morgan

Morgan is an HTTP request logger middleware for Node.js, maintained under the expressjs GitHub organization and used with both Express and plain Node HTTP servers. It logs incoming requests using predefined formats (combined, common, dev, short, tiny) or fully custom format strings built from named tokens.

Morgan supports custom tokens, conditional/skip logic for filtering which requests get logged, and configurable output streams, making it a lightweight, dependency-light choice for request-level observability in Node.js applications.

What You Get

• Predefined log formats (combined, common, dev, short, tiny) matching common web server log conventions
• Custom format strings built from named tokens (:method, :url, :status, :response-time, etc.)
• Support for registering custom tokens to log application-specific request/response data
• Configurable skip functions to conditionally omit logging specific requests
• Pluggable output streams for directing logs to files, external services, or stdout

Common Use Cases

• Logging every incoming HTTP request/response in an Express application for debugging
• Producing Apache-style combined/common access logs for production Node.js servers
• Filtering logs to only capture errors or slow requests via custom skip functions
• Piping request logs into a file or external log aggregator via a custom stream

Under The Hood

Architecture - Morgan is implemented as a single middleware factory function (index.js) that wraps on-finished to hook into the response lifecycle, on-headers to capture response timing before headers are sent, and a token registry (morgan.token()) that resolves format placeholders like :method or :status at log time. Tech Stack - Pure JavaScript with minimal runtime dependencies: basic-auth for the :remote-user token, debug for internal debug logging, depd for deprecation warnings, and on-finished/on-headers for response lifecycle hooks; no build step or TypeScript, published as CommonJS. Code Quality - The project has a single but extensive test suite (test/morgan.js, ~1,800 lines) covering all predefined formats, custom tokens, skip functions, and streaming behavior; linting is enforced via ESLint with the standard config, and coverage is tracked via Coveralls. API Design - The API is a single default export (morgan(format, options)) used directly as Express/Connect middleware, with an extremely low barrier to entry — a one-line app.use(morgan('combined')) covers the common case, while morgan.token() provides an escape hatch for custom fields.