moduleNameMapper with preset and additional config - override order

[shlomiassaf]

Do you want to request a feature or report a bug?
bug (or intended behavior)

What is the current behavior?
When using a preset and an additional configuration (on top of the preset) the moduleNameMapper property is merged from both preset and the additional configuration.
The preset moduleNameMapper is assigned before the user-defined moduleNameMapper.

What is the expected behavior?
User-defined moduleNameMapper should execute before preset defined moduleNameMapper.
The current behaviour traps preset moduleNameMapper values thus not allowing override.

The logic that implements the above can be found here

Switching lines 80 with 81 should solve it.

Please provide your exact Jest configuration and mention your Jest, node, yarn/npm version and operating system.

Node 7.4
Jest 20.0.1
yarn 19

[markwhitfeld]

I am experiencing the exact same issue and found the exact same lines to be the problem.
In particular, my issue is with the jest-preset-angular which provides this in the preset:

  "moduleNameMapper": {
    "(.*)": "<rootDir>/src/$1"
  },

Yip, you aren't going to get past that with the current setup.

Unfortunately switching those 2 lines won't fix it because the preset will always override your configuration if you specify the same regex. What we really need is for our configuration to be tested first but to still be able to override the preset.

I believe the following code change to those lines will do the trick:

options.moduleNameMapper = Object.assign(
    {},
    options.moduleNameMapper,
    preset.moduleNameMapper,
    options.moduleNameMapper);

This will add the settings from options first, then the ones from the preset, but then override with options in case one was overwritten by a preset.

I will submit a pull request for this fix shortly.

[github-actions]

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