API Testing Overview API testing validates HTTP endpoints by sending requests and asserting responses, covering status codes, headers, body content, and error handling. Supertest provides a fluent chainable API for integration testing against Express, Fastify, and Hono apps without starting a real server. MSW (Mock Service Worker) v2 intercepts outgoing HTTP requests at the network level, enabling realistic mocking of external services in both Node.js tests and browser environments. When to use: Integration tests for REST APIs, testing middleware pipelines, validating request/response contrac…