Under-the-hood of test runners (e.g. Mocha)

Test runners are a very important part to the modern JavaScript application. Without which we could not be able to run any tests at all. They are fairly straight forward to understand in terms of what they do, they run your tests and print output. However something which is not as straight forward is how they work and the mechanisms they utilise to enable the command-line interface that we are all so familiar with.

A video for this post can be found here. This is part of my “under-the-hood of” series:

• Web bundlers (e.g. Webpack)
• Type systems (e.g. TypeScript)
• VSCode auto formatters (e.g. Prettier)
• Source maps
• React hooks
• Apollo

I have been a member of the MochaJS core team for a little over 2 years so I feel I have a good understanding how the Mocha test runner works. This post aims to shine a light on how the Mocha test runner works and the codebase design, by walking through a slimmed down version of the tool. The real one is many thousands of lines of code so I have created a slimmed down version which can be found here. All of it has been taken from the real MochaJS codebase, however only the core functionality has been included. Hopefully this will help us focus on and learn about the individual aspects which are critical to its core functionality. It will be a mix of code snippets, links to mochas repository and links to my repository.