ESLint now turned on for all of the Firefox/Gecko codebase

About 4 years and 2 months ago, Dave Townsend and I landed a couple of patches on the Mozilla codebase that kick-started rolling out ESLint across our source code. Today, I’ve just landed the last bug in making it so that ESLint runs across our whole tree (where possible).

ESLint is a static analyser for JavaScript that helps find issues before you even run the code. It also helps to promote best practices and styling, reducing the need for comments in reviews.

Several Mozilla projects had started using ESLint in early 2015 – Firefox’s Developer Tools, Firefox for Android and Firefox Hello. It was clear to the Firefox desktop team that ESLint was useful and so we put together an initial set of rules covering the main desktop files.

Soon after, we were enabling ESLint over more of desktop’s files, and adding to the rules that we had enabled. Once we had the main directories covered, we slowly started enabling more directories and started running ESLint checks in CI allowing us to detect and back out any failures that were introduced. Finally, we made it to where we are today – covering the whole of the Firefox source tree, mozilla-central.

Along the way we’ve filed over 600 bugs for handling ESLint roll-out and related issues, many of these were promoted as mentored bugs and fixed by new and existing contributors – a big thank you to you all for your help.

We’ve also found and fixed many bugs as we’ve gone along. From small bugs in rarely used code, to finding issues in test suites where entire sections weren’t being run. With ESLint now enabled, it helps protect us against mistakes that can easily be detected but may be hard for humans to spot, and reduces the work required by both developer and reviewer during the code-review-fix cycle.

Although ESLint is now running on all the files we can, there’s still more to do. In a few places, we skipped enabling rules because it was easier to get ESLint just running and we also wanted to do some formatting on them. Our next steps will be to get more of those enabled, so expect some more mentored bugs coming soon.

Thank you again to all those involved with helping to roll out ESLint across the Firefox code-base, this has helped tremendously to make Firefox’s and Gecko’s source code to be more consistently formatted and contain less dead code and bugs. ESLint has also been extremely helpful in helping switch away from older coding patterns and reduce the use of risky behaviour during tests.

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