Long polling as fallback in Node.js - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

Understanding long polling fallback behavior

Consider a Node.js server that uses long polling as a fallback when WebSocket is unavailable. What will the client receive if the server holds the request open until new data arrives?

Node.js

const http = require('http');
let dataAvailable = false;

http.createServer((req, res) => {
  if (req.url === '/poll') {
    if (dataAvailable) {
      res.end('New data');
      dataAvailable = false;
    } else {
      // Hold request open
      setTimeout(() => {
        res.end('No new data');
      }, 5000);
    }
  } else {
    res.end('Hello');
  }
}).listen(3000);

A The client receives 'Hello' regardless of the URL requested.

B The client always receives 'No new data' immediately without waiting.

C The client request times out immediately with an error if dataAvailable is false.

D The client receives 'New data' immediately if dataAvailable is true; otherwise, it waits 5 seconds and receives 'No new data'.

💡 Hint

Think about how the server responds based on the dataAvailable flag and the timeout.

The server checks if dataAvailable is true. If yes, it responds immediately with 'New data'. If not, it holds the request open for 5 seconds before responding with 'No new data'. This simulates long polling fallback behavior.

📝 Syntax

intermediate

Identify the syntax error in long polling implementation

Which option contains a syntax error that would prevent a Node.js long polling server from running?

Node.js

const http = require('http');

http.createServer((req, res) => {
  if (req.url === '/poll') {
    setTimeout(() => {
      res.end('Data');
    }, 3000);
  } else {
    res.end('OK');
  }
}).listen(3000);

A Missing semicolon after setTimeout call.

B Missing curly braces around if block.

C Missing closing parenthesis for setTimeout function.

D Missing callback function in createServer.

💡 Hint

Check the parentheses matching in the setTimeout call.

Option C is correct because the setTimeout call is missing a closing parenthesis, causing a syntax error. Other options are either stylistic or incorrect.

❓ state_output

advanced

What is the output after multiple long polling requests?

Given this server code, what will the client receive on the second poll request?

Node.js

const http = require('http');
let dataQueue = ['first', 'second'];

http.createServer((req, res) => {
  if (req.url === '/poll') {
    if (dataQueue.length > 0) {
      res.end(dataQueue.shift());
    } else {
      setTimeout(() => {
        res.end('no data');
      }, 2000);
    }
  } else {
    res.end('welcome');
  }
}).listen(3000);

A The client receives 'second' immediately on the second request.

B The client receives 'first' again on the second request.

C The client waits 2 seconds and receives 'no data' on the second request.

D The client receives 'welcome' on the second request.

💡 Hint

Think about how dataQueue.shift() changes the queue after each request.

On the first poll, 'first' is removed and sent. On the second poll, 'second' is removed and sent immediately. Only after the queue is empty does the server wait 2 seconds and send 'no data'.

🔧 Debug

advanced

Why does this long polling server never respond?

Examine the code below. Why does the client never get a response?

Node.js

const http = require('http');
let hasData = false;

http.createServer((req, res) => {
  if (req.url === '/poll') {
    if (hasData) {
      res.end('Data ready');
      hasData = false;
    } else {
      // Missing response or timeout here
    }
  } else {
    res.end('Hello');
  }
}).listen(3000);

A Because the hasData variable is never set to true.

B Because the else block for no data does not send any response or end the request.

C Because the server listens on the wrong port.

D Because res.end is called twice in the if block.

💡 Hint

Check what happens when hasData is false.

The else block for when hasData is false does not send any response or close the connection. This causes the client to wait indefinitely, so no response is received.

🧠 Conceptual

expert

Why use long polling as a fallback in real-time apps?

Which reason best explains why long polling is used as a fallback method for real-time communication when WebSocket is unavailable?

A Long polling allows the server to push updates by holding client requests open until new data is available, simulating real-time communication over HTTP.

B Long polling reduces server load by closing connections immediately after sending data.

C Long polling uses WebSocket protocol internally to maintain a persistent connection.

D Long polling requires the client to constantly reconnect without waiting for server data.

💡 Hint

Think about how long polling mimics real-time updates over traditional HTTP.

Long polling keeps the client request open until the server has new data, allowing near real-time updates without WebSocket. It does not use WebSocket internally and does not close connections immediately.

Practice

1. What is the main purpose of using long polling in Node.js applications?

easy

A. To keep a client request open until new data is available

B. To immediately close the connection after sending data

C. To send multiple responses to a single request

D. To replace WebSocket connections entirely

Solution

1. Step 1: Understand long polling behavior

   Long polling keeps the client request open until the server has new data to send.

2. Step 2: Compare options to this behavior

   Only To keep a client request open until new data is available describes this behavior correctly; others describe different or incorrect behaviors.

3. Final Answer:

   To keep a client request open until new data is available -> Option A

4. Quick Check:

   Long polling = open request until data ready [OK]

Hint: Long polling holds request open until data arrives [OK]

Common Mistakes:

• Thinking long polling sends multiple responses per request
• Confusing long polling with WebSocket persistent connection
• Assuming long polling closes connection immediately

2. Which of the following is the correct way to implement a long polling endpoint in Node.js using Express?

easy

A. res.write('data'); // keep connection open without res.end()

B. res.send('data'); res.end();

C. res.json('data'); res.end();

D. res.send('data'); // then wait before closing connection

Solution

1. Step 1: Identify how to keep connection open

   In long polling, the server holds the connection open, so it should not call res.end() immediately.

2. Step 2: Check which option keeps connection open

   res.write('data'); // keep connection open without res.end() uses res.write() without res.end(), which keeps the connection open for long polling.

3. Final Answer:

   res.write('data'); // keep connection open without res.end() -> Option A

4. Quick Check:

   Keep connection open = no res.end() [OK]

Hint: Use res.write() without res.end() to keep connection open [OK]

Common Mistakes:

• Calling res.end() immediately closes connection
• Using res.send() closes connection right away
• Not sending any response causes timeout

3. Consider this Node.js long polling code snippet:

app.get('/poll', (req, res) => {
  setTimeout(() => {
    res.json({ message: 'Update available' });
  }, 5000);
});

What will the client experience when calling this endpoint?

medium

A. The client never receives a response

B. The client receives the update immediately

C. The client waits 5 seconds, then receives the update

D. The client receives multiple updates every 5 seconds

Solution

1. Step 1: Analyze the setTimeout delay

   The server waits 5 seconds before sending the JSON response.

2. Step 2: Understand client wait behavior

   The client request stays open during the 5 seconds, then receives the update once sent.

3. Final Answer:

   The client waits 5 seconds, then receives the update -> Option C

4. Quick Check:

   setTimeout delay = delayed response [OK]

Hint: setTimeout delays response, client waits that time [OK]

Common Mistakes:

• Assuming immediate response despite delay
• Expecting multiple responses from one request
• Thinking client never gets response

4. Given this long polling code snippet, what is the main issue?

app.get('/poll', (req, res) => {
  res.json({ message: 'Update' });
  setTimeout(() => {
    res.json({ message: 'Another update' });
  }, 3000);
});

medium

A. The first res.json() never sends data

B. Calling res.json() twice causes an error

C. setTimeout is not asynchronous

D. The server never responds to the client

Solution

1. Step 1: Understand res.json() behavior

   res.json() sends a response and closes the connection immediately.

2. Step 2: Analyze calling res.json() twice

   Calling res.json() a second time after the connection is closed causes an error.

3. Final Answer:

   Calling res.json() twice causes an error -> Option B

4. Quick Check:

   Multiple res.json() calls = error [OK]

Hint: Only one res.json() per request allowed [OK]

Common Mistakes:

• Thinking setTimeout blocks code
• Assuming multiple res.json() calls send multiple responses
• Ignoring connection close after first response

5. You want to implement long polling as a fallback in a Node.js app that first tries WebSocket. Which approach correctly handles fallback and client reconnection?

hard

A. Use WebSocket and long polling simultaneously, sending duplicate data to client

B. Use WebSocket only; if unavailable, do nothing and drop updates

C. Use long polling only; never try WebSocket to simplify code

D. Use WebSocket; if unavailable, client repeatedly sends GET requests that wait on server until data or timeout, then reconnect

Solution

1. Step 1: Understand fallback strategy

   WebSocket is preferred for real-time, but fallback uses long polling with repeated client requests.

2. Step 2: Identify correct client reconnection method

   The client must send repeated GET requests that wait on server until data or timeout, then reconnect to keep updates flowing.

3. Final Answer:

   Use WebSocket; if unavailable, client repeatedly sends GET requests that wait on server until data or timeout, then reconnect -> Option D

4. Quick Check:

   Fallback = repeated long polling requests [OK]

Hint: Fallback means repeated long polling requests after WebSocket fails [OK]

Common Mistakes:

• Ignoring fallback and dropping updates
• Sending duplicate data by using both methods simultaneously
• Not reconnecting client after long polling timeout