# Unit Testing Javascript

# Unit Testing Promises with Mocha, Sinon, Chai and Proxyquire

Here we have a simple class to be tested that returns a Promise based on the results of an external ResponseProcessor that takes time to execute.

For simplicty we'll assume that the processResponse method won't ever fail.

import {processResponse} from '../utils/response_processor';

const ping = () => {
  return new Promise((resolve, _reject) => {
    const response = processResponse(data);

module.exports = ping;

To test this we can leverage the following tools.

  1. mocha (opens new window)
  2. chai (opens new window)
  3. sinon (opens new window)
  4. proxyquire (opens new window)
  5. chai-as-promised (opens new window)

I use the following test script in my package.json file.

"test": "NODE_ENV=test mocha --compilers js:babel-core/register --require ./test/unit/test_helper.js  --recursive test/**/*_spec.js"

This allows me to use es6 syntax. It references a test_helper that will look like

import chai from 'chai';
import sinon from 'sinon';
import sinonChai from 'sinon-chai';
import chaiAsPromised from 'chai-as-promised';
import sinonStubPromise from 'sinon-stub-promise';


Proxyquire allows us to inject our own stub in the place of the external ResponseProcessor. We can then use sinon to spy on that stub's methods. We use the extensions to chai that chai-as-promised injects to check that the ping() method's promise is fullfilled, and that it eventually returns the required response.

import {expect}       from 'chai';
import sinon          from 'sinon';
import proxyquire     from 'proxyquire';

let formattingStub = {
  wrapResponse: () => {}

let ping = proxyquire('../../../src/api/ping', {
  '../utils/formatting': formattingStub

describe('ping', () => {
  let wrapResponseSpy, pingResult;
  const response = 'some response';

  beforeEach(() => {
    wrapResponseSpy = sinon.stub(formattingStub, 'wrapResponse').returns(response);
    pingResult = ping();

  afterEach(() => {

  it('returns a fullfilled promise', () => {

  it('eventually returns the correct response', () => {

Now instead let's assume you wish to test something that uses the response from ping.

import {ping} from './ping';

const pingWrapper = () => {
  ping.then((response) => {
    // do something with the response

module.exports = pingWrapper;

To test the pingWrapper we leverage

  1. sinon (opens new window)
  2. proxyquire (opens new window)
  3. sinon-stub-promise (opens new window)

As before, Proxyquire allows us to inject our own stub in the place of the external dependency, in this case the ping method we tested previously. We can then use sinon to spy on that stub's methods and leverage sinon-stub-promise to allow us to returnsPromise. This promise can then be resolved or rejected as we wish in the test, in order to test the wrapper's response to that.

import {expect}   from 'chai';
import sinon      from 'sinon';
import proxyquire from 'proxyquire';

let pingStub = {
  ping: () => {}

let pingWrapper = proxyquire('../src/pingWrapper', {
  './ping': pingStub

describe('pingWrapper', () => {
  let pingSpy;
  const response = 'some response';

  beforeEach(() => {
    pingSpy = sinon.stub(pingStub, 'ping').returnsPromise();

  afterEach(() => {

  it('wraps the ping', () => {

# Basic Assertion

At its most basic level, Unit Testing in any language provides assertions against some known or expected output.

function assert( outcome, description ) { 
    var passFail = outcome ? 'pass' : 'fail'; 
    console.log(passFail, ': ', description);
    return outcome;

The popular assertion method above shows us one quick and easy way to assert a value in most web browsers and interpreters like Node.js with virtually any version of ECMAScript.

A good unit test is designed to test a discreet unit of code; usually a function.

function add(num1, num2) { 
    return num1 + num2; 
var result = add(5, 20); 
assert( result == 24, 'add(5, 20) should return 25...'); 

In the example above, the return value from the function add(x, y) or 5 + 20 is clearly 25, so our assertion of 24 should fail, and the assert method will log a "fail" line.

If we simply modify our expected assertion outcome, the test will succeed and the resulting output would look something like this.

assert( result == 25, 'add(5, 20) should return 25...');

console output:

> pass: should return 25...

This simple assertion can assure that in many different cases, your "add" function will always return the expected result and requires no additional frameworks or libraries to work.

A more rigorous set of assertions would look like this (using var result = add(x,y) for each assertion):

assert( result == 0, 'add(0, 0) should return 0...');
assert( result == -1, 'add(0, -1) should return -1...');
assert( result == 1, 'add(0, 1) should return 1...');

And console output would be this:

> pass: should return 0...
> pass: should return -1...
> pass: should return 1...

We can now safely say that add(x,y)… should return the sum of two integers. We can roll these up into something like this:

function test__addsIntegers() {

    // expect a number of passed assertions
    var passed = 3;

    // number of assertions to be reduced and added as Booleans
    var assertions = [

        assert( add(0, 0) == 0, 'add(0, 0) should return 0...'),
        assert( add(0, -1) == -1, 'add(0, -1) should return -1...'),
        assert( add(0, 1) == 1, 'add(0, 1) should return 1...')

    ].reduce(function(previousValue, currentValue){

        return previousValue + current;


    if (assertions === passed) {

        console.log("add(x,y)... did return the sum of two integers");
        return true;

    } else {

        console.log("add(x,y)... does not reliably return the sum of two integers");
        return false;