Koa is a web framework for Node.js created by the team behind Express. Its documentation describes it as “a new web framework designed by the team behind Express, which aims to be a smaller, more expressive, and more robust foundation for web applications and APIs.” First released in 2013, Koa was a deliberate attempt to rethink the Express model in light of newer JavaScript language features, especially better tools for managing asynchronous control flow.
The central change is how middleware is written. Express middleware relies on callbacks and a “next” function passed through a linear chain. Koa instead builds its middleware around async functions, letting code use familiar try/catch error handling and await rather than nesting callbacks. The Koa site emphasizes that the framework leverages async functions “to ditch callbacks and greatly increase error-handling,” and its middleware flows in a stack-like manner that lets a function perform actions downstream and then manipulate the response upstream as control returns.
Koa is even more minimal than Express. The documentation notes that “Koa does not bundle any middleware within its core,” instead providing an elegant set of methods for building servers and leaving routing, body parsing, and everything else to separately installed modules. This keeps the framework’s core extremely small and unopinionated, pushing nearly all functionality out into the surrounding ecosystem of plugins.
Koa also cleans up the request and response abstractions. Rather than extending Node’s raw request and response objects in place, Koa wraps them in its own context object that bundles request and response together and adds convenience accessors. This gives middleware a single, consistent handle to the HTTP transaction and avoids some of the inconsistencies that came from augmenting Node’s native objects directly.
Though it never displaced Express in raw popularity, Koa was influential as a demonstration of how async functions could reshape server programming in Node, and its context-and-stack model informed how later Node frameworks thought about middleware.