The project's coverage tool

Coverage is measured by a tool tied to the project's language and test framework. This tool is the one the project already uses, found in the project rather than assumed.

Finding it

1. CI: the job that runs the tests with coverage on, and the report it keeps or uploads.
2. The task file and the config: a coverage script or target, a coverage section in the test framework's config, or the tool's own file (.coveragerc, .nycrc).
3. The dependency manifest: the coverage package among the test dependencies.

When these disagree, CI wins. When none of them names a tool, ask a person before adding one.

Using it

• Run the tests with coverage on, through the project's own test command where it can: for example coverage run -m pytest, npx jest --coverage or go test -coverprofile=cover.out ./....
• Write a common report format (LCOV, Cobertura XML), or convert to one, so other tools can read it.
• Measure the changed lines, not only the total: a diff-coverage tool reads the report with the diff from the main branch.
• Keep a floor in the config (fail_under in coverage.py, coverageThreshold in Jest) that only rises.

Examples: coverage.py (Python), c8 and Istanbul (JavaScript), JaCoCo (Java), go test -cover (Go); diff-cover for the changed lines.