Testing Node.js APIs with Jest: A Frontend Developer's Guide to Backend Testing

A practical guide for frontend developers familiar with Jest who want to test Express APIs. Covers setting up Jest with the node test environment, using Supertest to fire HTTP requests without starting a real server, database mocking with MongoDB Memory Server and SQLite in-memory, testing JWT authentication middleware, and integrating tests into GitLab CI/CD and GitHub Actions pipelines. Key pitfalls highlighted include always awaiting Supertest calls, separating app.js from server.js to avoid port conflicts, and properly closing database connections in afterAll hooks to prevent CI timeouts.