Implement async code transformations

EcmaScript Modules can (actually must, but easy to wrap the sync stuff in a promise) resolve asynchronously. While it’s in no way a blocker for ESM support in Jest, it would be nice to add support for async transforms to be used.

This logic should for now not impact anything in Jest beyond @jest/transform as Jest wouldn’t use it until ESM support is in place.

I’m thinking we can add a .transformAsync and .transformSourceAsync to the ScriptTransformer class.

There are some checks and verifications that process is a function - we need to make sure there is one clear code path for sync and one for async transformation that verify the provided reporter implements the correct interface.

Sort of reopening #5556 and all its linked issues.

[SimenB]

Gave this some more though when working on #11150 (leaving in an extra async that isn’t needed, but will be when this issue is fixed). And I think it’ll be quite straightforward to implement in jest-runtime once #9889 lands. I’ll do it soonish, definitely before a stable release of Jest 27 even though it shouldn’t be a breaking change. That way people with async transforms have a major release they can target

[SimenB]

That would break current transformers indeed. We can add some sort of “preload” to transformers where they can do whatever they want async so that they’re ready to transform code synchronously (e.g. do import('@babel/core')) like you suggest. We need to keep supporting synchronous transformation for JIT when people use require, but some sort of bootstrapping phase should be fine.

Can you open up a new issue with that feature request? SOrta orthogonal to async transformation