Running Headless JavaScript Testing with Electron On Any CI Server

April 23, 2020


This post comes from the work I’ve been working on recently on testing React code.

One issue I got while writing actually unit test code is that some of the code depends on a browser based environment because they need to access to some browser only object or APIs.

The first solution

Most of the unit tests nowdays are running with Nodejs, so in order to emulate a browser environment, jsdom showed up.

A JavaScript implementation of the WHATWG DOM and HTML standards, for use with Node.js.

Here’s a handy snippet that you could use before your testing code to prepare a DOM environment:

import jsdom from 'jsdom'

// This part inject document and window variable for the DOM mount test
export const prepareDOMEnv = (html = '<!doctype html><html><body></body></html>') => {
  if (typeof document !== 'undefined') {
  global.document = jsdom.jsdom(html)
  global.window = global.document.defaultView
  global.navigator = {
    userAgent: 'JSDOM'

And in your test code, you could just import it and use it by calling the fucntion.

import { prepareDOMEnv } from 'jsdomHelper'


If your code depends on some DOM helper function like jQuery, you may also need to include the source code of jQuery into the prepared environment, you could do:

import fs from 'fs'
import jsdom from 'jsdom'
import resolve from 'resolve'

const jQuery = fs.readFileSync(resolve.sync('jquery'), 'utf-8')

jsdom.env('<!doctype html><html><body></body></html>', {
  src: [jQuery]
}, (err, window) => {
  console.log('Voilà!', window.$('body'))
  // your actual test code here.

Notes: in the official jsdom github repo, they give an example of loading jQuery from the CDN which need an additional network request and can be unreliable and not work if without network. They also have an example loading jQuery source code with nodejs fs module but it’s not clean as you have to tell the path to jQuery.

Everthing looks OK so far, but why do we bother to having a real browser environment?

The reason is that once things get compliated, your code may depend on more browser based API. Of course you could fix your code but what if you are using 3rd party moudles from npm, and one of them happen to depends on XMLHttpRequest, it’s nearly impossible to “mock” everything, and to be honest, I feel uncomfortable to do this way as it’s really tricky and kinda dirty.

Let’s run it in a browser

Why not Phantomjs

From the problem we saw above, it’s pretty straight forward to think about to just run all the test in a real browser. If you search “headless browser testing” on Google, the first result will be PhantomJS.

I haven’t used phantomjs a lot and I’m not familar with how it works, but I’ve been heard bad things about it, “lagging behind more and more from what actual web browser do today”, “have 1500+ opened issues on Github”, “unicode encode issue for different language”.

The last concern is actually from my own experince and I mentioned it in my another blog post PDF generation on the web.

Last but not least, I’m not quite confident about how Phantomjs deal with Nodejs code. As my testing code is actually not browser only code, it needs to access to nodjes fs module as well.

Let’s talk about Electron

What is Electron?

Build cross platform desktop apps with web technologies. Formerly known as Atom Shell. Made with <3 by GitHub.

It will be another blog post to explain what is Electron and what it does, I have built a few projects with it and also have written a few blog posts on it, the short version and what it really matters to me is A Nodejs + Chromium Runtime, actively maintained by fine folks from Github and used by Atom editor, Slack etc. And to conclude I’ll quote from one of my favourite JavaScript developer dominictarr

Electron is the best thing to happen to javascript this year. Now we get root access to the browser!

Let’s run our code in browser

Please read the quick start guide and make sure you know how to write your first Electron App.

Since we are not building a real Electron app here but only want to run our JavaScript code in it, there’s a project called browser-run.

You can install it with npm install browser-run and use it like this:

$ echo "console.log('Hey from ' + location); window.close()" | browser-run
Hey from http://localhost:53227/

Run test in Electron

And if you are writting your test with tape, you could even pipe your testing result to a test reporter like faucet

browserify -t babelify test.js | browser-run -p 2222 | faucet

There are also a tool specific designed for tape named tape-run

A tape test runner that runs your tests in a (headless) browser and returns 0/1 as exit code, so you can use it as your npm test script.

With it is even easier to run your test.

browserify -t babelify test.js | tape-run | faucet

Tip: There’s also one module to run mocha test named electron-mocha.

Important notes

As the title indicate, this post is to running testing on any CI server. The reason is that most of the CI server are neither Mac or Windows, and there’s known issue to run Electron on Linux, you need a few setup to make it running on them.

Here’s a few notes copied from the repo and thanks to juliangruber to include my section on running it on gnu/linux there.

To use the default electron browser on travis, add this to your travis.yml:

      - xvfb
  - export DISPLAY=':99.0'
  - Xvfb :99 -screen 0 1024x768x24 > /dev/null 2>&1 &
  - npm install


For gnu/linux installations without a graphical environment:

$ sudo apt-get install xvfb # or equivalent
$ export DISPLAY=':99.0'
$ Xvfb :99 -screen 0 1024x768x24 > /dev/null 2>&1 &
$ browser-run ...

There is also an example docker machine here.

Final step

Once we have all setups ready, our test will be much simpler without need to “hack” a browser like environment:

import test from 'tape'
import React from 'react'
import jQuery from 'jquery'
import { render } from 'react-dom'

test('should have a proper testing environment', assert => {
  const $searchInput = jQuery('input')

  assert.true($searchInput instanceof jQuery, '$searchInput is an instanceof jQuery')


And you can put the test code in npm script and call it on your CI

  // ...
  "scripts": {
    "test": "browserify -t babelify test.js | tape-run | faucet"
   // ...


Voilà! That’s all we needed to running headless JavaScript test on any CI server. Of course your testing environment may different from mine but the idea is there.

As front-end development is changing rapidly recently with things like single page application, isomorphic universal app, also front-end tooling system like npm, brwoserify, babel, webpack, testing will become more complex. I hope these setups will make your life less suck and eaiser.

Last but not least, if you have any questions or better way for testing setups, let us know!