Creating a basic Express server

Introduction

Express helps you quickly make a web server that listens to requests and sends responses. It makes building websites and APIs simple and fast.

You want to create a simple website that shows pages or data.

You need a backend server to handle form submissions or user logins.

You want to build an API that other apps or websites can use.

You want to test how your frontend talks to a server locally.

You want to learn how web servers work in a friendly way.

Syntax

Express

import express from 'express';

const app = express();

app.get('/', (req, res) => {
  res.send('Hello World!');
});

app.listen(3000, () => {
  console.log('Server is running on http://localhost:3000');
});

express() creates the server app.

app.get(path, handler) listens for GET requests on the path.

Examples

Sends a welcome message when someone visits the home page.

Express

app.get('/', (req, res) => {
  res.send('Welcome to my site!');
});

Creates a new page at /about with a simple text response.

Express

app.get('/about', (req, res) => {
  res.send('About us page');
});

Starts the server on port 4000 instead of the default 3000.

Express

app.listen(4000, () => {
  console.log('Server running on port 4000');
});

Sample Program

This program creates a simple Express server. When you visit http://localhost:3000/ in your browser, it shows "Hello World!". The console logs a message to confirm the server is running.

Express

import express from 'express';

const app = express();

app.get('/', (req, res) => {
  res.send('Hello World!');
});

app.listen(3000, () => {
  console.log('Server is running on http://localhost:3000');
});

Important Notes

Make sure you have Node.js installed to run Express.

Run npm install express before running the server code.

Use Ctrl+C in the terminal to stop the server.

Summary

Express makes it easy to create a web server with just a few lines of code.

You define routes like app.get to respond to web requests.

Start the server with app.listen and visit it in your browser.

Practice

1. What is the main purpose of app.listen() in an Express server?

easy

A. To define a route for GET requests

B. To start the server and listen for incoming requests

C. To import the Express library

D. To send a response to the client

Solution

1. Step 1: Understand the role of app.listen()

   This function starts the Express server and makes it listen on a specified port for incoming requests.

2. Step 2: Compare with other functions

   Functions like app.get() define routes, but do not start the server. Importing Express or sending responses are done elsewhere.

3. Final Answer:

   To start the server and listen for incoming requests -> Option B

4. Quick Check:

   app.listen() starts server [OK]

Hint: Remember: listen means server waits for requests [OK]

Common Mistakes:

• Confusing route definition with server start
• Thinking app.listen() sends responses
• Mixing import statements with server functions

2. Which of the following is the correct way to import Express in a modern Node.js project?

easy

A. const express = require('express');

B. load express from 'express';

C. var express = import('express');

D. import express from 'express';

Solution

1. Step 1: Identify modern import syntax

   In modern Node.js with ES modules, the correct syntax is import express from 'express';.

2. Step 2: Check other options

   require() is CommonJS style, not modern ES modules. var express = import('express'); is invalid syntax. load is not a valid keyword.

3. Final Answer:

   import express from 'express'; -> Option D

4. Quick Check:

   Modern Node.js uses import [OK]

Hint: Use 'import' for modern Node.js, not 'require' [OK]

Common Mistakes:

• Using require() in ES module projects
• Trying to use 'load' keyword
• Incorrect use of dynamic import syntax

3. What will be the output when you visit http://localhost:3000/hello given this Express code?

import express from 'express';
const app = express();
app.get('/hello', (req, res) => {
  res.send('Hello World!');
});
app.listen(3000);

medium

A. Hello World!

B. Cannot GET /hello

C. 404 Not Found error

D. Server error

Solution

1. Step 1: Analyze the route definition

   The code defines a GET route at '/hello' that sends 'Hello World!' as a response.

2. Step 2: Understand server behavior on request

   When visiting /hello, the server matches the route and sends the defined response.

3. Final Answer:

   Hello World! -> Option A

4. Quick Check:

   Defined route sends 'Hello World!' [OK]

Hint: Check if route matches URL path exactly [OK]

Common Mistakes:

• Visiting a route not defined causes 404
• Confusing response text with error messages
• Forgetting to start server with listen()

4. Identify the error in this Express server code:

import express from 'express';
const app = express();
app.get('/', (req, res) => {
  res.send('Welcome!');
});
app.listen();

medium

A. Missing port number in app.listen()

B. Incorrect route path in app.get()

C. res.send() should be res.sendResponse()

D. Express not imported correctly

Solution

1. Step 1: Check app.listen() usage

   The app.listen() method requires a port number to start the server. Here, no port is given.

2. Step 2: Verify other parts

   The route path '/' is valid, res.send() is correct, and Express is imported properly.

3. Final Answer:

   Missing port number in app.listen() -> Option A

4. Quick Check:

   app.listen() needs port [OK]

Hint: Always specify a port in app.listen(port) [OK]

Common Mistakes:

• Omitting port number in app.listen()
• Using wrong method names for response
• Incorrect route paths syntax

5. You want to create an Express server that responds with JSON at /data and listens on port 8080. Which code snippet correctly does this?

hard

A. import express from 'express';
const app = express();
app.get('/data', (req, res) => {
res.send('Hello JSON');
});
app.listen(3000);

B. import express from 'express';
const app = express();
app.post('/data', (req, res) => {
res.send({ message: 'Hello JSON' });
});
app.listen(8080);

C. import express from 'express';
const app = express();
app.get('/data', (req, res) => {
res.json({ message: 'Hello JSON' });
});
app.listen(8080);

D. import express from 'express';
const app = express();
app.get('/data', (req, res) => {
res.json('Hello JSON');
});
app.listen(8080);

Solution

1. Step 1: Check route method and path

   The question asks for a GET route at '/data', so app.get('/data', ...) is correct.

2. Step 2: Verify response and port

   Response should be JSON, so res.json({ message: 'Hello JSON' }) is correct. The server must listen on port 8080.

3. Step 3: Eliminate alternatives with errors

   Alternatives fail due to using POST instead of GET, wrong port like 3000, res.send('string') instead of res.json(object), or res.json('string') instead of object.

4. Final Answer:

   The snippet with app.get('/data'), res.json({ message: 'Hello JSON' }), app.listen(8080) -> Option C

5. Quick Check:

   GET /data + res.json + port 8080 [OK]

Hint: Use app.get + res.json + correct port [OK]

Common Mistakes:

• Using POST instead of GET for route
• Sending string instead of JSON object
• Listening on wrong port number