How does jest --findRelatedTests work under the hood?

Question

Find and run the tests that cover a space separated list of source files that were passed in as arguments. Useful for pre-commit hook integration to run t

Answer 1

I'v been stuck with the same question for the last few days. After digging through the Jest source code, I think I have a pretty good idea of what's going on.

When running --findRelatedTests path/to/src-code.js, the first thing that happens is Jest creates an instance of an internal package, jest-resolve-dependencies. It's a pretty straightforward class with two methods: resolve and resolveInverse.

findRelatedTests calls resolveInverse on the paths you provided, looking up every source and test file that requires your file, in our example path/to/src-code.js. This lookup relies directly on some Jest configuration, specifically roots and/or rootDir, to help resolve paths.

If the found file is a test file, Jest runs it, simple enough. If the found file is a source file, call it found-file.js, then any test files that import found-file.js and the test files that import any of the source files that themselves import found-file.js will be run.

It's a clever implementation of, as the maintainers put it, a resolver of "transitive inverse dependencies". You can see for yourself in this while loop.