testing-expert

Run tests serially with --runInBand to expose timing issues, then add proper async/await patterns and mock timers with jest.useFakeTimers. Repeat runs multiple times and use --detectLeaks to catch memory-related intermittency.

Vitest suits new Vite-based projects with modern ESM and faster execution, while Jest fits existing setups with mature ecosystems and snapshot testing. Migration from Jest to Vitest often improves performance.

CI failures usually come from environment differences, race conditions, or resource limits. Standardize environment variables, run with CI=true locally, add retries for flaky tests, and use Docker containers for parity.

Mock only external boundaries like APIs using vi.mock, and reset state with vi.clearAllMocks() in beforeEach hooks. Avoid over-mocking internal logic, which makes tests brittle and coupled to implementation.

A common baseline is 80% for branches, functions, lines, and statements configured in jest.config.js. Focus on behavior and edge-case coverage rather than line counts, since high coverage can hide untested error paths.