What is middleware?
- Node.js course
- Intermediate
- Lesson 30/116
- 5 min
Learn the middleware concept and the next function.
What is middleware?
Middleware is a function inserted between a request and a response in Express. Each middleware takes three parameters: req (request), res (response) and next (pass to next). Example: function logger(req, res, next) { console.log(req.url); next(); }.
The next() call passes control to the next middleware or route. If next() is not called the request hangs; no response is ever sent. Middlewares form a chain: each runs in turn and calls next() to move on. This chain is the basis of request handling: you can split logging, authentication, data parsing into separate middlewares.
Middleware signature
A middleware function takes three parameters: (req, res, next). req is the incoming request, res the response, next the function to pass control.
Example logging middleware: function logger(req, res, next) { console.log(req.method + " " + req.url); next(); }. This prints the method and URL on every request and continues the chain with next(). We register it with app.use(logger) so it runs before all routes. If middleware is registered before route definitions it runs on every request; if after, only on routes that follow.
Practice Write the logger middleware
Define logger(req, res, next) with body console.log(req.method + " " + req.url); next();. Add it with app.use(logger). Define the route app.get("/", (req, res) => res.send("Hello")). The server closes itself.
Starter code
const express = require("express");
const app = express();
// Define a middleware named logger
// Print method and url, pass to next
// Add it to the app, respond on the root route
// write here
const server = app.listen(3000, () => {
const http = require("http");
http.get("http://localhost:3000/", (res) => {
let body = "";
res.on("data", (c) => body += c);
res.on("end", () => { console.log("Reply: " + body); server.close(); });
});
});Sample solution
const express = require("express");
const app = express();
function logger(req, res, next) {
console.log(req.method + " " + req.url);
next();
}
app.use(logger);
app.get("/", (req, res) => res.send("Hello"));
const server = app.listen(3000, () => {
const http = require("http");
http.get("http://localhost:3000/", (res) => {
let body = "";
res.on("data", (c) => body += c);
res.on("end", () => { console.log("Reply: " + body); server.close(); });
});
});Expected output
GET / Yanıt: Merhaba
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What does next() do?
function mw(req, res, next) { next(); }- Closes the server
- Passes control to the next one Correct answer
- Sends the response
- Deletes the request
Explanation: next() passes control to the next middleware.
2 Complete the logger body
function logger(req, res, next) {
console.log(req.method + " " + req.url);
____();
}function logger(req, res, next) {
console.log(req.method + " " + req.url);
next();
}Explanation: next() continues the chain.
3 Match each piece to its role
| Term | Meaning |
|---|---|
req |
Incoming request |
res |
Response object |
next |
Pass to next |
app.use |
Registers middleware |