When writing JavaScript applications or websites a lot of developers write unit tests to ensure the code does what it is supposed to do. The hottest code uses ES6 and transpiles to ES5 with Babel. Mocha is pretty popular as a testing framework these days. Bringing these tools together can be cumbersome. Here is how you do it.

My work on JavaScript applications usually includes writing unit tests.

With some of my latest projects I wrote my JavaScript applying some of ES6's sugar and utilize Babel with Browserify to transpile the code to ES5.

Setting up a testing infrastructure with Mocha I discovered some impediments which I had to work around. Let’s have a look at the details.

ES6 Modules

ES6 Modules provide a way for developers to define dependencies between different JavaScript files. Modules export certain properties for other programs to use. These programs can import whole modules or their functionality selectively. ES6 Modules are designed to replace current dependency management approaches as AMD and CommonJS.

If you want to know more about ES6 Modules, read Axel Rauschmayer's article.

Example Module and Spec

Here is an example module and specification.


[module] --> app/index.js --> test/index.js <-- Mocha
[module]                                          ^

Browserify and Babel

Compiling with Browserify using Babel is pretty easy. You just need to run

browserify -t [ babelify ] app/index.js -o dist/index.js

This requires browserify and babelify to be installed:

npm i --save browserify babelify

You can now use the first command as a script in your package.json.

I cover the usage of npm as a build tool in another article.

With this we have set up the ES6-to-ES5 compile step which generates a new file which we can use in out application. The next step is to set up testing with Mocha.

Testing with Mocha

You can install Mocha – it's always the same – with npm. You need an assertion library too. I use Chai. You can install both tools like this:

npm i --save mocha chai

Running tests with mocha is as easy as

mocha test/index.js

If you don’t use ES6 and don’t need to transpile to ES5 you’re done with the Mocha setup. Otherwise you will have to do more.

There is an option for Mocha to treat JS as ES6: --harmony. Unfortunately this does not implement ES6 Modules and other features. This is why we want to use Babel to be utilized by Mocha to compile JavaScript. Mocha has an option --compilers which helps with this:

mocha --compilers js:babel/register test/index.js

Now Mocha compiles JavaScript while running and we’re save to use the latest ES6 features.

Furthermore we need an initial file which is our entry point for Mocha running on Node.js. I named this file test/index.js, as you can see. All it does is it requires Chai and the test suites:

global.chai = require('chai');
global.expect = global.chai.expect;

// Load test suites

You can do more test specific stuff here as for example bootstrapping jsdom.

Test Coverage with Istanbul

It gets a bit tricky if you want to generate coverage reports using Istanbul. You need to pass the compiler to the files you want to test and use _mocha instead of mocha in order to get it to use Babel to compile the JS.

istanbul cover _mocha -- --compilers js:babel/register test/index.js

If you don’t use ES6 and don’t need to transpile to ES5 you can use

istanbul cover mocha test/index.js

Don't forget to install Istanbul:

npm i --save istanbul

To exclude the specification file from the coverage report you can pass the option -x to Istanbul:

istanbul cover _mocha -x '**/*.spec.js' -- --compilers js:babel/register test/index.js


If you searched Google for an hour to fix this workflow and you can finally use all nice features ES6 provides, setting up Mocha with Babel is totally worth it. But luckily I have already done this search.

Have you used Mocha with the latest ES6 features? Which tools did you use? Let me know and tweet me at @drublic.