ts-testing

Unit tests for TypeScript validate individual functions with focused test cases. Use Jest or Vitest with TS-friendly syntax, write behavior-focused tests that verify outputs for given inputs, keep tests fast by avoiding unnecessary mocks, and ensure each test checks one thing so failures are clear and quick to diagnose.

Jest and Vitest are separate test frameworks with different configurations and runners. Choose one framework aligned with your project's existing setup rather than mixing both, as they have distinct module resolution and execution models that can conflict.

Mutation testing validates test quality by introducing code changes and verifying tests catch them. Run mutation tests alongside Jest or Vitest to measure test effectiveness, identify gaps in coverage where mutations survive, and refactor tests to catch more edge cases and regressions.

Debugging failing tests means isolating the test failure, reviewing assertion output, and checking if the implementation matches expected behavior. Use clear failure diagnostics from your test framework, add focused test cases for the specific failure, and refactor code only after confirming the test intention is correct.

TypeScript test suites apply unit tests for isolated functions, integration tests for module interactions, and end-to-end tests for full workflows. Align test types with your repository's existing tooling and typical scenarios—unit tests provide fast feedback, integration tests verify component cooperation, and end-to-end tests catch real-world issues.

Refactor TypeScript code for testability when functions are tightly coupled or hard to isolate, but write tests first to prevent regressions during refactoring. Use failing tests to drive incremental improvements, then implement minimal code changes to pass tests safely.