Get started with Mocha testing

Testing Node.js packages is easy. Here’s the condensed 1-minute guide to setting up tests on a Node.js project.

Start with a package

Make a package.json file if you don’t have one yet, then install your weapons of choice.

npm init
# just keep pressing enter.
# this will create the file `package.json`

npm install --save-dev mocha chai

Write tests

Make your first test file test/my_test.js:

/* test/my_test.js */
var expect = require('chai').expect;

describe('my test suite', function () {
  it('fails majestically', function () {
    expect(3).to.eql(2);
  });
});

Update scripts

Update your package.json to use mocha.

  "scripts": {
-   "test": "echo \"Error: no test specified\" && exit 1"
+   "test": "mocha"
  },

Run tests

Type npm test to run your tests. It should fail. Now go write tests that will pass!

  my test suite
    1) fails majestically


  0 passing (17ms)
  1 failing

  1) my test suite fails majestically:

      AssertionError: expected 3 to deeply equal 2
      + expected - actual

      +2
      -3

      test/test.js:5:18: Context.<anonymous>

Learn a bit more

Here’s a quick Mocha cheatsheet. Also see mochajs.org.

describe('test suite', function () {
  beforeEach(function() { /*...*/ });
  afterEach(function() { /*...*/ });

  before(function() { /*...*/ });
  after(function() { /*...*/ });

  it('a basic test', function() {
    /*...*/ });

  it('a test with a promise', function() {
    return somePromiseObject; });

  it('an asynchronous test', function(next) {
    if (success) { next(); } else { next(error); }
  });

  xit('use "xit" for pending tests', function() {
    /*...*/ });
});

Expectations with Chai

Here’s a quick Chai cheatsheet. See chaijs.com for other expect()ations.

expect(3).to.eql(2);

expect(obj).to.be.a('string');
expect(obj).to.be.null;
expect(obj).to.be.true;
expect(obj).to.be.false;
expect(obj).to.be.undefined;

expect(list).to.include("item");
expect(list).to.have.length(3);
expect(list).to.have.length.gt(0);