Karma Is Not Just For AngularJS

The AngularJS team created Karma, but Karma isn’t tied to AngularJS. As a test runner, I can use Karma to run tests against any JavaScript code using a variety of testing frameworks in a variety of browsers. All I need is Node.js.

Pretend I am writing code to represent a point in two dimensional space. I might create a file point.js.

var Point = function(x,y) {
    this.x = x;
    this.y = y;

I’ll test the code using specifications in a file named pointSpecs.js.

describe("a point", function() {
    it("initializes with x and y", function() {
        var p1 = new Point(3,5);

What Karma can do is:

- Provide web browsers with all of the HTML and JavaScript required to run the tests I’ve written.

- Automate web browsers to execute all the tests

- Tell me if the tests are passing or failing

- Monitor the file system to re-execute tests whenever a file changes.


The first step is using npm to install the Karma command line interface globally, so I can run Karma from anywhere.

npm install karma-cli –g

Then I can install Karma locally in the root of my project where the Point code resides.

npm install karma --save-dev

Karma requires a configuration file so it knows what browsers to automate, and which files I’ve authored that I need Karma to load into the browser. The easiest way to create a basic configuration file is to run karma init from the command line. The init command will walk you through a series of questions to create the karma.conf.js file. Here is a sample session.

>karma init 
Which testing framework do you want to use ?
Press tab to list possible options. Enter to move to the next question.
> jasmine 
Do you want to use Require.js ?
This will add Require.js plugin.
Press tab to list possible options. Enter to move to the next question.
> no 
Do you want to capture any browsers automatically ?
Press tab to list possible options. Enter empty string to move to the next question.
> PhantomJS
> Chrome
What is the location of your source and test files ?
You can use glob patterns, eg. "js/*.js" or "test/**/*Spec.js".
Enter empty string to move to the next question.
> js/**/*.js
> specs/**/*.js
Should any of the files included by the previous patterns be excluded ?
You can use glob patterns, eg. "**/*.swp".
Enter empty string to move to the next question.
Do you want Karma to watch all the files and run the tests on change ?
Press tab to list possible options.
> yes 
Config file generated at "C:\temp\testjs\karma.conf.js".

I can open the config file to tweak settings and maintain the configuration in the future. The code is simple.

module.exports = function(config) {
    basePath: '',
    frameworks: ['jasmine'],
    files: [
    exclude: [
    preprocessors: {
    reporters: ['progress'],
    port: 9876,
    colors: true,
    logLevel: config.LOG_INFO,
    autoWatch: true,
    browsers: ['PhantomJS', 'Chrome'],
    singleRun: false

Note that when I enter the location of the source and test files, I want to enter the location of the source files first. It’s helpful if you can organize source and test files into a directory structure so you can use globbing (**) patterns, but sometimes you need to explicitly name individual files to control the order of loading.

At this point I can start Karma, and the tests will execute in two browsers (Chrome, which I can see, and PhantomJS, which is headless). I’d probably tweak the config file to only use Phantom in the future.

AngularJS Abstractions: Scope

The parameter to the controller function is named $scope.

var AboutController = function($scope) {
    // ...    

The name $scope is important since it allows the AngularJS dependency injector to know what type of object you are asking for, in this case an object that will contain the view model. Any plain old JavaScript you attach to $scope (properties, functions, objects, arrays) is eligible to use from the expressions inside the view.

In the last post our model was relatively “flat” in the sense that properties and functions were added directly to $scope:

$scope.rabbitCount = 2;
$scope.increase = function() {
    $scope.rabbitCount *= $scope.rabbitCount;

This allowed us to use rabbitCount and increase() in the markup that was inside the view scope (the div) of the AboutController:

<div data-ng-controller="AboutController">
    <div>Number of rabbits in the yard: {{rabbitCount}}</div>
    <button ng-click="increase()">More rabbits</button>

You can think of $scope as the execution context for the expressions in the view. Saying ng-click=”increase” results in a call to $scope.increase. The only thing tricky to understand about $scope is that having a controller inside a controller, or a controller inside an application (which you’ll always have), will result in nested $scopes, and a nested $scope will prototypally inherit from it’s parent scope by default. This is why $scope is injected by angularJS – the framework sets up the prototype chain before giving your controller the $scope object to use as a model.

Inheritance means a view has access to it’s own scope as well as any inherited scope. In the following example, the view inside the ChildController markup can use an expression like {{rabbitCount}}, and this expression will read the rabbitCount property of AboutController’s scope ($scope.rabbitCount will follow the prototype chain).

<div data-ng-controller="AboutController">    
    <div>Number of rabbits in the yard: {{rabbitCount}}</div>
    <div data-ng-controller="ChildController">
        Number of rabbits in the yard: {{rabbitCount}}
        Number of squirrels in the yard: {{squirrelCount}}

The one place to be careful with the inherited scope is with 2 way data binding. The way JavaScript prototypes work is that writing to the rabbitCount property of the ChildController $scope will add a rabbitCount property to the ChildController $scope and effectively hide the parent property. $scope.rabbitCount no longer needs to follow the prototype chain to find a value. More details and pictures on this scenario in “The Nuances of Scope Prototypal Inheritance”.

AngularJS Abstractions: Filters

Filters in AngularJS can form a pipeline to format and adapt model data inside a view. The built-in filters for AngularJS are date (format a date), filter (select a subset of an array), currency (format a number using a currency symbol for the current locale), and many more. I say filters can form a pipeline because you place them inside expressions in a view using a pipe symbol. The canonical example of a filter is the filter filter, which in the following markup filters a list of items according to what the user types in the search input (try it live here).

<input type="search" placeholder="Filter" ng-model="searchTerm"/>    
<ul ng-repeat="item in items | filter:searchTerm | limitTo:maxItems">

Notice the output of filter is also piped to a limitTo filter, which restricts the number of items in the list to maxItems (where maxItems would need to be defined in the model).

Custom Filters

Custom filters only require a filter function to be registered with Angular. The filter function returns a function that will perform the actual work. For example, a plurify filter to add the letter s to the output:

(function() {
    var plurify = function() {
        return function(value) {
            return value + "s";
           .filter("plurify", plurify);

Now plurify is available to use in a view:

<ul ng-repeat="item in items | filter:searchTerm | limitTo:maxItems">
    <li>{{item | plurify }}</li>

A filter can also take one or more optional parameters.

var plurify = function() {
    return function(value, strength) {          
       strength = strength || 1;
       return value + Array(strength + 1).join("s");

Now we can add 5 s characters to the output:

<li>{{item | plurify:5 }}</li>

The beauty of filters is how you can package up small pieces of testable code to make the job of the model and view a bit easier, and the syntax to use a filter is easy and intuitive.