[Feature]: Support asynchronous createTransformer

[lachrist]

🚀 Feature Proposal

Support asynchronous functions for createTransformer in the transformer API.

Motivation

Support asynchronous operations in createTransformer. In my case, I need to perform some pre/post processing around an existing transformer which is an esm modules.

Example

export const createTransformer = async (options) => {
  const { process } = await import(options.path);
  return {
    process: (content, path, options) => {
      console.log("pre-processing here");
      const result = process(content, path, options);
      console.log("post-processing here");
      return result;
    },
  };
};

Pitch

Currently there is now way to perform asynchronous operations in createTransformer which is needlessly restrictive. Indeed, it seems this would only require but minimal code changes because the createTransformer is already called inside an asynchronous function.

packages/jest-transform/src/ScriptTransformer.ts#L272

transformer = transformer.createTransformer(transformerConfig);

transformer = await transformer.createTransformer(transformerConfig);

[SimenB]

PR very much welcome! As you mention, support should be fairly trivial

[lachrist]

Hi @SimenB thanks for the swift reply.
I will try to get that done today.

Cheers

[SimenB]

https://github.com/facebook/jest/releases/tag/v29.4.0

[github-actions]

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