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Testing in Browsers and Node with Mocha, Chai, Sinon, and Testem

So you’re writing some JavaScript code that has to run in multiple web browsers as well as Node. How can you effectively test this code? Effective testing means that you should be able to run all tests in all environments on demand, i.e., with the press of a single button the tests should run in Node, Chrome, Firefox, Internet Explorer, and whatever other environments you have set up for testing, and we should avoid code duplication whenever possible. Continue reading and I’ll walk you through setting up a basic testing environment using Mocha as a testing framework, Chai for assertions, Sinon for mocking, and Testem as a universal test runner. Basic knowledge of testing, asserting, and mocking in languages other than JavaScript is assumed. Those of you who are familiar with testing, asserting, and mocking in JavaScript will want to skip to the Testem section below – that’s where the meat and potatoes of pulling everything together into a working example is.


Mocha is a testing framework for JavaScript which provides a flexible, intuitive, and consistent interface for testing JavaScript code, both synchronous and asynchronous. Additionally, it is designed to run in both major browsers and Node.

Without further to do, a basic test suite looks like the following:

describe('Library Tests', function () {
  before(function () {
    // set up the environment

  it('Should add numbers', function () {
    // make assertions

The above code should be fairly self-explanatory. describe is used to describe a test suite, and it is used to define actual tests. I am also making use of a before hook to set up the environment prior to running any tests. In addition to the before hook, Mocha makes available the after hook, which executes after all tests have run. The before and after hooks additionally have beforeEach and afterEach counterparts, which, as their names imply, rather than running before or after all tests run before and after each individual test.

Asynchronous Testing

One of the advantages of using Mocha as opposed to certain other testing frameworks is that it makes asynchronous testing really easy. Consider the following test:

it('should foobar', function (done) {
  // Simulate calling an asynchronous method with `setTimeout`
  setTimeout(function () {
    // Make assertions
  }, 1000);

The only change to the test is that the it function now accepts a done argument. done is a function you should call when the test has completed (after all assertions). Additionally, done accepts an Error object as its first argument if you have one to give it. The previously mentioned before, beforeEach, after, and afterEach hooks can also be set up to run asynchronously in the same fashion:

describe('async tests', function () {
  // Before Hooks
  before(function (done) {
    setTimeout(done, 1000);

  beforeEach(function (done) {
    setTimeout(done, 1000);

  // Tests
  it('should foo', function (done) {
    setTimeout(done, 1000);

  it('should bar', function (done) {
    setTimeout(done, 1000);

  // After Hooks
  afterEach(function (done) {
    setTimeout(done, 1000);

  after(function (done) {
    setTimeout(done, 1000);

The above code will take approximately 8 seconds to run and will follow a serial execution path as such:

  • before
  • beforeEach
  • it should foo
  • afterEach
  • beforeEach
  • it should bar
  • afterEach
  • after

Of course, you can mix synchronous and asynchronous hooks and tests and everything will run serially with no extra effort.

You may have noticed that I haven’t been making any assertions. That is because Mocha is designed to let you use your own assertions library. Node provides an assert module, but we want to use the same tests and assertions in all of our environments, not just Node. This is where Chai comes in.


Chai is an assertions library which provides multiple styles of assertions for your to use as you see fit. I prefer expect-style assertions which read like sentences. See the following examples:

expect([1, [2, [3]]]).to.deep.equal([1, [2, [3]]]);
expect(new Error('foo')).to.be.instanceOf(Error);

As you can see, the expect style of assertions are very readable and even self-documenting. The lack of parentheses after true in to.be.true is not a mistake. Chai’s expect style makes use of JavaScript getters and setters meaning to.be.true is not a function, but a property getter. If you need to support an environment which does not support getters and setters, you will have to use to.equal(true) or another assertions library which is purely functional.

Obviously, the above sample does not cover everything you can do in Chai. There is a list of all available chains and assertions available through Chai’s documentation. The assertions provided by Chai cover everything you may want to assert, including (deep) (in)equality, inclusion, existence, ranges, regular expression matching, and (static) method respondence. Additional functionality which extends the core of Chai is available in community-provided plugins.


On to everybody’s favorite topic: mocking! Sinon is a library which provides spies, stubs, and mocks for JavaScript. Let’s cover each of those in order.


A spy is a function which automatically records various metrics regarding its execution including arguments passed to it, return value, the value of this, and any exceptions thrown for all of its calls. See the following example:

var func = sinon.spy();
func(1, 'foo');
// The following two assertions are identical
// The following two assertions are identical
expect(func.firstCall.calledWith(sinon.match.number, sinon.match.string)).to.be.true;
expect(func.getCall(0).calledWith(sinon.match.number, sinon.match.string)).to.be.true;
// You can even verify that a function was called with specific arguments (not just types)
expect(func.firstCall.calledWith(1, 'foo')).to.be.true;

As you can see, Sinon spies are extremely flexible. This example only begins to scratch the surface of spies in Sinon. calledOnce is accompanied by calledTwice and calledThrice which are all aliases for checking the callCount. In addition to firstCall, Sinon spies provide secondCall and thirdCall properties which are all aliases for getCall(0), getCall(1), and getCall(2), respectively. Anything beyond the third call must be accessed with getCall(n) where n is the number call you want to access. Calls can additionally be inspected, in this case to make sure they were called with a number and a string. Sinon provides even more matchers for the following situations:

  • All basic JavaScript primitives
  • Truthy / falsy values
  • Referential equality to a given object
  • Making sure an argument is an instance of a particular constructor
  • Making sure an argument has a property (or its own property) which optionally match additional expectations

Matchers can even be combined; for example, you can check to make sure an argument is either a string or a number. To learn more about this functionality check out Sinon’s matchers documentation.

Spies can wrap themselves around existing methods on existing methods. Consider the following example utilizing jQuery:

// Spy on the `ajax` method of the `jQuery` object
sinon.spy(jQuery, 'ajax');
// Call `jQuery.get(...)` which calls `jQuery.ajax` down the chain
// Assert that `jQuery.ajax` was called with the given url.
// Unwrap `jQuery.ajax` so other tests will be unaffected and can get up their own spies

In this example I referenced the passed argument directly and checked a property on it for equality to what was passed. I also called the restore() method on the spy to return jQuery.ajax to its original functionality. This is an important step in testing (often run in after or afterEach hooks) to make sure tests and their spies do not interfere with each other.

For more information on spy functionality in Sinon, including verifying order of calls to multiple spies, check out the Sinon spy documentation.


A stub is a spy with pre-programmed functionality. You should use a stub when you need to control the behavior of a function you are spying on. You can very easily program a stub to throw an exception, return a given value, return and argument which was passed to it, or even call an argument which was passed to it – with or without arguments, synchronously or asynchronously. Stubs can even be programmed to do different things when different arguments are passed to them. Consider the following example:

var func = sinon.stub();


When func is passed 42, it will return 1. Otherwise, it will throw an exception.

Just like how you can spy on methods of objects, you can stub methods of objects. Consider the following example of stubbing the readFile method on Node’s filesystem module:

// Stub the file system module
sinon.stub(fs, 'readFile');
// Asynchronously call the third argument with a null error and some text when passed certain arguments
  .withArgs('foo.txt', 'utf8', sinon.matchers.func)
  .callsArgWithAsync(2, null, 'Foo!');
// Asynchronously call the third argument with an error when passed certain arguments
  .withArgs('bar.txt', 'utf8', sinon.matchers.func)
  .callsArgWithAsync(2, new Error('File not found!'));
// Do your testing
// Afterwards make sure you restore `fs.readFile` to its original functionality.

Now everything that calls fs.readFile('foo.txt', 'utf8', callback) will have callback called with null for an error and 'Foo!' for data and everything that calls fs.readFile('bar.txt', 'utf8', callback) will have callback called with an Error object. The callbacks are called asynchronously so all other functionality of your program is preserved, but your tests are no longer dependent on the file system. Stubbing methods which rely on external resources such as file systems or network connections is very common in testing as your tests will run faster and will be more portable overall.

To learn more about Sinon stubs, read the Sinon stub documentation.


Mocks provide fake methods with pre-programmed functionality and with pre-programmed expectations. For example, you can program a mock to expect a certain number of calls, at most a certain number of calls, at least a certain number of calls, to never be called, to be called with certain arguments, and more. After your test is done you can call your mock’s verify method which will throw an exception if the preset expectations are not met. To learn more about mocks and expectations in Sinon you can read Sinon’s mock documentation.

Chai Assertions

Sinon provides its own assertions. Despite this, for the sake of consistency, I was using Chai assertions in the examples above. There is a plugin available for Chai which can streamline expectations for Sinon spies, stubs, and mocks. Using the plugin, my first Sinon example from above would look like the following:

var func = sinon.spy();
func(1, 'foo');
expect(func).to.have.been.calledWith(sinon.match.number, sinon.match.string);
expect(func).to.have.been.calledWith(1, 'foo');

For more information, check out the Sinon-Chai plugin documentation.

More Sinon Functionality

Sinon provides a plethora of other features specific to JavaScript including fake timers, fake XMLHttpRequests, and fake servers. To learn more about these features take a look at Sinon’s official documentation.


This is where we tie everything together. Testem is a test runner which can automatically run tests in multiple web browsers and other environments such as Node. By default tests will run when you connect a browser or other environment and can be rerun in all connected environments with the press of a button. By default, tests are also automatically rerun when a change is detected on the file system. Testem is even capable of giving you native system notifications meaning you can just code away in your favorite editor and immediately see test results upon saving without having to do anything.

Testem provides default HTML pages for running and auto-rerunning as well as collecting test results. Setting it up to run tests in Node requires a little bit of set up; however, it is entirely worth it, trust me. Once everything is set up not only are tests run automatically but all results are collected centrally so you can instantly see what happened where. In order to streamline this process I have made a project available on github.

First, you need to install Node. My preferred method is to use nvm but you can set it up however you like.

Next, clone the project down to your local workspace, change into the project directory, and install all dependencies using the following commands:

$ git clone https://github.com/KenPowers/testing-in-browsers-and-node.git
$ cd testing-in-browsers-and-node
$ npm install

Optionally, you can install Testem and Mocha globally (they are both installed locally as a part of the project, but installing them globally will allow you to use them elsewhere):

$ npm install -g testem mocha

Be aware that as of the writing of this article, if you are running Windows then you will need to follow one extra step.

Now we can take a look at how everything works together. First, testem.json:

  "framework": "mocha",
  "src_files": [
  "launchers": {
    "node": {
      "command": "./node_modules/.bin/mocha -r test/setup.js -R tap test/*.test.js",
      "protocol": "tap"
  "launch_in_dev": [

Let’s walk through this file key-by-key. First we have the framework option set to mocha. This tells Testem to load Mocha into all connected web browsers. Next we have a src_files option. These are the files which are monitored for changes as well as served (in order) to all connected web browsers. We explicity list test/setup.js before *.test.js to make sure our setup file is served and executed before the tests. Next, we set up launching tests in Node. This is required because by default Testem just sets up a server for browsers to connect to. All we need to do to get Node working with Testem is get it running in a similar setup that the browsers will have. The command ./node_modules/.bin/mocha -r test/setup.js -R tap test/*.test.js runs Mocha (node) telling it to require the file test/setup.js, use tap as the protocol (so Testem can read the results), and run all files matching the pattern test/*.test.js as tests. If you have Mocha installed globally then you can simply use mocha instead of ./node_modules/.bin/mocha. Finally, launch_in_dev tells Testem to run our Node launcher.

Now we just have to take a look at one section of package.json:

"scripts": {
  "test": "./node_modules/.bin/testem"

With this piece of configuration you can run npm test which will run the local installation of Testem which will read testem.json and start running the tests. Alternatively, if you have testem installed globally then you can just run testem directly.

One last file needs a walk though: test/setup.js:

// Export modules to global scope as necessary (only for testing)
if (typeof process !== 'undefined' && process.title === 'node') {
  // We are in node. Require modules.
  expect = require('chai').expect;
  sinon = require('sinon');
  num = require('..');
  isBrowser = false;
} else {
  // We are in the browser. Set up variables like above using served js files.
  expect = chai.expect;
  // num and sinon already exported globally in the browser.
  isBrowser = true;

This file exports the libraries we are using as global variables. I know what you are thinking: “NO! NOT GLOBAL VARIABLES! GLOBAL VARIABLES ARE EVIL!” The truth of the matter is that when we are testing in multiple environments there are different ways to load these dependencies – we can either do this for each test suite or we can do it once up front. Take your pick. This file additionally sets a boolean value isBrowser which you can use in the tests if you need to test things differently in browsers than you do in Node. You can even set up tests to only run in certain environments by using code like the following:

describe('multi-environment testing', function () {
  if (isBrowser) {
    it('should foo', function () {
      // This test will only run in browsers.
  } else {
    it('should bar', function () {
      // This test will only run in Node.
  it('shold baz', function () {
    // This test will run everywhere.

Take a moment to look through the other files included in the project. There is no need to walk through them here because after reading this article their contents should be fairly intuitive. That being said, let’s do a quick rundown of each file. lib/num.js contains a basic math library. It has a square method, which squares a given number, and a squareRandomNumber method, which depends on a method that asynchronously retrieves a random number. test/num.test.js tests each of the previously mentioned methods, making use of assertions and stubs as necessary.

Go ahead and run either npm test (or testem if you installed globally) now.

Now that Testem is running you’ll see a few things. One thing you’ll see is a tab with the test results for node. Another thing you’ll see is a URL you can visit to connect a web browser for testing. After opening the URL in a few web browsers Testem should look like the following:

Testem Screenshot

By using the left and right arrow keys you can see test results and error messages for all browsers and connected launchers. Pressing enter will rerun tests in all connected environments, and pressing q will quit Testem.

Try making a change to lib/num.js that you think will make one or more of the tests fail. You should see Testem automatically rerun the tests and further see which tests failed in which environments.

Other features in Testem

Testem does more than just run tests in multiple environments very easily. You can specify preprocessors that run before tests run (e.g., compiling CoffeeScript or running a CSS preprocessor), separate source files (monitored for changes) and served files (not monitored for changes), and more (such as the previously mentioned system notifications). To learn about these features take a look at Testem’s documentation.


This article is meant to get you familiar with the basics of multi-environment testing in JavaScript. More articles about each of the above projects featuring more advanced features are in the pipeline. In the meantime, you can read the official documentation of each project here:

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