Headless Firefox with Karma


I recently started revisiting some vue.js code that I was working on at work and when I ran the unit-tests all of them failed with pretty much the same error.

actual.should.equal is not a function. (In 'actual.should.equal(expected)', 'actual.should.equal' is an instance of ProxyObject)

I'm using the chai BDD 'should' style of testing, so this should have worked (because it was working before). To debug it I ran it with FireFox and when I did, all the tests mysteriously started working. It turns out I was using PhantomJS, which has been abandoned, and somewhere along the line something in chai stopped working with it, so I decided to set up Firefox headless as my browser instead. This is how I did it.

Set Up

Install the plugin

I already had this installed for debugging, but I'll mention it anyway, for completeness.

npm install karma-firefox-launcher --save-dev

Set it up in Karma

I'm running the tests using karma and I had hoped that I would just have to change the name of the browser in the configuration, but it turns out to need a little setup in the karma.conf.js file. First get rid of the PhantomJS reference. Delete this:

/home/athena/.virtualenvs/necromuralist.github.io/bin/python3: No module named virtualfish
browsers: ['PhantomJS'],

and replace it with this.

browsers: ['FirefoxHeadless'],

The FirefoxHeadless browser isn't actually defined, so you have to add it (I put it right after the browsers declaration).

customLaunchers: {
      'FirefoxHeadless': {
        base: 'Firefox',
        flags: [

This tells karma to run firefox and pass in the -headless flag, as shown in the Mozilla documentation about firefox's headless mode. I found out how to do it from a question on Stack Overflow.


PhantomJS breaks the Chai's BDD style of testing, but since both Firefox and Chrome have headless modes, using them instead makes it work without actually needing to have the GUI launch.