jest-babel

Jest and nyc with the Babel plugin

This example uses oxc-coverage-instrument/babel in place of babel-plugin-istanbul, in two test setups:

• Jest with coverageProvider: 'babel' and babel-jest.
• nyc with mocha and @babel/register.

The sources are TypeScript and JavaScript (src/). src/badge.js has JSX in a .js file, as many React projects do. One file, src/report.ts, has no test. Jest collectCoverageFrom and nyc all still report it.

Run it

Build the native binding first (see the repository AGENTS.md), then:

npm install
npm run coverage            # Jest, oxc-coverage-instrument/babel
npm run coverage:istanbul   # Jest, babel-plugin-istanbul
npm run nyc                 # nyc + mocha, oxc-coverage-instrument/babel
npm run nyc:istanbul        # nyc + mocha, babel-plugin-istanbul
npm run verify
npm run typecheck           # the types of oxc-coverage-instrument/babel

npm run verify compares each pair of coverage-final.json files:

• Jest: the two files must be equal byte for byte.
• nyc: the coverage data must be equal. nyc also writes contentHash, a hash of the instrumented code. The two plugins emit different code, so the check ignores that field.

The script also confirms that, in the Jest setup, this plugin instruments the files and babel-plugin-istanbul skips them.

npm run typecheck compiles the files in types/ with module: nodenext. It checks the types of the plugin for a default import from an ES module and for require() from CommonJS.

The configuration

babel.config.js selects the plugin. COVERAGE_PLUGIN=istanbul selects babel-plugin-istanbul, for the comparison runs.

For Jest, the plugin has onlyWithIstanbul: true. babel-jest adds babel-plugin-istanbul only for jest --coverage, so a test run without coverage does not instrument. With coverage, this plugin takes the options of babel-plugin-istanbul, and babel-plugin-istanbul skips the file.

For nyc, the plugin has no options. It reads include, exclude, and extension from the NYC_CONFIG environment variable that nyc sets. The nyc config in package.json sets "instrument": false and "sourceMap": false, because Babel instruments the files.