Jest 29 moduleNameMapper CSS mock skipped when CI=true is set

Answer the core question first

In Jest 29 the moduleNameMapper resolver is identical regardless of whether CI=true is set. Jest does not alter the mapping algorithm or the order in which mocks are looked up when the CI flag is present. The failure you see is almost always caused by an environmental difference that coincides with CI, not by the flag itself.

Likely culprits that surface only in CI

• Case-sensitive file system: macOS and Windows use case-insensitive file systems by default, while most CI runners (Linux) are case-sensitive. A mismatch such as styleMock.js vs StyleMock.js will resolve locally but break in CI.
• Missing or ignored mock file: The mock may be omitted from the CI checkout because it’s listed in .gitignore, .dockerignore, or a pipeline artifact exclusion rule.
• Different rootDir or working directory: If the CI job runs from a subdirectory or sets rootDir differently, the relative path in the mapper may point to a non-existent file.
• Stale Jest cache: A stale jest-cache can cause the resolver to skip a fresh mapping. This is more likely when CI runs with a clean build environment but the cache is still present from a previous run.
• Transform or module system differences: If the project uses ESM (NODE_OPTIONS=--experimental-vm-modules) or a custom transformer that behaves differently on CI, the mapper may be bypassed.

How to force the mock unconditionally

1. Keep a single, explicit mapping that covers all stylesheet extensions:

   moduleNameMapper: {
     '\\.(css|less|scss)$': '/__mocks__/styleMock.js'
   }
   

2. Use an absolute path anchored to <rootDir> so the file is found regardless of the current working directory. Avoid relying on the __mocks__ adjacency convention.
3. Verify the mock file’s name and location are committed exactly as referenced. On CI run:

   npx jest --showConfig --json > ci-config.json
   

   and compare the moduleNameMapper and rootDir values with a local run.
4. Clear the Jest cache before each CI run:

   npx jest --clearCache
   

   This guarantees the resolver uses the latest mapping.
5. If you use ESM, add --experimental-vm-modules consistently in both environments or switch to CommonJS for the test files that import CSS.
6. Add a minimal test that imports the CSS file and runs with --no-cache to isolate cache vs. resolution issues:

   npx jest path/to/test.js --no-cache
   

Does jest-environment-jsdom alter stylesheet resolution?

No. The environment only provides DOM globals (window, document, etc.). The moduleNameMapper resolver runs before the environment is instantiated, so it will apply the mock regardless of the environment chosen. If the mock is not applied, the issue lies elsewhere, as listed above.

One diagnostic detail needed

Could you confirm the exact filename and path of styleMock.js in the repository, and whether it appears in the CI build’s file list? A case mismatch or missing file is the most common hidden cause.