Scala, Javascript vs Craftsmanship

Integrating Polymer web-component-tester, saucelabs and jenkins

To test polymer components, the recomended way is to use web-component-tester if you want to run your tests on multiple browsers, web-component-tester (wct) includes a saucelabs plugin by default. The problems start when you want to automatically run your tests after each push or after each realease.

At work we use jenkins for continuous integration tasks, we naturally investigated how to automate our polymer tests from it. The saucelabs documentation on how to properly integrate selenium tests started from jenkins is comprehensive but the bit on Setting Up Reporting between Sauce Labs and Jenkins assumes you have access to the webdriver instance:

  • to set the job name and job number you are expected to directly set them in the capabilites

    DesiredCapabilities capabilities = new DesiredCapabilities();
    // ...
    capabilities.setCapability("build", System.getenv("JOB_NAME") + "__" + System.getenv("BUILD_NUMBER"));
  • the let jenkins display the saucelabs tests results you have to make webdriver write a specific line to stdout or stderr

    SauceOnDemandSessionID=<session id> job-name=<some job name>

Neither the capabilities nor the session id are readily accessible in wct or in wct-sauce.

However wct-sauce includes a travis-specific integration to report job name and number. All you have to do to make it work from jenkins is set 2 environment variables TRAVIS_JOB_NUMBER and TRAVIS_JOB_NAME from the corresponding variables in jenkins.

Making webdriver write the correct line on one of the standard outputs is not so easy. You will have to write a wct plugin such as this one :

module.exports = function(wct, pluginOptions, plugin) {
   wct.on('browser-end', function(def, error, stats, sessionId, browser) {
     wct.emit('log:info', 'SauceOnDemandSessionID=' + sessionId + ' job-name=' + process.env.BUILD_NUMBER);

How to write a plugin is almost documented in the wct’s readme. I couldn’t find a reference documentation of the available events and had to go through the code to find the browser-end event published by wct. As I write this, I am pretty sure it is the only event which contains the sessionId or a connected browser.

It is far from perfect : if the test times out or throws, there is a good chance that browser-end will not be emitted.

The next step is configuring the plugin in wct which is inferrable from the readme since plugins is plural it accepts a list of plugins, like this:

  "plugins": {
      "disabled": false,
    "sauce": {
      "disabled": true,
      "browsers": [...

The last step and most frustrating one for me was to get this all to work together. As it happens, just the js code isn’t enough to make a wct plugin. You also need a line in a package.json somewhere. This is documented in the readme adding this was enough:

  // ...
  "wct-plugin": {
      "cli-options": {
  // ...

Before you ask, no I have not tried removing the cli-options item. It’s in the readme and that’s the limit of my confidence and the time I have to waste on this. Feel free to comment if it works for you without.

Of course package.json means an npm module with its own directory (as far as I understand anyway, I’ll be happy to be shown wrong). I placed all this in :

├── lib
│   └── plugin.js
└── package.json

So you need 6 lines of boilerplate in two separate files in their own directory structure to write a line to the standard output. Javascript tooling is amazing. I have no intention of publishing and maintaining npm packages. After a bit more digging I found that if I could put it in the toplevel directory of my project the module it could be picked up by adding this to my project’s package.json’s devDependencies section :


Then I was able to load the module with a require at the top of the guplfile :

var wct_jenkins  = require('wct-jenkins');

The saucelabs results finally got displayed in the jenkins build.

I still feel this was harder than it should have been but at least it works. Any suggestions to simplify the process by a npm,gulp,wct,js specialist are more than welcome.