Allow Async Transforms

[joezimjs]

I'd like to suggest a feature:
Currently, transform scripts (through the transform configuration) must be synchronous. However, there are some transformers out there that must run asynchronously that I'd like to use in my transform scripts, but until Jest supports asynchronous transforms, this isn't possible.

[cpojer]

This is not possible, Jest transforms when a module is being required (like "require('Foo')") which has to be synchronous.

[ryancat]

I have encountered this use case. Basically the test file needs to be compiled as it's written in coffeescript and using google closure library to require/provide dependencies. I managed to use transform with coffee module to compile the source code to javascript, but cannot use webpack to deal with google closure library as the compilation is asynchronous.

I need to either find a sync way to compile webpack, or to find a async way to transform the file.

[SimenB]

#4725

[github-actions]

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