1 · Structure and rendering · 30 MIN

Express routing and middleware order

Middleware executes in registration order.

Express layers routing and middleware over Node’s HTTP server. A middleware can finish a response or call next to continue. This read-only example adds a request ID before the route runs, then registers a final not-found handler. Ordering matters: a catch-all response placed first would prevent later routes from running. Request IDs help correlate behavior, but do not log credentials or entire request bodies. A client-supplied ID also needs validation if you choose to accept one.

Confirm the request ID is present on success and 404.

Read the example

import express from 'express';
import {randomUUID} from 'node:crypto';
const app=express();
app.use((req,res,next)=>{res.set('X-Request-ID',randomUUID());next()});
app.get('/courses',(req,res)=>res.json([{id:'react',title:'React foundations'}]));
app.use((req,res)=>res.status(404).json({error:'Not found'}));
app.listen(3000,'127.0.0.1');

GET /courses returns one course and an X-Request-ID header. An unknown route returns 404.

Your challenge

Add a route-specific timing middleware and verify it does not interfere with response completion.

Solution cost: O(m) middleware traversal for m layers. time · O(1) request metadata. space

Common trap

Calling next after sending a response can let later middleware attempt a second response.

import express from 'express';
import {randomUUID} from 'node:crypto';
const app=express();
app.use((req,res,next)=>{res.set('X-Request-ID',randomUUID());next()});
app.get('/courses',(req,res)=>res.json([{id:'react',title:'React foundations'}]));
app.use((req,res)=>res.status(404).json({error:'Not found'}));
app.listen(3000,'127.0.0.1');

Further reading: Official documentation