Testing is a very important part of the development process. It allows us to verify the functionality of our projects as well as judge the quality of our work.

At Mozilla, we have multiple ways of testing our code, including:

  • Unit tests and integration tests, which are automated tests that verify that pieces of code work as expected.
  • End-to-end tests, automated tests which check the functionality of a project as a whole. For example, simulating clicks in a web browser to test how a site functions.
  • Manual testing, which is performed by a human and involves verifying features work as expected and exploratory tests.

Assessing and managing risk

The end goal of testing is to manage the risk of something going wrong with your project. To this end, one of the first steps you should take is to assess the risk of each area of your project.

More concretely, some parts of your project are going to be more likely to fail than others. Also, some parts of your project are more important than others, and it may be more harmful for them to fail than less important parts.

A risk assessment lists out the different parts of your project (such as certain web pages or parts of an API) and ranks them based on their importance. For example, a news site would rank being able to read existing articles as more important than being able to submit new articles. This kind of ranking allows you to make better decisions about which parts of your project to test more and what kind of tests to run.

Unit and integration tests

As a developer, the most common type of tests you will write are unit tests and integration tests. Unit tests test the smallest possible chunk of functionality and are isolated from each other, whereas integration tests test the interaction between these chunks.

In practice, any changes you make to a project should be tested in some automated way if it’s reasonable to do so. While each project varies, generally Webdev isn’t picky about having perfect unit tests or perfect test isolation. If you’re unsure, look at the project’s existing tests for guidance on the preferred style.

Mozilla uses TaskCluster to run these tests automatically for continuous integration. Other projects rely on Travis CI for executing their tests.

For Django projects, these tests live within the tests module of each included Django application. For Node-based projects, they normally live in a directory named test or tests at the root of the repository. Refer to your project’s documentation for more details.

End-to-end tests

End-to-end tests simulate how your project will be used by end users and verify that it behaves as expected. This is most commonly applied to websites, where we use tools like Selenium to simulate users interacting with the website.

For many sites, these tests are written by Web QA contributors and run against the various server environments.

Manual testing

Manual testing is good old-fashioned human-powered testing, where a living, breathing human uses your project and checks for any errors. Typically this is either for verifying that a new feature works as expected, or for free-form exploratory testing.

In addition to writing automated tests, you almost certainly should be manually testing any changes you make to a project.

Testing tools

The following is a non-exhaustive, possibly-out-of-date list of tools and libraries that may aid you in testing your projects.


  • Jenkins is a continuous integration server that builds and/or tests software projects continuously.
  • Travis CI is a hosted continuous integration service that integrates with GitHub.
  • Selenium is a tool for automating browsers, often for testing purposes.


  • nose is a highly recommended testing library for Python.

  • factory_boy replaces test fixtures with factories that generate test objects easily. It integrates with the Django ORM to generate model instances with a very convenient syntax.

  • Mock is one of the most popular libraries for replacing parts of the system you’re testing with mock objects and asserting things about their behavior.

Node / JavaScript

  • Mocha is a framework for running tests on Node.js and in the browser.
  • Chai is an assertion library with many interfaces to accommodate different testing styles.
  • Karma allows you to execute JavaScript code in multiple real browsers.