Add compile() method to complement process() for transformers

🚀 Feature Proposal

Add support for a compile() like method to complement process() for transformers.

Motivation / Pitch

I’m writing an esm loader and trying to make it work as the ESM support layer for Jest (without using Babel). I found I could make esm provide a process() method and be used as a "transform": "esm" which is great! However, esm anchors its runtime to the module object created and ran through the wrapper(). Normally this isn’t a problem, since esm owns the implementation it handles this in Module#compile or Module#_extensions. However, Jest constructs modules from scratch so there is no place for esm to hook into and decorate the module object. A hook like compile or init or beforeCompile would be one way of enabling this.

Related to https://github.com/facebook/jest/issues/4842#issuecomment-377134208.

[thymikee]

Ok, cool! I’m just not sure if we squeeze this in for v24. But since this is non breaking, I guess we can give it some more time.

[SimenB]

Ah, that’s pretty neat!

@cpojer @aaronabramov do you have any thoughts on the API here? I would still like for this to be possible to use together with e.g. Babel, but if this is an OK first go we should do it.

We could also go crazy and have an --esm flag so it’s provided out of the box (like --coverage is). Would be coupled to this implementation, but I think that’s fine?