Add "dependencies" in jest transformers

[Schniz]

Hey! 👋
Thanks for all the hard work you're putting in Jest. I love it. So thanks ❤️
I'm not quite sure that what I'm asking for is right, but I hope so - I haven't thought about a different way to solve my problem.

🚀 Feature Proposal

Adding a way to declare code dependencies in jest transform steps, that will eventually help jest to know when to invalidate cache (for babel compilation, etc)

Motivation

Imagine that you use prepack for some of your code and you want to test it. You'd probably write a babel plugin (or a custom transformer) to do that. Unfortunately, prepack, like any other "bundler" with runtime, will leverage dependencies. This means, that the cache that jest has on the file (which is basically its file name) isn't good enough.

It's not just for "prepack" or bundlers, if we'll take a look at @kentcdodds's import-all.macro, we'll see that you can import a directory using importAll('./dir/*.js'). If we can say "this code depends on any file in this directory".

Even when declaring getCacheKey, for some reason the cache doesn't invalidate while in watch mode.

Currently, if you use some kind of "bundler" or any babel plugin/macro like this, you need to use --no-cache in jest.

Example

Currently, a custom jest transformer can compile files using custom steps:

module.exports = {
  process(fileContents, fileName) {
    return compileTheFile(fileContents)
  }
}

I'm suggesting adding a method or data structure that will provide a way to include the dependencies:

module.exports = {
  process(fileContents, fileName) {
    const {code, dependencies} = runPrepackSynchronously(fileName);
    return {code, dependencies: [...dependencies, maybeSomeOtherFilenameHere]}
  }
}

This way jest will know when to invalidate the cache (whenever one of these files changes!)

we can still support the string return value from process for simpler usages!

Pitch

Supporting explicit dependencies will help making our apps more testable. Changes in sass files could be watched in Jest. Prepack, webpack configurations, etc, will be able to be tested too. Wider babel plugins support.

Actually, what I'd like more is a way of sharing these dependencies between babel plugins and jest without writing a custom transformer. but maybe that'll be a PR for babel-jest after landing this feature! 😸

[SimenB]

Does #7313 (and #7349, #7350) cover this use case? It allows you to tell jest-haste-map what dependencies a file has.

It'll be available in the next major of Jest (currently at jest@beta, jest@24.0.0-alpha.9)

[Schniz]

Yes, it sounds like it solves that. Thanks @SimenB 😸

cc @yanivefraim

[SimenB]

Awesome! Closing then 🙂 Happy to reopen if it isn't enough. Docs: https://jestjs.io/docs/en/next/configuration#dependencyextractor-string

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