Improve documentation about transformer and moduleNameMapper

🐛 Bug Report

The current documentation about is missing important details about transform and moduleNameMapper which would be crucial for understanding how to configure Jest.

To Reproduce

Steps to reproduce the behavior:

• Navigate to the configuration page

Expected behavior

I expected to understand

• Does moduleNameMapper transformation happens before transform?
• Does transform runs on files matched by moduleNameMapper?
• Does the order of the transform keys matter for the pattern matching? If so
  • Does the file gets apply all the matching transformers or just the first matching as moduleNameMapper works?

Notes

Right now I am trying to add these two transformers

{
  '\\.(js|jsx|ts|tsx)$': require.resolve('./transforms/babel.js'),
   '^(?!\\.(js|jsx|ts|tsx|json))$': require.resolve('./transforms/file.js');
}

Since I don’t have a way to know if all the matching will get applied to the files, I am not sure if I have to ignore everything from the first transformer as I am doing today or match everything that didn’t match before.

[PiotrSiatkowski]

@yordis I think it’s better to check that by experiment. And I think moduleNameMapper takes precedence over transform.

In project, I have ‘identity-obj-proxy’ moduleNameMapper. Tests work.

I have a custom transformer as well, which is broken by design. When moduleNameMapper is turned off it crashes as it should. When I turn on my mapper again, everything works like a charm.

I think this should serve as a proof that these two options are exclusive with moduleNameMapper taking precedence.

[yordis]

Waiting for some confirmation but seems

• Does moduleNameMapper transformation happens before transform? Yes, it happens when Jest try to mock the files.

• Does transform runs on files matched by moduleNameMapper? Yes, it seems that since the transfomer do not know about the moduleNameMapper and viceversa, the transformers will actually transform the files matched by moduleNameMapper