Supertest is a Node.js library designed for automating API testing by sending HTTP requests and asserting responses, built on the SuperAgent HTTP client, and integrating seamlessly with Express and test runners like Jest or Mocha. It allows developers and QA engineers to validate APIs, catch regressions, and ensure backend reliability before code reaches production. Supertest supports all standard HTTP methods and provides chainable assertions for status codes, headers, and response bodies, with no need for port binding as it handles server startup internally. It is compatible with popular JavaScript testing frameworks, allowing tests to be written using async/await or Promises for clarity. The library enables testing of complex authentication flows, stateful scenarios, file uploads, and schema validations, providing full control over endpoints and assertions. Compared to GUI-based tools like Postman, Supertest is ideal for automated testing in CI/CD pipelines due to its code-driven nature, while still offering comprehensive response validation and integration with additional testing tools for mocking external API calls.