Mlewand blog Thoughts on programming.

Debugging Karma tests with VSCode

tl;dr: I was able to put it as a video on YouTube, however it was my first video, and it started to take bit too much time, so it’s not a super pro one:


After talking with one fellow programmer I decided to get back to my old mouselessNavigator chrome extension. The code was not maintained for couple of years, and it was written in good old ECMAScript 3 spirit.

The extension is working heavily with DOM, and since I’d like to also push it to browsers other than Chrome, it made sense to try out Karma for cross browser local testing. Since I had already some decent unit tests in the project, this meant that I’ll have to port these tests to Karma.

While adjusting tests I found one skipped failing test, and since I was totally unfamiliar with the code base, I decided that the sooner I’ll use debugger, the more benefits I’ll have - as I really, really love VSCode’s debugger. In case you need more information on this see:


All right, so in order to get this going we’ll need three things:

  1. (Obviously) VSCode installed on our desktop.
  2. debugger-for-chrome extension.
  3. (Obviously) a project using Karma.

VSCode Installation

You can grab your installation file at Visual Studio Code home page.

Debugger for Chrome Extension

You can install it simply by pressing ctrl/cmd+p and typing ext install debugger-for-chrome and hit enter.

Karma Project

I’ll use Yeoman Karma generator to create a new project.

So first off install Yeoman and the generator:

npm install -g yo generator-karma

Then make a directory for your project and use generator. Note that I’m explicitly marking that I want to use Karma with Chrome in --browsers argument, otherwise PhantomJS would be installed. To see information on other arguments, see generator-karma docs.

mkdir karma-and-vscode-debugging
cd karma-and-vscode-debugging
yo karma --browsers "Chrome" --app-files "src/**/*.js" --test-files "test/**/*.js" --base-path ".."

Let it create all the necessary files and download the dependencies. Once that’s done you want to open it’s config with VSCode, so just use:

code . test/karma.conf.js


The whole trick is to make Chrome Debugger extension to listen to Chrome instance launched by Karma, using Chrome Debugging Protocol. This is very simple, you just need to make sure you’re using a correct port.

Karma Configuration

First let’s set the port in Karma’s configuration.

Go back to your Karma’s configuration file test/karma.conf.js and add customLaunchers property, like so:

  customLaunchers: {
    ChromeDebugging: {
      base: 'Chrome',
      flags: [ '--remote-debugging-port=9333' ]

Note --remote-debugging-port flag, which sets listening port to 9333.

Now change browsers property value to:

    browsers: [

So that Karma will launcher settings that were specified in listing above.

VSCode Configuration

Press f1 to invoke command toolbox and type > Debug: Open launch.json and press enter. This will open your .vscode/launch.json file.

By default it will be created with some standard/preset content, we’ll override it to keep it clean.

  "version": "0.2.0",
  "configurations": [
      "type": "chrome",
      "request": "attach",
      "name": "Attach Karma Chrome",
      "address": "localhost",
      "port": 9333,
      "pathMapping": {
        "/": "${workspaceRoot}",
        "/base/": "${workspaceRoot}/"

Note how config specifies pathMapping property, as by default Karma will serve your files prefixed with /base/ directory. Actually there’s one lesson I learned while doing this: currently your pathMapping entires must end with tailing slash.


Configuration is done. Now it’s time to use it in practice.

Add a dummy file that will create a global myFunction function.

window.myFunction = function( e ) {
  if ( e >= 10 ) {
    return true;
  } else {
    return false;

Save it as src/myFunction.js. Now let’s create an actual test.

describe( "A suite is just a function", function() {
  it( "and so is a spec", function() {
    let res = myFunction( 15 );

    expect( res ).toBe( true );
  } );
} );

Save it as test/example.js file.

Now run npm test to start watching tests.

It’s Debugging Time!

Enable debugger using either:

Now the fun part! Put a breakpoint in test/example.js file, say at 3rd line. And just press ctrl/cmd+r to refresh browser window attached to the debugger. As it gets to your line it will stop the execution, and let you inspect your variables and do all your debugging business.


I have placed all the files in karma-and-vscode-debugging repository, so you can check it out for full sources.

At this point you might want to customize your paths, as for instance I like to have karma.conf.js file sitting in the root directory. So after moving the file I need to adjust basePath property of karma.conf.js and pathMapping of .vscode/launch.json file. In case you have troubles matching the files use .scripts debugger command for easier troubleshooting.

You also might want to have karma instance automatically started if not running, but I guess I’ll cover it some time later.

comments powered by Disqus