koa

Koa is a web framework for Node.js — a runtime that lets you run JavaScript on a server. Like Express (one of the most popular Node.js frameworks), Koa helps you build web applications and HTTP APIs by handling incoming requests and sending responses. The key difference is that Koa was designed from the ground up to use modern JavaScript async/await syntax, making it easier to write code that performs multiple operations in sequence without the callback-heavy patterns that older frameworks required.

Koa is intentionally minimalist: the core library is extremely small (around 570 lines of code) and comes with no built-in middleware. Middleware is the chain of functions that process each incoming request — things like reading request bodies, handling cookies, authentication, logging, and so on. In Koa, these are all separate packages you add as needed, giving you complete control over what your application does. Middleware functions in Koa flow in a stack-like pattern: each function can do work before passing control to the next function, then do more work on the way back after all downstream functions have finished.

Koa provides each middleware function with a context object (usually called ctx) that neatly wraps the request and response together, along with helper methods for common tasks like checking what content types a client accepts or redirecting to another URL.

A developer building a custom HTTP API or web server in Node.js who wants complete control over the middleware stack and prefers modern async syntax over older callback patterns would reach for Koa. The tech stack is JavaScript running on Node.js version 18 or higher.