Middleware
- Node.js course
- Intermediate
- Lesson 68/116
- 6 min
Add logging and error middleware.
Middleware
The fourth lesson adds middleware: functions that sit between the request and the response. You define one with app.use((req, res, next) => { ... }). It runs on every request; calling next() passes control to the next middleware or route.
A common use is logging: with console.log(req.method, req.url) you print the method and path of every request. For example, when a GET /books request arrives, GET /books is logged. Middleware must be defined BEFORE routes; otherwise the request reaches the route and the middleware is skipped. If next() is not called, the request hangs. So every middleware must call next() or send a response.
Logging middleware
A logging middleware records every request. You define it with app.use((req, res, next) => { console.log(req.method, req.url); next(); }). It runs on all requests.
First console.log prints the method and path, then next() continues the flow. If you forget next(), the request never reaches any route and the client gets no response. Middleware order matters: a middleware added with app.use only affects routes defined AFTER it. So you must place the logging middleware before the routes. This way each request logs first, then runs the route; both monitoring and debugging become easier.
Practice Add a logging middleware
Define a logging middleware. With app.use((req, res, next) => { ... }) print console.log(req.method, req.url) and call next(). The middleware must come BEFORE the routes. The self-request is ready.
Starter code
const books = [];
books.push({ id: 1, title: "Node.js" });
const express = require("express");
const app = express();
app.use(express.json());
// Logging middleware — print method and url, pass to next step
// write here
app.get("/books", (req, res) => {
res.json(books);
});
// Self-request (boilerplate)
const server = app.listen(3000, () => {
const http = require("http");
http.get("http://localhost:3000/books", (res) => {
let body = "";
res.on("data", (c) => body += c);
res.on("end", () => { console.log(body); server.close(); });
});
});Sample solution
app.use((req, res, next) => {
console.log(req.method, req.url);
next();
});Expected output
GET /books
[{"id":1,"title":"Node.js"}]
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What is called to pass control to the next middleware?
next()- res.send()
- return
- app.use()
- next() Correct answer
Explanation: next() passes control to the next middleware or route.
2 Call next
console.log(req.method, req.url); ____();console.log(req.method, req.url); next();Explanation: next() passes the flow to the next step.
3 Match each piece to its role
| Term | Meaning |
|---|---|
app.use |
Adds middleware |
req.method |
HTTP method |
req.url |
Request path |
next() |
Passes to next step |