Koajs

Koa, also known as Koa.js, is an open-source web framework for Node.js designed to be small, expressive, and robust. It was created by the team behind Express.js to provide a more modern foundation for web applications and APIs. Koa emphasizes unblocking developer experience through a lightweight core and a middleware-based architecture.

The framework uses a stack of asynchronous middleware. Each middleware function receives a context object (ctx) and a next function; invoking await next() passes control to the next middleware in the stack. This approach enables centralized error handling, better flow control, and easier cancellation of downstream handlers.

Koa exposes a unified context (ctx) that exposes request and response properties (ctx.request, ctx.response) as well as conveniences for cookies, headers, and streaming. Rather than formatting its own routing or templates, Koa leaves such concerns to optional middleware, keeping the core minimal.

Versioning has evolved with asynchronous programming. Koa v2 uses async/await for middleware, while early versions used generator-based middleware with a compatibility layer. This aligns with modern Node.js patterns and promotes cleaner code.

Usage and ecosystem: Developers typically create a Koa application by instantiating a new app, registering middleware with app.use, and then hooking the app to an HTTP server. The ecosystem includes routing, body parsing, authentication, and other utilities via separate modules (for example, koa-router, koa-bodyparser). Koa is widely used to build APIs and web services in Node.js projects, valued for its explicit control flow and minimal core.