Use V8 builtin coverage reporting

[thymikee]

🚀 Feature Proposal

V8 provides a native way of getting code coverage. Recently a c8 project by @bcoe was released to use this feature in Node land. Would be cool to experiment with this edgy tech.

Here's a blog post about it.

Motivation

Make coverage 2-3x faster and babel-free.

[SimenB]

@aaronabramov you had tried this, or just looked at it?

[bcoe]

👋 I'd love to help with this as best I can, there are a few outstanding issues that make the experience a little bit clunky, but shouldn't block a proof of concept:

nodejs/node#22919

A strategy for handling source-maps also needs to be developed. I've been chatting with @jdalton about some of these issues, as they relate to ESM -- perhaps we can aim for a community effort.

[SimenB]

We talked a tiny bit internally last week, and it was the sourcemaps that we figured would be hard part. How much of that is handled in devtools? Or, do you have a write-up about it somewhere?

[bcoe]

@SimenB in nyc, we handle source-maps with a fairly tiny abstraction on top of Mozilla's source-map library.

Some of this might be able to be torn out of nyc ... it might be better to start over with fresh eyes, especially since source-map has switched to an async API (nyc is pinned on the old sync API).

[SimenB]

For coverage I suppose the async api works, but jest is also stuck on the sync api because of test errors which must be collected synchronously. Although I have hope for mozilla/source-map#331

[kwonoj]

for mozilla/source-map#331, I have created POC as discussion mozilla/source-map#369 - while I have low expectation if it could be accepted due to some hacky workaround to achieve sync interfaces. /cc @SimenB for fyi.

[azz]

How can we get the ball rolling on this? Using V8 coverage and using native ES modules would remove the need for Jest to use babel out-of-the box, right?

[SimenB]

You'd still need it to get mock hoisting, but I suppose that's something people can live without.

This was actually unblocked by bcoe/c8#85, so if anyone wants to work on this, that'd be awesome! 🙂

[bcoe]

@SimenB 👋 one issue we still have is source-maps that are injected dynamically, because c8 doesn't hook module.require; I'd love to get the ball rolling on implementing source-maps in V8/Node.js itself.

[SimenB]

Jest controls the source code and manually executes it in a sandbox, so we should be able to inject it still, right?

Built-in sourcemap support in the engine(s) would be absolutely amazing!

[bcoe]

@SimenB I think with some fiddling we should be able to figure out a way to provide the source-map, might take some work in v8-to-istanbul; the main limitation faced by c8, is that it doesn't intercept source-code requires itself (which is a deprecated API any ways).