Support Object transformer for transform and moduleNameMapper configs

🚀 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.

[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

[github-actions[bot]]

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