Handling connection events in Express - Mini Project: Build & Apply

Handling connection events

📖 Scenario: You are building a simple Express server that listens for client connections. You want to handle connection events to log when a client connects and disconnects.

🎯 Goal: Create an Express server that listens on port 3000 and logs messages when clients connect and disconnect.

📋 What You'll Learn

Create an Express app instance

Set up a server to listen on port 3000

Handle the 'connection' event on the server

Log a message when a client connects

Log a message when a client disconnects

💡 Why This Matters

🌍 Real World

Servers often need to know when clients connect or disconnect to manage resources or log activity.

💼 Career

Understanding connection events is important for backend developers working with network servers and real-time applications.

1

Create Express app and HTTP server

Create a variable called express that requires the 'express' module. Then create a variable called app by calling express(). Finally, create a variable called server by requiring the 'http' module and calling createServer(app).

Express

const express = require('express');
const app = express();
const http = require('http');
const server = http.createServer(app);
// Your code here

Hint

Use require('express') to import Express. Then call express() to create the app. Use require('http') and createServer(app) to create the server.

2

Set the server to listen on port 3000

Add a line to make the server listen on port 3000 by calling server.listen(3000).

Express

const express = require('express');
const app = express();
const http = require('http');
const server = http.createServer(app);

// Make the server listen on port 3000
// Your code here

Hint

Use server.listen(3000) to start the server on port 3000.

3

Handle the 'connection' event on the server

Add an event listener on server for the 'connection' event using server.on('connection', (socket) => { ... }). Inside the callback, log the message 'A client connected'.

Express

const express = require('express');
const app = express();
const http = require('http');
const server = http.createServer(app);

server.listen(3000);

// Add connection event listener
// Your code here

Hint

Use server.on('connection', (socket) => { ... }) to listen for new connections and log a message inside the callback.

4

Log when a client disconnects

Inside the 'connection' event callback, add a listener on socket for the 'close' event using socket.on('close', () => { ... }). Inside this callback, log the message 'A client disconnected'.

Express

const express = require('express');
const app = express();
const http = require('http');
const server = http.createServer(app);

server.listen(3000);

server.on('connection', (socket) => {
  console.log('A client connected');
  // Add close event listener on socket
  // Your code here
});

Hint

Inside the connection callback, use socket.on('close', () => { ... }) to detect when the client disconnects and log a message.

Practice

1. What does io.on('connection', callback) do in an Express app using Socket.IO?

easy

A. Runs the callback when a user disconnects

B. Runs the callback when the server starts

C. Runs the callback when a new user connects to the server

D. Runs the callback when the server shuts down

Solution

1. Step 1: Understand the event name 'connection'

   The 'connection' event triggers when a new client connects to the server.

2. Step 2: Understand the role of the callback

   The callback runs code to handle the new connection, such as setting up listeners.

3. Final Answer:

   Runs the callback when a new user connects to the server -> Option C

4. Quick Check:

   io.on('connection') = new user connected [OK]

Hint: Remember 'connection' means user joined [OK]

Common Mistakes:

• Confusing 'connection' with 'disconnect'
• Thinking it runs on server start
• Assuming it runs on server shutdown

2. Which of the following is the correct syntax to listen for a user disconnect event on a socket?

easy

A. io.on('disconnect', () => { /* code */ })

B. socket.on('disconnect', () => { /* code */ })

C. socket.emit('disconnect', () => { /* code */ })

D. io.emit('disconnect', () => { /* code */ })

Solution

1. Step 1: Identify the object to listen on

   The disconnect event is listened on the individual socket, not the io server object.

2. Step 2: Use the correct method and event name

   Use socket.on('disconnect', callback) to run code when that socket disconnects.

3. Final Answer:

   socket.on('disconnect', () => { /* code */ }) -> Option B

4. Quick Check:

   socket.on('disconnect') = user left [OK]

Hint: Listen on socket, not io, for disconnect [OK]

Common Mistakes:

• Using io.on instead of socket.on for disconnect
• Using emit instead of on to listen
• Listening disconnect on wrong object

3. What will be logged when this code runs and a user connects then disconnects?

io.on('connection', (socket) => {
  console.log('User connected');
  socket.on('disconnect', () => {
    console.log('User disconnected');
  });
});

medium

A. User connectedUser disconnected

B. User disconnectedUser connected

C. User connected

D. User disconnected

Solution

1. Step 1: When user connects

   The 'connection' event triggers, logging 'User connected'.

2. Step 2: When user disconnects

   The socket's 'disconnect' event triggers, logging 'User disconnected'.

3. Final Answer:

   User connected
   User disconnected -> Option A

4. Quick Check:

   Connect then disconnect logs both messages [OK]

Hint: Connect logs first, disconnect logs second [OK]

Common Mistakes:

• Assuming disconnect logs before connect
• Missing that disconnect logs separately
• Thinking only one message logs

4. Identify the error in this code snippet that tries to log when a user disconnects:

io.on('connection', (socket) => {
  socket.on('disconnect', () => {
    console.log('User disconnected');
  });
});

socket.on('disconnect', () => {
  console.log('Another disconnect');
});

medium

A. The second socket.on('disconnect') is outside the connection handler and socket is undefined

B. The first socket.on('disconnect') should be io.on('disconnect')

C. The disconnect event should be 'disconnected' not 'disconnect'

D. The console.log statements are missing semicolons

Solution

1. Step 1: Check where socket is defined

   Socket is only defined inside the connection callback, so outside it socket is undefined.

2. Step 2: Identify the error

   The second socket.on('disconnect') is outside the connection handler, causing a runtime error.

3. Final Answer:

   The second socket.on('disconnect') is outside the connection handler and socket is undefined -> Option A

4. Quick Check:

   socket undefined outside connection callback causes error [OK]

Hint: Use socket only inside connection callback [OK]

Common Mistakes:

• Trying to use socket outside connection event
• Confusing event names 'disconnect' vs 'disconnected'
• Ignoring scope errors

5. You want to broadcast a message to all connected users except the one who just connected. Which code inside io.on('connection', (socket) => { ... }) correctly does this when a user connects?

hard

A. socket.on('message', 'A new user joined')

B. io.emit('message', 'A new user joined')

C. socket.emit('message', 'A new user joined')

D. socket.broadcast.emit('message', 'A new user joined')

Solution

1. Step 1: Understand broadcast

   socket.broadcast.emit sends to all except the current socket.

2. Step 2: Compare options

   io.emit sends to all including current socket; socket.emit sends only to current socket; socket.on listens, not sends.

3. Final Answer:

   socket.broadcast.emit('message', 'A new user joined') -> Option D

4. Quick Check:

   Broadcast excludes sender, perfect for this case [OK]

Hint: Use socket.broadcast.emit to exclude sender [OK]

Common Mistakes:

• Using io.emit which sends to sender too
• Using socket.emit which sends only to sender
• Confusing emit with on