nock

Nock is an HTTP server mocking and expectations library for Node.js that intercepts outgoing HTTP requests at the network level and returns predefined responses. Unlike approaches that mock the HTTP client library itself, Nock overrides Node.js's http.request and https.request functions, intercepting all HTTP traffic regardless of which HTTP client library (axios, node-fetch, got, superagent) is used to make the request. Nock allows you to define interceptors that match requests by URL, HTTP method, request headers, query parameters, and request body, then respond with specific status codes, headers, and response bodies. The library supports recording real HTTP interactions and replaying them in tests, simulating network errors and timeouts, defining sequential responses for the same endpoint, and setting up persistent interceptors for frequently called endpoints. Nock is essential for unit testing code that makes HTTP requests, ensuring tests are fast, deterministic, and independent of external services. The library's matching system supports exact strings, regular expressions, and custom filter functions for flexible request matching.