Koa

Koa is a web framework for Node.js designed to be a smaller, more expressive, and robust foundation for building web applications and APIs. It was developed by the Express.js team as an alternative to Express, focusing on a minimal core and a rich ecosystem of middleware rather than bundled features. Koa emphasizes a simple, extensible architecture that encourages explicit control flow and robust error handling.

Architecturally, Koa uses a middleware stack that is composed with asynchronous functions. Each middleware runs in a layered fashion, with the ability to perform actions before and after delegating to the next middleware via a next() call. This onion-like model provides more precise error handling and a clearer flow for asynchronous code using async/await.

In usage, Koa provides a context object (ctx) that encapsulates the request and response, along with situational helpers such as ctx.request, ctx.response, and ctx.state for sharing data across middleware. The framework itself does not impose routing or body parsing; instead, developers commonly add middleware such as a router implementation (eg, koa-router) and a body parser (eg, koa-bodyparser). This modular approach allows teams to assemble a stack tailored to their needs.

Koa is maintained as an open-source project with ongoing updates and community contributions. It remains a popular choice for developers who prefer a lightweight core with explicit middleware composition and modern JavaScript features in Node.js environments.