Unit and Integration Testing Front-end Javascript

— 8 minute read

tl;dr= Reference/starter project can be found at github.com/bguiz/front-end-js-testing

Testing NodeJs vs Testing Front-end permalink

When developing a NodeJs application, testing it can be quite straight forward, in the sense that you simply need to to interact one thing: your Javascript modules.

This is fairly easy to do if your test runner or framework is also written in NodeJs - all you have got to do is require() the necessary modules in, anything from an individual function to the entire application, and run your tests against them.

It is not quite so simple with front-end Javascript, because there is an additional thing that you have to deal with: the browser.

It does not suffice to point your test runner or framework at your Javascript modules and expect them to run. Inevitably, they'll need access to window, document, a DOMElement of some variety, or any number of other things that only a browser would give you access to.

Browser-based Test Runners permalink

The solution to the problem above is to use test runners which have browser capabilities. That is, the test runner is able to start and stop browsers that are installed on the local system (or even on remote ones), and run test cases within the browser while they are open.

This way, you can test your front-end Javascript code in an actual browser environment.

In any type of software testing, there are two paradigms: there are unit tests, and there are integration tests. Different browser-based test runners lend themselves more naturally to each of these paradigms, and we'll take a look at those next.

Unit Testing using Karma permalink

Unit testing is sometimes referred to as white-box testing.

It is where you do not concern yourself with the overall application, but rather the individual bits and pieces used to build the application. The focus here is on testing individual methods and functions, and getting very specific on on how each of them perform in isolation. Testing each unit of software, hence the name unit testing.

Karma is a browser-based test runner that is very well suited to doing unit testing.

Integration testing using WebdriverIO permalink

Integration testing is sometimes referred to as black-box testing.

It is where you do not concern yourself with the individual bits and pieces used to build the application, but rather the overall application. The focus here is on testing the application as a whole, similar to how an end-user might interact with your application. Testing the how the entire software works together, hence the name integration testing.

Webdriver.io is a browser-based test runner that us very well suited to doing integration testing.

Starter/ Reference Project permalink

I have put together a trivial front-end Javascript application. It has two buttons, and pressing each one of them results in a different piece of text getting put on the screen. (When I said trivial, I meant trivial).

The intent was, of course, not the application itself, but rather to expose the mechanics of testing it, and to that end, it is sufficient.

The complete project, including the setup and config for both Karma and WebdriverIO, and sample unit tests and sample integration tests are available at: github.com/bguiz/front-end-js-testing

Asynchronous Assertions using Chai permalink

The first thing to note is that in the Javascript world, nearly everything is asynchronous. There quite likely are a mix of callback functions, promises, generator functions, and async functions in most Javascript applications.

For a primer on asynchronous Javascript, check out: Async Ascendance in Javascript

This means that our tests need to take that into account.

For this, in the starter project, we will extend our assertions library, chai, with chai-as-promised, so that we can use expect(somePromise).to.eventually.equal(someValue).

NodeJs Test Runner within a Browser-based Test Runner permalink

Just because we are using browser-based test runners, does not mean that we need to learn a complete new structure for writing our test cases.

Test runner libraries such as Mocha and Jasmine can be included within both Karma and WebdriverIO as a framework, and therefore you can continue writing test cases using syntax that is already familiar.

For this, in the starter project, we will be including Mocha as a framework within both Karma and WebdriverIO.

Front-end Framework Agnostic permalink

It is worth pointing out at this juncture that the set up for this project is intentionally agnostic to the Javascript framework that the application may have been written with.

BackboneJs, AngularJs, EmberJs, ReactJs, et cetera, and all of their accompanying library and tool stacks quite often come bundled with their own testing utilities. Where this is not the case, the community of developers usually has a consensus around their preferred testing utilities.

This project however chooses to work with testing utilities and tools that are not tied to any one particular framework. In theory, they would work just as well with vanilla Javascript applications, as they would with Javascript application built using one of these front-end frameworks.

Set up for the System Under Test permalink

The front-end Javascript application we will be testing in this case in the system under test.

npm install --no-progress
npm run global-install
npm run build-dev

In the directory of the starter project, run the commands above. This sets up both the local and global dependencies, and uses Webpack to bundle the Javascript sources, and uses http-server to serve static files on localhost.

This step does not have anything to do with testing, just to do with the system under test. By all means, use any other set of tools to get this done, as there are a myriad of different way to do this.

Set up and run Karma Unit Testing permalink

Look in package.json, in the devDependencies section, and you'll find these:

"karma": "^0.13.8",
"karma-chrome-launcher": "^0.2.0",
"karma-mocha": "^0.2.0",
"karma-sinon-chai": "^1.0.0",
"karma-webpack": "^1.7.0",

These have already been installed as part of the previous step.

Next look in karma.conf.js, and you'll find this:

files: [

This tells Karma to run any *.spec.js files found in test/unit or its subdirectories.

Finally let us look in this folder, and we see test/unit/sample-unit.spec.js, which contains a couple of unit tests for src/function-to-test.js.

In order to run these unit tests, we can enter the following command:

npm run unit-test-background

Take a look in the scripts section of package.json to see exactly what this does.

A new browser window should open, run through the tests once, and then idle. Switch back to the command prompt, and you will see the test output.

Try modifying the test file, test/unit/sample-unit.spec.js by inserting a new test case, or modifying one that is already there. When you save the file, the tests should re-run, giving you potentially different results.

If we wish for the tests to just run through once, and exit right away, instead of idling, and waiting for some changes, we can run the following command instead:

npm run unit-test

Set up and run WebdriverIO Integration Testing permalink

Take a look in the scripts section of package.json, and you'll find the following lines:

"global-install": "npm i --global selenium-standalone@5.0.0 http-server@0.9.0 && selenium-standalone install",
"start-selenium-server": "selenium-standalone start",

Look in package.json, in the devDependencies section, and you'll find these:

"wdio-mocha-framework": "^0.2.11",
"webdriverio": "^3.4.0",

These commands did various global and local installations. WebdriverIO uses a well known 3rd-party browser testing protocol, the Selenium protocol, in order to do its integration testing. This requires it to have a Selenium server installed and running before running the tests.

To start the Selenium server now, we run:

npm run start-selenium-server

Leave that process running, while we examine the project further.

Open up wdio.conf.js, and you will see:

specs: [

This tells WebdriverIO to run any *.spec.js files found in test/integration or its subdirectories.

Finally let us look in this folder, and we see test/integration/sample-integration.spec.js, which contains a couple of integration tests for the application as a whole.

In order to run these integration tests, we can enter the following command:

npm run integration-test

You should see, similar to when we ran Karma unit tests previously, the browser window open while running through the tests. The results of the tests will be visible in the terminal as well.

The difference that you'll notice, from when running the unit tests, is that we actually see the system under test appearing in the browser, and the browser automatically whirring through each of the steps. Usually this is slow enough for us to see each of the steps briefly, but faster than any human would be able to execute by hand.

Walkthrough permalink

We have just gone through the basics of setting up, configuring, and running both unit tests and integration tests for system under test - in this case also known as "the world's most rudimentary front-end Javascript application".

We have discussed neither the configuration, nor the syntax of the test cases, in detail. That is perhaps something for a follow up post!