feat: bundle Jest's modules with webpack

[SimenB]

Summary

Bundling our modules allows us to improve our startup time by avoiding a bunch of extra I/O. Optimized code also should run faster (in theory), but might be worth it to drop that for readable code in node_modules?

Anyways, there are (at least) 2 outstanding issues.

1. jest-worker needs to pass the path to a file to child_process so it can be executed. I've been unable to make Webpack split out a separate file on disk, then return an absolute path to it. Might need to do very custom stuff just for jest-worker, not sure (if possible, keep just plain babel run for this module for now, if everything else works. Hard to tell since without jest-worker, we cannot run anything at all)
2. There are some places we reach into build on purpose (such as for jest-circus/runner). We need to do something Clever ™️ here. (I haven't bothered thinking too much about as I've swapped between fighting rollup and webpack the last few days).

Any and all help greatly appreciated!

Problem areas: https://github.com/facebook/jest/blob/29534331e2d3d3f70cf153d7e53569d9dd80ecfe/packages/jest-worker/src/workers/NodeThreadsWorker.ts#L61 & https://github.com/facebook/jest/blob/29534331e2d3d3f70cf153d7e53569d9dd80ecfe/packages/jest-worker/src/workers/ChildProcessWorker.ts#L86

Note that worker_threads can receive source code as string, so I wondered if just inlining could work (albeit still a separate bundle), but child_process does not, so doesn't help
Another alternative is to inline as bundled string, then write to disk during load. Feels wrong tho

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Speaking of rollup, I cannot make it understand require, and it also barfs on import thing = require('thing') even though babel is supposed to compile it away. Anyways, let's try webpack first.

Test plan

Green CI (at some point)

[SimenB]

getting webpack to to leave require.resolve alone (as far as I can tell it never behaves like node's builtin one, which is the behavior I want for everything not a local require) is painful and this is spotty. Seems to work fine, tho

[merceyz]

Speaking of rollup, I cannot make it understand require, and it also barfs on import thing = require('thing') even though babel is supposed to compile it away

You might need to enable the transformMixedEsModules option from @rollup/plugin-commonjs
https://github.com/rollup/plugins/tree/9874740ca685b1600e4319365c21e8c1131e9d95/packages/commonjs#transformmixedesmodules

[SimenB]

(just rebased after #12636, I'm not currently planning to work on this)

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[SimenB]

Green! 🎉

[SimenB]

On my machine this shaves off about 10% when running yarn jest packages/expect in this repo.

main:

  Time (mean ± σ):      3.457 s ±  0.351 s    [User: 4.286 s, System: 0.496 s]
  Range (min … max):    3.249 s …  4.429 s    10 runs

This PR:

  Time (mean ± σ):      3.151 s ±  0.060 s    [User: 3.684 s, System: 0.480 s]
  Range (min … max):    3.073 s …  3.249 s    10 runs

Turning on minification seems to improve the results slightly.

  Time (mean ± σ):      3.130 s ±  0.077 s    [User: 3.650 s, System: 0.495 s]
  Range (min … max):    3.019 s …  3.248 s    10 runs

Hard to tell if that's any real improvement or not, tho. And having minification caused quite a bit of tests to fail (I suspect it's due to munging, but not 100% sure) so I'd like to explore that in a separate PR anyways.

[SimenB]

Since we use exports I'm thinking this is not a breaking change. If people wanna use something internal that's not exposed we can revisit (and either revert until Jest 30 or expose whatever it is people are using).

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[liuxingbaoyu]

A full test run on my machine goes from ~88 seconds to ~73 seconds (without transform cache). Promising 🙂

I was investigating another issue and found that it seems debugging is not working, we are not generating source maps for bundled.
I'm not sure if the increased speed has anything to do with it. 🤦‍♂️

To be honest, I'm not sure what I should do.
In theory it would be more convenient to not bundle when developing locally, but this would require keeping both build processes and having the tests accept their line numbers.
Do you have any suggestions?

[SimenB]

Happy to produce sourcemaps, but it breaks some tests with filenames in the snapshots. Probably worth it?

[liuxingbaoyu]

If we also bundle during development, then just emitting the source map all the time should not break tests.

[liuxingbaoyu]

But there is still a problem in this case. Maybe we should not let users load the source map by default, which may cause us to test two situations at the same time.

[SimenB]

Yep.

Debugging is broken due to using workers, I think. #14085 (comment)

[liuxingbaoyu]

I'm not sure if what I'm experiencing is related to this, I've been debugging under multi-threading before this. 🤦‍♂️
While I can put breakpoints in the tests, it doesn't work in the jest source code unless I put the debugger manually.

[SimenB]

I just worked with the debugger now while putting together #14578 - breakpoints in jest sources worked fine. I use --inspect-brk and hit Continue once the debugger connects, maybe that's the difference? No manual debugger statements

[liuxingbaoyu]

Unfortunately I still can't get a break point in the ts file.

[SimenB]

Ah, within the ts file! That probably needs source maps. I always debug in the chrome dev tools, never within the IDE. So haven't noticed 😅

[liuxingbaoyu]

Yeah. VSCode's debugger can automatically attach to child processes. :)

I'll try to get the tests to accept the source map, it shouldn't be too complicated,

[SimenB]

Awesome!

We have some "absolute path to relative path" stuff for stacktraces, and stripping internal frames. That's where I'd assume any issues would arise - beyond that it should be fine 🙂

[SimenB]

One regression: nodejs/cjs-module-lexer#88

In practice, this breaks jest-watch-typeahead.

Unfortunate that TS types don't have a default export of all the things - not sure how to reconcile named exports in type exports with broken named exports in native ESM...

[SimenB]

Fix for the above was to have our own ESM exports, bypassing the lexer entirely: #14661

Released in https://github.com/jestjs/jest/releases/tag/v30.0.0-alpha.1

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