Test-Driven Development in Node.js With Mocha

Who needs Test-Driven Development?

Imagine that you need to implement a complex feature on top of an existing interface, e.g., a ‘like’ button on a comment. Without tests you’ll have to manually create a user, log in, create a post, create a different user, log in with a different user and like the post. Tiresome? What if you’ll need to do it 10 or 20 times to find and fix some nasty bug? What if your feature breaks existing functionality, but you notice it 6 months after the release because there was no test!

Don’t waste time writing tests for throwaway scripts, but please adapt the habit of Test-Driven Development for the main code base. With a little time spent in the beginning, you and your team will save time later and have confidence when rolling out new releases. Test Driven Development is a really really really good thing.

Quick Start Guide

Follow this quick guide to set up your Test-Driven Development process in Node.js with Mocha.

Install Mocha globally by executing this command:

$ sudo npm install -g mocha

We’ll also use two libraries, Superagent and expect.js by LeanBoost. To install them fire up npm commands in your project folder like this:

$ npm install superagent
$ npm install expect.js   

Open a new file with .js extension and type:

var request = require('superagent');
var expect = require('expect.js');

So far we’ve included two libraries. The structure of the test suite going to look like this:

describe('Suite one', function(){
  it(function(done){
  ...
  });
  it(function(done){
  ...
  });
});
describe('Suite two', function(){
  it(function(done){
  ...
  });
});

Inside of this closure we can write request to our server which should be running at localhost:8080:

...
it (function(done){
  request.post('localhost:8080').end(function(res){
    //TODO check that response is okay
  });
});
...

Expect will give us handy functions to check any condition we can think of:

...
expect(res).to.exist;
expect(res.status).to.equal(200);
expect(res.body).to.contain('world');
...

Lastly, we need to add done() call to notify Mocha that asynchronous test has finished its work. And the full code of our first test looks like this:

var request = require('superagent');
var expect = require('expect.js');
  
describe('Suite one', function(){
 it (function(done){
   request.post('localhost:8080').end(function(res){
    expect(res).to.exist;
    expect(res.status).to.equal(200);
    expect(res.body).to.contain('world');
    done();
   });
  });
});

If we want to get fancy, we can add before and beforeEach hooks which will, according to their names, execute once before the test (or suite) or each time before the test (or suite):

before(function(){
  //TODO seed the database
});
describe('suite one ',function(){
  beforeEach(function(){
    //todo log in test user
  });
  it('test one', function(done){
  ...
  });
});

Note that before and beforeEach can be placed inside or outside of describe construction.

To run our test simply execute:

$ mocha test.js

To use different report type:

$ mocha test.js -R list
$ mocha test.js -R spec