author: Krasimir Tsonev

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Using Mocha with ES6 spec files

The new ES6 specification of JavaScript simply works better for me. It helps expressing my ideas in a shorter and cleaner way. So, I want to write ES6 everywhere. Even while I’m testing my applications.

As a testing framework I like using Mocha. There are tons of plugins for it and the reporters are nice. The problem is that it doesn’t accept ES6 written JavaScript out of the box. We need some setup before that. Here is how the process looked like so far:

           spec files written in
           plain ES5 JavaScript
             |
             |
mocha ---> reading the files and
           running the tests
             |
             |
           test results

However, if I write my tests in ES6 this doesn’t work anymore because Mocha don’t understand the input.

The best transpiler of ES6 to ES5 is Babel. And that’s what I used at the end. The script that I added to my package.json file looked like that:

// package.json
"scripts": {
  "test": "mocha --compilers js:babel-core/register ./test/**/*.spec.js"
}

The final bit was adding a .babelrc file to my root directory. As you may guess this is a configuration file for babel. The latest versions of the transpiler are no longer bundled with compilers by default. We need to install and use them separately.

// .babelrc
{
  "presets": ["es2015"]
}

Once we run mocha in the terminal we get the following workflow:

           spec files written in
           plain ES6 JavaScript
             |
             |
mocha ---> reading the files
             |
babel ---> compiles the files
           from ES6 to ES5
             |            
mocha ---> running the tests
             |
             |
           test results

The full list of dependencies is placed below. babel-preset-es2015 contains several plugins that cover the most used ES6 features. If you are interested check out this link.

// package.json
"devDependencies": {
  "babel": "6.3.13",
  "babel-core": "6.1.18",
  "babel-preset-es2015": "6.3.13",
  "mocha": "2.3.4"
  ...
}

Notice the versions of babel and mocha. If you come to this article after a month you may see the API of these two modules changed.

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