fix(jest): stricter regex for 'server-only' in default config

[sbougerel]

NextJest default moduleNameMapper configuration for server-only matches any string that contains this substring, causing mocks of matching dependencies to overwrite each others and the empty mock for import 'server-only';. This changes makes the regex for the module name more strict to ensure only the full string server-only is a match.

Why?

On my project, we thought it'd be a good idea to store our server-only files under the directory @/app/lib/server-only. Unfortunately, this made testing with Jest difficult due to nextJest's default configuration for server-only. Given a test file with:

jest.mock('@/app/lib/server-only/one, () => ({
  fnOne: jest.fn(),
}));
jest.mock('@/app/lib/server-only/two, () => ({
  fnTwo: jest.fn(),
}));

The mocks for the dependency @/app/lib/server-only/two would overwrite the mock of @/app/lib/server-only/one (that was overwriting the empty mock for server-only) and thus fnOne would be undefined.

In our project, we've currently fixed the issue by patching the configuration generated by nextJest using the stricter regexp in this PR:

// ...At the end of jest.config.ts
module.exports = async () => {
  const jestConfig = await createJestConfig(config)();
  if (
    typeof jestConfig.moduleNameMapper !== 'undefined' &&
    'server-only' in jestConfig.moduleNameMapper
  ) {
    const value = jestConfig.moduleNameMapper['server-only'];
    jestConfig.moduleNameMapper['^server-only$'] = value;
    delete jestConfig.moduleNameMapper['server-only'];
  }
  return jestConfig;
};

Verifications

I've checked that pnpm build and pnpm test-unit were unaffected. This is the result of pnpm test-unit:

Test Suites: 199 passed, 199 total
Tests:       2 skipped, 1312 passed, 1314 total
Snapshots:   188 passed, 188 total
Time:        11.697 s
Ran all test suites matching /test\/unit\/|packages\/next\/|packages\/font/i.