Jest Config Generator

jest.config.ts is the TypeScript version of Jest's configuration file. You need it whenever you want to customize how Jest finds and runs your tests — changing the test environment, adding coverage, mapping path aliases, or configuring transforms. If you're using TypeScript in your project, jest.config.ts gives you full type safety on your Jest configuration via the Config type from 'jest'.

ts-jest is the recommended choice for TypeScript projects. It runs TypeScript files directly using the TypeScript compiler, giving you real type-checking in tests. babel-jest is faster because it strips types without checking them (using @babel/preset-typescript), but it won't catch type errors in your test files. For new TypeScript projects, use ts-jest unless build speed is a bottleneck.

Use jsdom for React component tests — it simulates a browser DOM inside Node.js and is required by @testing-library/react. For React testing with Next.js, the framework configures this automatically. Use the node environment only for pure utility functions, API handlers, or non-DOM code. happy-dom is a faster alternative to jsdom but has less complete browser compatibility.

moduleNameMapper serves two purposes: (1) mapping path aliases like '@/' to their real directories so Jest can resolve imports the same way TypeScript does; (2) mocking non-JS imports like CSS files and images that Jest cannot parse. If your project uses tsconfig.json paths (e.g. '@/*': ['./src/*']), you must mirror those in moduleNameMapper, or use the ts-jest pathsToModuleNameMapper utility.

Add --coverage to your test command (jest --coverage) in CI. Use --coverageReporters=lcov,text-summary to get both machine-readable (lcov) and human-readable output. Set coverageThreshold in jest.config.ts to fail the build when coverage drops below your minimums — this enforces coverage as a quality gate. Upload the lcov.info file to Codecov or Coveralls for PR coverage annotations.

Jest uses CommonJS by default and cannot import ESM packages directly. The fix is to add transformIgnorePatterns to tell Jest to transform specific node_modules: transformIgnorePatterns: ['/node_modules/(?!(your-esm-package)/)'] . For packages like chalk v5, nanoid v3+, or any pure ESM library, add them to the exception list. Alternatively, pin to the last CJS version of the package (e.g. chalk@4).