Incorrect behaviour when using Jest's Array type config (collectCoverageFrom)

I don’t know whether this is the expected behaviour or not, however i just report this in case someone face the same problem…

in my package.json

  "jest": {
    "collectCoverageFrom": [
      "src/components/**/*.{js,jsx}"
    ]
  }

after apply react-app-rewired, the collectCoverageFrom become [ 'src/components/**/*.{js,jsx}', 'src/**/*.{js,jsx,mjs}' ], which is not the default behaviour of react-scripts, as it won’t include the jest default 'src/**/*.{js,jsx,mjs}' .

[dawnmist]

This has occurred due to a combination of two things.

First - most jest configuration options are not permitted by react-scripts. How react-app-rewired works around this is it takes a copy of all your defined jest options from package.json, and then passes into react-scripts everything except your jest options (i.e. it deletes the jest options part). This means that react-scripts isn’t seeing the overrides for even the few fields it does permit overrides for.

React-app-rewired then merges any array or object values defined in package.json with the original values from react-scripts instead of overwriting them. I don’t know the history of why it was done this way rather than overriding them.

What you can do to make an override setting instead of a setting to be merged is to apply the setting inside the jest function in config-overrides, which is run after the merge has occurred. This lets you make a full replacement instead of a merge replacement.

module.exports = {
  jest: (config) => {
    config.collectCoverageFrom = ["src/components/**/*.{js,jsx}"];
    return config;
  }
}

[hardyscc]

Thank you very much, IMO the purpose of react-app-rewired is to temporary modify the unsupported config, and remove it once react-scripts is supported… so I think it shouldn’t break the default behaviour of any untouched config unless it has a bigger reason.