Javascript: testing mediators using sinon and inject-loader

Unit testing in javascript is not always easy, especially when it comes to testing asynchronous code. And searching the internet might not always give you an answer or, at least, the best answer…

I personally like the mediator pattern and I use it a lot in my projects. Mediators allow you to decouple your front-end components from your services, and by doing so, testing all the different parts of your application becomes easier.

structure of the Mediator pattern

I like to use Postal.js for communication between the different parts of my applications, especially between the front-end components, the mediators and the services. Postal.js is a very well known in-memory message bus inspired by AMQP and allows you to post and subscribe to messages in a very simple fashion.

Now, testing front-end components and services is relatively straightforward, but testing mediators is not so easy. And the reason is that you mediators will normally call services to do some kind of logic, and you need to mock those services, which is not trivial if you’re using bundlers like webpack or requirejs.

If you’re using webpack, fortunately, a very simple library is available that allows you to mock the required dependencies that your mediator uses. So if you imagine you have a service like this:

exports default () => {
    this.doWork = () => {
        return 'doing some work...';

and a mediator like this:

import postal from 'postal';
import myService from 'myService';

var channel ='');

channel.subscribe('', () => {
       console.log(myService.doWork()); // doing some work...

Then you can easily test this mediator like this:


var postal = require('postal');
var sinon = require('sinon/pkg/sinon');
var expect = require('chai').expect;

describe('My mediator', function(){
      var myServiceStub, myServiceMock;
      var channel ='');

            var MyModuleInjector = require('inject!myMediator');
            //Create stub for doWork
            myServiceStub = sinon.stub().returns('Mock do work...');

            //Creating mock to be injected
            myServiceMock = function (){
                 this.doWork = myServiceStub;

            //Injecting the mock
                   'myService': myServiceMock

      it('should use mock service', function(){

Some clarifications about the code above.

First, in the mediator test where I do something like require(‘inject!myMediator’), I’m using the inject-loader an awesome webpack loader that allows you to mock your required dependencies. With the inject loader you can mock all or just the dependencies you want, which makes it extremely flexible!

Second, I’m using sinon to create my stubs, because it’s an amazing library for stubbing, mocking and spying. Really, if you don’t know what it is you should look it out, it is extremely easy to use and is agnostic to your application infrastructure, so you can always use it.

As you see, if you’re using webpack, testing your mediators should be simple enough, and now you don’t have an excuse not to test them. 🙂