`app.use()`

Adds a middleware function (or async generator) to the application middleware stack.

Syntax

app.use(fn)

Parameters

Name	Type	Required	Description
`fn`	`function \| AsyncFunction`	Yes	An async middleware function with signature `async (ctx, next) => {}`. Call `await next()` to pass control to the next middleware.

Returns

Application — The app instance, enabling chaining.

Examples

import Koa from 'koa';

const app = new Koa();

app.use(async (ctx, next) => {
  const start = Date.now();
  await next();
  const ms = Date.now() - start;
  ctx.set('X-Response-Time', `${ms}ms`);
});

app.use(async (ctx) => {
  ctx.body = 'Hello, Koa!';
});

app.listen(3000);

Output

GET / → 200 "Hello, Koa!" with header X-Response-Time: 1ms

// Conditional middleware
app.use(async (ctx, next) => {
  if (ctx.path.startsWith('/api')) {
    ctx.set('Content-Type', 'application/json');
  }
  await next();
});

Output

GET /api/users → Content-Type: application/json

Notes

Middleware is executed in the order it is added (onion model): the first middleware added is the outermost layer. Always `await next()` unless you intend to short-circuit. Koa does not support Express-style `(err, ctx, next)` error middleware; use `app.on('error', handler)` or try/catch instead.