Handling connection events in Express

Introduction

Handling connection events lets your app know when users connect or disconnect. This helps you manage real-time interactions smoothly.

You want to greet users when they join a chat room.

You need to track how many users are online.

You want to clean up resources when a user leaves.

You want to log connection times for analytics.

Syntax

Express

const express = require('express');
const http = require('http');
const { Server } = require('socket.io');

const app = express();
const server = http.createServer(app);
const io = new Server(server);

io.on('connection', (socket) => {
  // code to run when a user connects
  socket.on('disconnect', () => {
    // code to run when a user disconnects
  });
});

server.listen(3000);

The connection event runs when a user connects.

The disconnect event runs when a user leaves.

Examples

This logs a message every time a user connects.

Express

io.on('connection', (socket) => {
  console.log('A user connected');
});

This logs a message when a user disconnects.

Express

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

This sends a welcome message to the user right after they connect.

Express

io.on('connection', (socket) => {
  socket.emit('welcome', 'Hello!');
});

Sample Program

This program starts a server that listens for users connecting and disconnecting. It logs their connection status and sends a welcome message when they join.

Express

const express = require('express');
const http = require('http');
const { Server } = require('socket.io');

const app = express();
const server = http.createServer(app);
const io = new Server(server);

io.on('connection', (socket) => {
  console.log('User connected:', socket.id);
  socket.emit('welcome', 'Welcome to the server!');

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

server.listen(3000, () => {
  console.log('Server listening on port 3000');
});

Important Notes

Each user connection has a unique socket.id to identify them.

Use connection events to manage real-time features like chat or notifications.

Summary

Connection events let you detect when users join or leave.

Use io.on('connection') to run code on user connect.

Use socket.on('disconnect') to run code on user disconnect.

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