Jest coverage

Version baseline: Jest 30.x, with 30.4.2 as the current stable release at this review. Inspect the installed patch, Node.js support, module system, test environment, and transformer compatibility before changing configuration.

Coverage is evidence about executed code paths, not a substitute for useful assertions. Keep the test scope explicit and separate measurement from any threshold or source-code change.

Collect coverage

Start with the project's existing coverage script. If none exists, forward the flag through the detected package manager:

npm test -- --coverage
yarn test --coverage
pnpm test --coverage
bun run test --coverage

Useful focused variants include:

<project-test-command> path/to/file.test.ts --coverage
<project-test-command> --findRelatedTests src/changed.ts --coverage
<project-test-command> --coverage --collectCoverageFrom='src/**/*.{js,ts,tsx}'

Confirm the resolved coverageDirectory, collectCoverageFrom, exclusions, provider, reporters, and thresholds before interpreting the percentages.

Read the report

Inspect statements, branches, functions, and lines separately. Find the uncovered file and line ranges, then trace each range to a behavior or branch:

• If behavior is untested, add or request a focused test.
• If the file is generated, an entrypoint, or intentionally excluded, verify the repository's exclusion policy before changing it.
• If the run selected too few tests, rerun the relevant suite before concluding that coverage is missing.
• If the threshold fails, report the exact metric and configured threshold.

Coverage thresholds can be global or per path. Never lower one just to make a run green. If the user asks to change thresholds, show the before/after config and run the full gate afterward.

Guardrails

• Do not delete coverage/ or change reporters without an explicit request.
• Do not claim a coverage percentage without naming the scope and config.
• Do not collect coverage with a mocked module and treat the mock as source behavior; explain what was replaced.
• Use the repository's configured coverage provider. Do not switch between babel and v8 to hide a discrepancy.

References

• Jest CLI options
• Jest configuration