Learn how to create and run a web server using Express.js, handle browser requests, send responses, and configure a server port.
An Express server is a web server created with the Express.js framework. It listens for HTTP requests from browsers, mobile applications, or other clients and sends suitable responses.
For example, when a user opens:
http://localhost:3000The browser sends a request to the server. Express processes the request and returns a response.
Sends HTTP request
Processes the request
Receives the response
First, create a project folder.
mkdir express-server
cd express-serverInitialize the Node.js project:
npm init -yInstall Express.js:
npm install express
Now create a file named app.js in the
express-server folder.
const express = require('express');
const app = express();
app.get('/', (req, res) => {
res.send('My First Express Server');
});
app.listen(3000, () => {
console.log('Server running on port 3000');
});require('express') imports Express.js.express() creates the Express application.app.get() handles a GET request to the specified route.req contains information about the incoming request.res.send() sends a response to the client.app.listen(3000) starts the server on port 3000.Open your terminal in the project folder and run:
node app.jsExpected terminal output:
Server running on port 3000Open this URL in your browser:
http://localhost:3000My First Express Server
Ctrl + C in the terminal
to stop the server.
An Express server can respond differently depending on the URL requested by the client.
const express = require('express');
const app = express();
app.get('/', (req, res) => {
res.send('Welcome to the Home Page');
});
app.get('/about', (req, res) => {
res.send('This is the About Page');
});
app.get('/contact', (req, res) => {
res.send('Contact Us');
});
app.listen(3000, () => {
console.log('Server running on port 3000');
});Test these URLs:
http://localhost:3000/http://localhost:3000/abouthttp://localhost:3000/contact
You can use res.send() to send HTML content
to the browser.
const express = require('express');
const app = express();
app.get('/', (req, res) => {
res.send(`
<html>
<head>
<title>My Website</title>
</head>
<body>
<h1>Welcome Student!</h1>
<p>This page is served by Express.js.</p>
</body>
</html>
`);
});
app.listen(3000, () => {
console.log('Server running on port 3000');
});The browser renders the HTML instead of displaying the HTML tags as ordinary text.
JSON is commonly used to exchange data between a server and a website or mobile application.
const express = require('express');
const app = express();
app.get('/student', (req, res) => {
res.json({
id: 101,
name: 'Amit',
course: 'Node.js'
});
});
app.listen(3000, () => {
console.log('Server running on port 3000');
});Open:
http://localhost:3000/studentExpected response:
{
"id": 101,
"name": "Amit",
"course": "Node.js"
}A port is a communication endpoint used by applications on a computer. You can configure Express to listen on a different port.
const express = require('express');
const app = express();
app.get('/', (req, res) => {
res.send('Express server running on port 5000');
});
app.listen(5000, () => {
console.log('Server running on port 5000');
});Visit:
http://localhost:5000When a client requests a URL that does not match any route, you can return a custom 404 response.
const express = require('express');
const app = express();
app.get('/', (req, res) => {
res.send('Home Page');
});
app.use((req, res) => {
res.status(404).send('404 - Page Not Found');
});
app.listen(3000, () => {
console.log('Server running on port 3000');
});
Open http://localhost:3000/unknown
to test the 404 response.
An environment variable allows the port to be configured without hard-coding it in the application.
const express = require('express');
const app = express();
const PORT = process.env.PORT || 3000;
app.get('/', (req, res) => {
res.send('Express server is running');
});
app.listen(PORT, () => {
console.log(`Server running on port ${PORT}`);
});
If PORT is provided by the environment,
Express uses that value. Otherwise, it uses port 3000.
| Error | Possible Cause | Solution |
|---|---|---|
| Cannot find module 'express' | Express is not installed in the project. | Run npm install express. |
| Cannot find module './app.js' | Wrong folder or filename. | Check your current directory and filename. |
| Port already in use | Another process is using the port. | Stop that process or select another port. |
| Cannot GET /about | No matching GET route exists. | Create the route or check the URL. |
Create an Express server with the following requirements:
my-website./about route displaying your course./services route listing three services./student route returning student data as JSON.app.listen(), handle multiple routes, send HTML and
JSON responses, choose a port, and handle unknown routes.
Select the correct answer for each question.