app.listen
- Node.js course
- Intermediate
- Lesson 28/116
- 5 min
Learn to start the server with app.listen.
app.listen
app.listen(port, callback) starts the server listening on a given port. Until this call is made, the Express application does not accept requests. The second parameter is a callback called when the server starts.
Example: app.listen(3000, () => console.log("Server started"));. This starts listening on port 3000 and prints Server started when ready. The app.listen callback usually prints a confirmation message, so you know the server is ready. Common ports in development are 3000, 8000, 8080; in production 80 (HTTP) or 443 (HTTPS). Express calls http.createServer behind the scenes; you do not deal with it directly.
Starting the server
After app.listen is called, Express listens for incoming requests on the specified port. Each request is matched against your routes and the appropriate handler is called.
Example: const server = app.listen(3000, () => console.log("Server started"));. app.listen returns a server object; you can assign it to a server variable. With server.close() you stop the server. This is useful in tests or temporary servers. During development the server keeps running; you stop it from the terminal with Ctrl+C. Even with many routes, a single app.listen starts the whole application.
Practice Start the server
Import express and create app. Define a route with app.get("/", (req, res) => res.send("Running")). Start the server with const server = app.listen(3000, () => console.log("Server started")). The ready-made code below sends a request and prints the result.
Starter code
// Import express, create app and add a "/" route
// write here
// Start the server on port 3000, callback prints "Server started"
// write here
setTimeout(() => {
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(); });
});
}, 100);Sample solution
const express = require("express");
const app = express();
app.get("/", (req, res) => res.send("Running"));
const server = app.listen(3000, () => console.log("Server started"));
setTimeout(() => {
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(); });
});
}, 100);Expected output
Sunucu başladı Yanıt: Çalışıyor
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Complete the app.listen call
app.____(3000, () => console.log("Sunucu başladı"));app.listen(3000, () => console.log("Sunucu başladı"));Explanation: app.listen starts the server.
2 Match each piece to its role
| Term | Meaning |
|---|---|
app.listen |
Starts the server |
port (3000) |
Port number |
callback |
Called when server starts |
server.close |
Stops the server |
3 What is the second parameter of app.listen?
app.listen(3000, ?)- A port number
- Bir rota yolu
- A module name
- A callback called when the server starts Correct answer
Explanation: The second parameter is a callback.