Support Object transformer for transform and moduleNameMapper configs

[yordis]

🚀 Feature Proposal

Allow specifying a JavaScript object that represents the transformer on transform and moduleNameMapper configuration.

Motivation

I created a factory function that creates some Jest configurations. In the JavaScript section, I have something like this

transform: {
  '\\.(js|jsx|ts|tsx)$': require.resolve('./transforms/babelTransform.js'),
}

Because the transforms are set up by specifying a file I lose the factory configuration when I call the function.

const createJestConfigFactory = (opts)=> {
  return {
    transforms: {
       // this is not a function call so I lose the scope for `opts`
       '\\.(js|jsx|ts|tsx)$': require.resolve('./transforms/babelTransform.js')
    }
  }
};

Example

An example of how this could be fixed:

const transformCreator = (opts)=> {
  return {
    // this object is a transformer
    process: ()=> {
      // ...
    }
  };
};

const createJestConfigFactory = (opts)=> {
  return {
    transforms: {
       // this is not a function call so I lose the scope for `opts`
       '\\.(js|jsx|ts|tsx)$': transformCreator(opts)
    }
  }
};

Pitch

Adding features like this one give more flexibility and power to advanced configurations.

[yordis]

Worth to mention, I need this feature for this transformer https://github.com/straw-hat-team/jest-config/blob/a8a02da278df3169273345546fb669beb405f6c2/transforms/fileTransform.js#L8-L22

I want to be able to specify if I am in React ecosystem or not so I could ignore that transform.

[SimenB]

Specifying a transform directly instead of pointing to a module Jest requires seems like a great idea! We need to make sure it's not an issue when serializing the config to and from workers, though

[yordis]

@SimenB for my personal knowledge, what do you mean by this?

We need to make sure it's not an issue when serializing the config to and from workers, though

I don't understand the underline architecture and I would like to if you don't mind to point me to some documentation about it or elaborate a little bit more about it, please.

[yordis]

Definitely, the workers are an issue right now. 😢

[jharris4]

Hi, I just made a PR with a similar feature to what was discussed here: #7398

I was able to get the config to support transforms as functions, and added unit tests to show that it works.

That said, I'm not sure what this issue with workers is, and whether I need to address anything related to that... ?

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