Koa

Minimal web framework for Node.js that provides a small, expressive foundation for APIs and web applications. Built by the Express team, Koa leverages async functions to replace callbacks and simplify error handling, with zero middleware bundled in core. Applications compose middleware in a cascading, stack-like manner and use a unified context object for request and response handling.

Koa targets developers who want a lean alternative to Express, without bundled routing or body parsing. Its cascading design lets middleware yield downstream and then run logic upstream, which was difficult with Node callbacks but natural with async/await. Unlike Express, Fastify, or NestJS, Koa keeps the core tiny and leaves middleware selection entirely to the developer. The ecosystem includes community packages for routing, body parsing, and session management.

Key capabilities:

• Async-first middleware using ES2017 async functions for clear control flow and error handling
• Cascading middleware execution with downstream and upstream phases
• Unified context object for request, response, and common helpers such as content negotiation and redirection
• No bundled middleware: choose routing, body parsing, and other concerns from the ecosystem
• Proxy and cookie signing support via app settings, with optional AsyncLocalStorage for request-scoped state

Well-suited for REST APIs, microservices, and custom web backends where you prefer explicit middleware composition over an opinionated stack. Fits into existing Node toolchains and can be mounted inside Connect or Express apps when needed.