Jest Coverage Config Generator

Lines: what percentage of executable code lines were executed during tests. Functions: what fraction of functions were called. Branches: what fraction of conditional branches (if/else, ternary, switch arms) were exercised — this is the hardest to hit 100% on. Statements: similar to lines but counts individual statements, not just lines. Branches coverage is most valuable because untested branches are where bugs hide.

Start with 70-80% for branches and 80-90% for lines/functions/statements. 100% coverage is rarely worth pursuing — it often leads to testing implementation details. A good rule: require 80% global coverage, plus require 100% on critical modules (auth, payments) using per-file thresholds. Add thresholds gradually — start where you are and increase by 5% per quarter.

lcov generates two outputs: a data file (lcov.info) for CI tools like Codecov/SonarQube, and an HTML report in coverage/lcov-report/. html generates only the interactive HTML report (coverage/index.html) without the lcov.info data file. For local development, html is most useful (open in browser). For CI/CD badge integration, use lcov. Use both for full coverage: coverageReporters: ['text', 'lcov'].

Without collectCoverageFrom, Jest only counts coverage for files that are imported during tests — files not touched by any test show 0% but aren't included in totals, giving a falsely optimistic number. With collectCoverageFrom: ['src/**/*.ts'], Jest includes ALL matching files in the report, even untested ones. This gives accurate coverage numbers. Always set this to your source directory for honest reporting.

In a monorepo, run jest from each package with its own jest.config.ts, or use a root jest.config.ts with projects array. For coverage, add coveragePathIgnorePatterns to exclude other packages' files. Use rootDir to scope collectCoverageFrom: ['<rootDir>/src/**/*.ts']. For combined coverage across packages, collect lcov reports from each package and merge with lcov --add-tracefile in CI.

ts-jest is a Jest transformer that compiles TypeScript to JavaScript for Jest without a separate build step. You need it if you write tests in TypeScript and don't use Babel. Install: npm install --save-dev ts-jest @types/jest. Configure preset: 'ts-jest' in jest.config.ts. Alternatively, use babel-jest with @babel/preset-typescript — faster but less type-safe (skips type checking). For Vite projects, consider vitest instead of jest.