Handling connection events in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when a client connects to this Express server?

Consider this Express server code. What will be logged when a client connects to the server?

Express

const express = require('express');
const app = express();
const server = app.listen(3000, () => console.log('Server started'));

server.on('connection', (socket) => {
  console.log('New client connected');
});

A The message 'Server started' is logged each time a client connects.

B The message 'New client connected' is logged each time a client connects.

C No message is logged when a client connects because 'connection' event is not supported.

D An error is thrown because 'server.on' is not a valid method.

💡 Hint

The 'connection' event is emitted by the underlying HTTP server when a new TCP connection is made.

The 'connection' event on the server object is triggered each time a client connects. The callback logs 'New client connected'.

📝 Syntax

intermediate

Identify the syntax error in this Express connection event handler

Which option correctly fixes the syntax error in this code snippet?

Express

const express = require('express');
const app = express();
const server = app.listen(3000);

server.on('connection' (socket) => {
  console.log('Client connected');
});

A Replace 'connection' with 'connect': server.on('connect' (socket) => {...})

B Remove the parentheses around socket: server.on('connection', socket => {...})

C Add a comma after 'connection': server.on('connection', (socket) => {...})

D Change 'on' to 'addListener': server.addListener('connection' (socket) => {...})

💡 Hint

Check the syntax of the event listener function parameters.

The event name and callback function must be separated by a comma. Missing comma causes syntax error.

🔧 Debug

advanced

Why does this Express server not log connection events?

This code is intended to log when a client connects, but nothing is logged. What is the cause?

Express

const express = require('express');
const app = express();

app.on('connection', (socket) => {
  console.log('Client connected');
});

app.listen(3000);

A 'app' is an Express application and does not emit 'connection' events; the server object does.

B The 'connection' event is deprecated and no longer works in Express.

C The 'listen' method must be called with a callback to enable events.

D The 'connection' event only fires on HTTPS servers, not HTTP.

💡 Hint

Consider which object emits the 'connection' event in Node.js HTTP servers.

The Express app object does not emit 'connection' events. The underlying HTTP server returned by app.listen does.

❓ state_output

advanced

What is the value of 'activeConnections' after three clients connect and one disconnects?

This Express server tracks active connections. What is the value of 'activeConnections' after three clients connect and one disconnects?

Express

const express = require('express');
const app = express();
const server = app.listen(3000);

let activeConnections = 0;

server.on('connection', (socket) => {
  activeConnections++;
  socket.on('close', () => {
    activeConnections--;
  });
});

// Assume three clients connect, then one disconnects

A 2

B 3

C 1

D 0

💡 Hint

Each connection increments, each close decrements the counter.

Three connections increase activeConnections to 3, one disconnect decreases it to 2.

🧠 Conceptual

expert

Which event should you listen to for detecting when an HTTP request is fully received in Express?

You want to run code when the entire HTTP request body has been received by your Express server. Which event is the best to listen for?

A 'connection' event on the server object

B 'request' event on the server object

C 'data' event on the request object

D 'end' event on the request object

💡 Hint

Think about the event that signals the end of data streaming on a request.

The 'end' event on the request object fires when the full request body has been received.