JS Unit Testing with Mocha and Chai

A walk through for JS unit tests using Mocha as our test runner, Chai as our assertion library & nyc as a coverage reporter

Setting up for tests using Mocha & Chai

• If you've not already, run npm init to start a new package for your project
• Install mocha as a dev dependency npm install mocha --save-dev
• Install chai as a dev dependency npm install chai --save-dev
• Make a /test folder and make your first spec file eg spec.js
• Bring chai's 'expect' library to your spec.js with const expect = require('chai').expect;
• Decide what you want to test! Let's make a function that adds two numbers together.

BONUS Use rewire to streamline the sharing of code to you test suite. \

• npm install rewire --save-dev
• In your spec file, add

const rewire = require('rewire');

• In your spec file, make a reference to the file your code is in that you wish to test. Note it is using a relative path.

let app = rewire('../theNumberGames');

Writing your first test in Mocha / Chai

Invoke mocha's describe function which takes two arguments: a string & a function.
The first argument (the string) says what you're testing - this will be used in the output.
The second argument (the function) will say how we're going to test it. \

describe ('addThese', () => {} )

Now let's fill out that function block (the bit inbetween the curly braces)

describe ('addThese', () => {
  // adding code here!
})

If you are using rewire, now is the time to grab the specific function you need to test.

describe ('addThese', () => {
  let add = app.__get__('add');
})

Now we have access to that function, we can make some assertions about it.
For each assertion, we will call on another mocha function, it, which takes 2 arguments just as describe does - a string and a function. \

describe('addThese', () => {
    let add = app.__get__('add');

    it('should exist', () => {
        // we need to make our assertion here
    })
})

Now we'll start making use of chai's library. Chai has lots of really great, semantically beautiful, offerings. Using chai we can practically write in English! expect(add).to.be.a('function'); - Wow that's nice! Have a look at the chai docs for more!

Our first is now looking like this and is ready to go!

describe('addThese', () => {
    let add = app.__get__('add');
    it('should exist', () => {
        expect(add).to.be.a('function');
    })
})

Running your tests

You can run mocha with mocha <your-test-file>
Optional commands include --reporter=nyan for some nice output formatting and --bail to force and exit after the first failure.

Why not make an npm script to run this for you? In your package.json file, add (or update) a "test" script to look something like this:

 "scripts": {
    "test": "mocha --reporter=nyan --bail"
  },

Mocha will automatically look for a folder called test and run through all the files at the top level. If you want it to run recursively through sub-folders as well, add --recursive to your test command.

Checking your test coverage

To get a check on what percentage of your code has test coverage, istanbul is one of several options.
Istanbul's CLI app, nyc is easy to get up and running:

• Install as a dev dependency: npm install nyc --save-dev
• Add it to your test script: "test": "nyc reporter=text-summary mocha --reporter=nyan --bail"
• If you prefer, add a separate script eg.

 "scripts": {
    "test": "mocha --reporter=nyan --bail"
    "test-with-coverage": "nyc --reporter=text-summary mocha --reporter=nyan --bail"
  },