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Testing Your JavaScript/CoffeeScript, Part 2 of 2: Writing Tests

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In part 1 of this series, you learned why testing your JavaScript or CoffeeScript code is important, and how to set up testing in a Ruby on Rails project. Mark Bates, author of Programming in CoffeeScript, concludes this series by showing how to use Mocha, Chai, and other libraries to write your tests.
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In part 1 of this series, you learned why testing your JavaScript or CoffeeScript code is important. Now Mark Bates, author of Programming in CoffeeScript, shows you how to use Mocha, Chai, and other libraries to write your tests. (Feeling thirsty yet?)

In "Testing Your JavaScript, Part 1 of 2: Setting Up a Test Framework in Ruby on Rails," I showed you how to set up Mocha and Chai in a Ruby on Rails application so you can test your JavaScript or CoffeeScript code.

If you don't use Ruby on Rails, don't worry; this article has very little Rails coverage in it. We're going to focus on using Mocha and Chai, as well as a few other libraries, to test our code. Since that's the case, you should be able to follow along.

Getting Started

To demonstrate testing some "real" code, including DOM interactions, AJAX requests, conditionals, and so on, we'll need some code. I've created a simple Todo List application and placed the source code, along with all of the tests we'll write, on GitHub.com, ready for you to download. After downloading the files, play with the code as you like.

The front-end code for this application was written in CoffeeScript, using the Backbone.js JavaScript framework. We'll be testing this Backbone code.

In part 1 of this series, we set up the testing frameworks and even got a basic test to pass. Now we're going to be testing something more interesting than a simple Greeter class.

The first thing we need to do is to create a file called spec_helper.coffee in the specs/javascripts folder. That file should look something like this:

# Require the appropriate asset-pipeline files:
#= require application
#= require_tree ./support

# Any other testing specific code here...
# Custom matchers, etc....

Konacha.mochaOptions.ignoreLeaks = true

beforeEach ->
  window.page = $("#konacha")

The spec_helper file will be required by all the test files we create, giving all of them access to a common set functionality and code.

Rails alert! This material is strictly related to Rails.

The Konacha gem gives us access to the Rails asset-pipeline. With that, we can require all of our application's JavaScript and CoffeeScript code simply by adding the following line:

#= require application

Next, we'll automatically require any files in the spec/javascripts/support directory (you'll need to create this folder):

#= require_tree ./support

The support directory will hold any third-party scripts (or scripts we write) that are helpful to our tests, but not helpful to the main application.

End of Rails! That's it for the Rails stuff, folks.

The first interesting bit of code you'll notice is the following line:

Konacha.mochaOptions.ignoreLeaks = true

While it might look like an odd thing to do, I've set the ignoreLeaks option to true because it doesn't seem to like when I add properties to the window object, as I'm about to do in the next bit of code. This flag prevents a whole series of warnings and errors from being thrown.

beforeEach ->
  window.page = $("#konacha")

The Mocha library has beforeEach and afterEach hooks that will be executed at their respective times in the test lifecycle. For example, the beforeEach function we've defined here will be run before every test.

In this beforeEach function, I'm defining the window.page property to equal that of the DOM element $("#konacha"). What is this DOM element? It's inserted into the DOM by the Konacha gem to give tests a "playground" DOM. After each test, this element will be wiped clean (think database transactions for the DOM). I like to set it with the window.page property because I feel it makes my tests easier to read. Also, if the DOM element changes names later, I can change it in one place and not have to worry about it.

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