The Tool Desk
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What Koa does
Koa provides a way to compose HTTP middleware in a Node.js application. Middleware can inspect a request, perform work, pass control to another function, and set the response. Koa describes its core as deliberately small and does not bundle middleware, so developers choose the additional packages and structure their application needs.
That makes Koa a framework rather than a ready-made collection of every web feature. The project’s official README documents installation and the runtime requirement; the official guide and API reference explain its application and middleware interfaces.
Install Koa and start a server
Use Node.js 18.0.0 or higher, as specified in the official README. In a project directory, install Koa and create a minimal server:
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npm install koa- Create
app.jswith the following code:const Koa = require('koa'); const app = new Koa(); app.use(async (ctx) => { ctx.body = 'Hello, Koa'; }); app.listen(3000); - Run
node app.js, then openhttp://localhost:3000. The response body should beHello, Koa.
Here, app.use() adds middleware to the application, ctx.body sets the response body, and app.listen() starts an HTTP server. The API reference describes app.listen() as convenience syntax around Node’s HTTP server and Koa’s app.callback().
How Koa middleware works
A Koa middleware function receives (ctx, next). The context, ctx, groups request and response helpers and exposes the underlying Node HTTP objects. Calling and awaiting next() passes execution to downstream middleware. Once that middleware completes, execution resumes after await next(), letting earlier middleware do work on the way back out.
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app.use(async (ctx, next) => {
console.log('before downstream middleware');
await next();
console.log('after downstream middleware');
});
app.use(async (ctx) => {
ctx.body = 'Response set downstream';
});For this request, the first message is logged before the second middleware runs. The second middleware sets the body; then the first resumes and logs its later message. This downstream-then-upstream flow is the defining shape of Koa’s middleware composition. The official middleware guide covers authoring middleware and this asynchronous flow.
What Koa does not include
Koa does not bundle middleware for common application features. You add the packages your application needs—for example, a router for URL-to-handler mapping or a parser for incoming request bodies—and register their middleware in the order the app requires. This leaves the stack flexible, but means a basic Koa installation alone does not provide every feature a larger web application may need.
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Koa or Express?
Koa and Express both serve Node.js web applications, but they use different middleware interfaces and offer different abstractions over Node’s HTTP modules. Traditional Express middleware uses (req, res, next); it is not directly compatible with Koa’s (ctx, next) interface. Choose based on the framework interface and application structure you want, and account for middleware compatibility when adopting or combining packages. The official comparison describes the project’s perspective on the two frameworks.
Upgrading to Koa v3
For an existing application, review the official v2-to-v3 migration guide before upgrading. Koa v3 requires Node.js 18 or higher and removes the older generator middleware signature. The migration guide also addresses changes to ctx.throw() associated with an http-errors update, and support for WHATWG Blob, ReadableStream, and Response response bodies. Which changes matter depends on the application and its middleware.
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