Organising client JavaScript with Rollup.js

Rollup.js is a great way to add structure to a large client-side code base.

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You might not be wanting to tackle all of EcmaScript(ES6), yet still want to separate your JavaScript code into modules. Previously the best way to do this was Browserify with common.js modules. Rollup.js is the logical next step as it replaces the common.js modules with ES6 modules. It is also a component of Babel (a full ES6 transpiler) so there is an easy next step to get working with the rest of ES6.

I have recently introduced Rollup.js into my development workflow. The simple command line interface means it can be used without the overhead of taskrunners like(gulp). It is easy to integrate with existing tools for testing. I use the Karma test runner to execute my test framework of choice, which is currently Jasmine.

This post walks through my setup of Jasmine, Karma and rollup. Want to jump straight to the code? Visit this repository to see the code.

I will assume that you already have node and npm installed, need help see Installing Node.js and npm.

Setting up a node project

We first need to create a package.json file to identify our project as a node project. This can be done by hand or by executing npm init, which walks through setup. The only setup we need is to give the project a name, let’s call it calculator.

Next is to fetch the development dependencies we will use. Fetch all the dependencies by executing the following.

$ npm install --save-dev \
    rollup \
    jasmine \
    karma \
    karma-jasmine \

At this point make sure that you have added node_modules to your .gitignore file. It is not a good idea to commit all these node files to version control-something that I have done on too many occasions.

Setting up Karma.js

The Karma test runner also comes with a handy project initializer which will create a karma.conf.js file. Run the karma initializer with the following options.

$  ./node_modules/.bin/karma init

Which testing framework do you want to use ?
Press tab to list possible options. Enter to move to the next question.
> jasmine

Do you want to use Require.js ?
This will add Require.js plugin.
Press tab to list possible options. Enter to move to the next question.
> no

Do you want to capture any browsers automatically ?
Press tab to list possible options. Enter empty string to move to the next question.
> Firefox

What is the location of your source and test files ?
You can use glob patterns, eg. "js/*.js" or "test/**/*Spec.js".
Enter empty string to move to the next question.
> test/bundle.js
11 11 2015 17:04:52.298:WARN [init]: There is no file matching this pattern.


Should any of the files included by the previous patterns be excluded ?
You can use glob patterns, eg. "**/*.swp".
Enter empty string to move to the next question.

Do you want Karma to watch all the files and run the tests on change ?
Press tab to list possible options.
> no

Config file generated at "/home/peter/Projects/Calculator/karma.conf.js".


Creating our first test

Let’s create a single test to check out Karma setup. We will create a main test file and add an example test.

// test/main.js

describe("A suite", function() {
  it("contains spec with an expectation", function() {

To bundle this file use Rollup with its default settings and store the output. At this point the output will be unchanged, this is because we are yet to import any modules

$ ./node_modules/.bin/rollup test/main.js > test/bundle.js

Now our bundle is available we can run the test.

$ ./node_modules/.bin/karma start --singleRun

This will show one passing test. As test/bundle.js is just a derived file it does not belong in version control. Add a line for it in the .gitignore.

Simple test execution

Typing all those commands to execute the tests would be painful. We can make life much easier by declaring them as npm scripts. First one change is needed in karma.conf.js, we will set singleRun configuration to be true.

// karma.conf.js

module.exports = function(config) {
    // Continuous Integration mode
    // if true, Karma captures browsers, runs the tests and exits
    singleRun: true,

A lot of functionality can end up in npm scripts and it is easy to write magical single line scripts that no-one understands. My advice is to write small contained scripts for bundling and running the tests. Then have a third script that coordinates the small component scripts. Add these scripts into package.json.

// package.json
  "scripts": {
    "test": "npm run -s test:build && npm run -s test:run",
    "test:build": "rollup ./test/main.js > test/bundle.js",
    "test:run": "karma start"

To run the project tests we can just execute the npm test script.

$ npm -s test

This single simple test command follows npm conventions and is easy to remember. Having a trivial test command is important if test driving the development of a project.

Testing some real code

At the moment we have Rollup.js setup but it is redundant as we have no modules. It is time to test some real code for a calculator. The first feature a calculator should have is the ability to add two numbers. Let’s adjust the test file so that it imports the calculator and add a meaningful test.

// test/main.js

// Import all exported functions from the calculator source file
// They will be available as part of the calculator namespace
import * as calculator from "../src/calculator";

describe("A Calculator", function() {
  it("should be able to add two numbers", function() {
    expect(calculator.add(1, 2)).toEqual(3);

The tests should now be failing, there is no code after all. There are several features a good calculator might need. To keep features separate the calculator module will import modules to gain its utility.

// src/calculator.js

// Import add from the `calculator/add.js` file
import add from "./calculator/add";

// Export add as one of the exports for the calculator module
export { add };
// src/calculator/add.js

// export the function add as the default for this module
export default function add(a, b) { return a + b; }

With the tests passing again our impressive calculator is taking shape.

Building a distribution

The final thing, now we have working calculator components, is to bundle the source into a single file. This is so it can be consumed in all the browsers that are not yet supporting ES6 modules.

Using Rollup.js again we bundle the distribution and save it in index.js.

$ ./node_modules/.bin/rollup \
    --format iife \
    --name calculator \
    src/calculator.js >  index.js

We pass the format and name options so that our final bundle is available in the global namespace and can be used by other js code.

Again to keep our version control neat we add index.js to .gitignore.


This has so far been a convenient way for me to add some structure to my projects. It has been much simpler to setup than Browserify with Gulp or Grunt. Check out the code for the calculator project the result of this walk through. Tell me what you think about this approach, if you would use it or if anything is missing.