Testing browser extensions

I practise two different ways of testing my browser extensions:

  • Unit tests
  • Integration test


I will use the cross-browser YouTube Lyrics by Rob W extension as an example throughout this answer. The core of this extension is written in JavaScript and organized with AMD modules. A build script generates the extension files for each browser. With r.js, I streamline the inclusion of browser-specific modules, such as the one for cross-origin HTTP requests and persistent storage (for preferences), and a module with tons of polyfills for IE.

The extension inserts a panel with lyrics for the currently played song on YouTube, Grooveshark and Spotify. I have no control over these third-party sites, so I need an automated way to verify that the extension still works well.


During development:

  1. Implement / edit feature, and write a unit test if the feature is not trivial.
  2. Run all unit tests to see if anything broke. If anything is wrong, go back to 1.
  3. Commit to git.

Before release:

  1. Run all unit tests to verify that the individual modules is still working.
  2. Run all integration tests to verify that the extension as whole is still working.
  3. Bump versions, build extensions.
  4. Upload update to the official extension galleries and my website (Safari and IE extensions have to be hosted by yourself) and commit to git.

Unit testing

I use mocha + expect.js to write tests. I don’t test every method for each module, just the ones that matter. For instance:

  • The DOM parsing method. Most DOM parsing methods in the wild (including jQuery) are flawed: Any external resources are loaded and JavaScript is executed.
    I verify that the DOM parsing method correctly parses DOM without negative side effects.

  • The preference module: I verify that data can be saved and returned.

  • My extension fetches lyrics from external sources. These sources are defined in separate modules. These definitions are recognized and used by the InfoProvider module, which takes a query, (black box), and outputs the search results.

    • First I test whether the InfoProvider module functions correctly.
    • Then, for each of the 17 sources, I pass a pre-defined query to the source (with InfoProvider) and verify that the results are expected:
      • The query succeeds
      • The returned song title matches (by applying a word similarity algorithm)
      • The length of the returned lyrics fall inside the expected range.
  • Whether the UI is not obviously broken, e.g. by clicking on the Close button.

These tests can be run directly from a local server, or within a browser extension. The advantage of the local server is that you can edit the test and refresh the browser to see the results. If all of these tests pass, I run the tests from the browser extension.
By passing an extra parameter debug to my build script, the unit tests are bundled with my extension.

Running the tests within a web page is not sufficient, because the extension’s environment may differ from the normal page. For instance, in an Opera 12 extension, there’s no global location object.

Remark: I don’t include the tests in the release build. Most users don’t take the efforts to report and investigate bugs, they will just give a low rating and say something like “Doesn’t work”. Make sure that your extension functions without obvious bugs before shipping it.


  • View modules as black boxes. You don’t care what’s inside, as long as the output matches is expected or a given input.
  • Start with testing the critical parts of your extension.
  • Make sure that the tests can be build and run easily, possibly in a non-extension environment.
  • Don’t forget to run the tests within the extension’s execution context, to ensure that there’s no constraint or unexpected condition inside the extension’s context which break your code.

Integration testing

I use Selenium 2 to test whether my extension still works on YouTube, Grooveshark (3x) and Spotify.

Initially, I just used the Selenium IDE to record tests and see if it worked. That went well, until I needed more flexibility: I wanted to conditionally run a test depending on whether the test account was logged in or not. That’s not possible with the default Selenium IDE (it’s said to be possible with the FlowControl plugin – I haven’t tried).

The Selenium IDE offers an option to export the existing tests in other formats, including JUnit 4 tests (Java). Unfortunately, this result wasn’t satisfying. Many commands were not recognized.

So, I abandoned the Selenium IDE, and switched to Selenium.
Note that when you search for “Selenium”, you will find information about Selenium RC (Selenium 1) and Selenium WebDriver (Selenium 2). The first is the old and deprecated, the latter (Selenium WebDriver) should be used for new projects.

Once you discovered how the documentation works, it’s quite easy to use.
I prefer the documentation at the project page, because it’s generally concise (the wiki) and complete (the Java docs).

If you want to get started quickly, read the Getting Started wiki page. If you’ve got spare time, look through the documentation at SeleniumHQ, in particular the Selenium WebDriver and WebDriver: Advanced Usage.
Selenium Grid is also worth reading. This feature allows you to distribute tests across different (virtual) machines. Great if you want to test your extension in IE8, 9 and 10, simultaneously (to run multiple versions of Internet Explorer, you need virtualization).

Automating tests is nice. What’s more nice? Automating installation of extensions!
The ChromeDriver and FirefoxDriver support the installation of extensions, as seen in this example.

For the SafariDriver, I’ve written two classes to install a custom Safari extension. I’ve published it and sent in a PR to Selenium, so it might be available to everyone in the future: https://github.com/SeleniumHQ/selenium/pull/87

The OperaDriver does not support installation of custom extensions (technically, it should be possible though).
Note that with the advent of Chromium-powered Opera, the old OperaDriver doesn’t work any more.

There’s an Internet Explorer Driver, and this one does definitely not allow one to install a custom extension. Internet Explorer doesn’t have built-in support for extensions. Extensions are installed through MSI or EXE installers, which are not even integrated in Internet Explorer. So, in order to automatically install your extension in IE, you need to be able to silently run an installer which installs your IE plugin. I haven’t tried this yet.

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