the framework can readily be integrated with almost any existing application, regardless of the module structure it employs.

There was one problem though: Creating 32 variants of a build took about 16 minutes - way too long for us.

We started searching for a solution, but didn’t find anything that worked for us. Thus, we created our own solution: parallel-webpack.

By using parallel-webpack we were able to reduce our build performance down to 2 minutes - a massive improvement that has made our lives a lot easier. Naturally, the improved speed comes at the price of a higher CPU usage. By default, parallel-webpack will use all available CPU cores to build your assets as fast as possible.


The Installation of parallel-webpack is as simple as installing any other npm module:

npm install parallel-webpack --save-dev

parallel-webpack works with any standard webpack.config.js and will automatically run all provided configurations in parallel.

module.exports = [{
    entry: 'pageA.js',
    output: {
        path: './dist',
        filename: 'pageA.bundle.js'
}, {
    entry: 'pageB.js',
    output: {
        path: './dist',
        filename: 'pageB.bundle.js'

Once you run parallel-webpack for this, you’ll notice that it builds pageA.bundle.js at the same time as pageB.bundle.js.

Often, you’ll just want to build variations of a standard configuration, and parallel-webpack can support you with that as well. All you have to do is call the createVariants function.

// Import the createVariants helper.
var createVariants = require('parallel-webpack').createVariants;

// Those options will be the same for every variant.
var baseOptions = {
    devtool: 'eval'

// This object defines the potential option variants
// the key of the object represents the option name, its value must be an array
// which contains all potential values of your build.
var variants = {
    minified: [true, false],
    debug: [true, false],
    target: ['commonjs2', 'var', 'umd', 'amd']

// This function transforms the given options variant into the actual webpack configuration.
function createConfig(options) {
    var plugins = [
        new webpack.optimize.DedupePlugin(),
        new webpack.optimize.OccurenceOrderPlugin(),
        new webpack.DefinePlugin({
            DEBUG: JSON.stringify(JSON.parse(options.debug))
    if(options.minified) {
        plugins.push(new webpack.optimize.UglifyJsPlugin({
            sourceMap: false,
            compress: {
                warnings: false
    return {
        entry: './index.js',
        output: {
            path: './dist/',
            filename: 'MyLib.' +
                (options.minified ? '.min' : '') +
                (options.debug ? '.debug' : '')
                + '.js',
        plugins: plugins

// Export the final build variants.
module.exports = createVariants(baseOptions, variants, createConfig);

The above configuration will create 16 variants of your build configuration and parallel-webpack will build them in parallel, producing an output like this for you:

[WEBPACK] Building 16 targets in parallel
[WEBPACK] Started building MyLib.umd.js
[WEBPACK] Started building MyLib.umd.min.js
[WEBPACK] Started building MyLib.umd.debug.js
[WEBPACK] Started building MyLib.umd.min.debug.js

[WEBPACK] Started building MyLib.amd.js
[WEBPACK] Started building MyLib.amd.min.js
[WEBPACK] Started building MyLib.amd.debug.js
[WEBPACK] Started building MyLib.amd.min.debug.js

[WEBPACK] Started building MyLib.commonjs2.js
[WEBPACK] Started building MyLib.commonjs2.min.js
[WEBPACK] Started building MyLib.commonjs2.debug.js
[WEBPACK] Started building MyLib.commonjs2.min.debug.js

[WEBPACK] Started building MyLib.var.js
[WEBPACK] Started building MyLib.var.min.js
[WEBPACK] Started building MyLib.var.debug.js
[WEBPACK] Started building MyLib.var.min.debug.js

parallel-webpack also supports watching and allows you to configure how often it should retry a build that failed before giving up. You can find the source code at GitHub and, as with any of our open source projects, we’d be more than happy to read up on your feedback.


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