Long polling as fallback in Node.js - Mini Project: Build & Apply

Long Polling as Fallback in Node.js

📖 Scenario: You are building a simple chat server that tries to use WebSocket for real-time messages. But if WebSocket is not available, it should use long polling as a fallback to keep the chat messages updated.

🎯 Goal: Create a Node.js server that supports long polling as a fallback method for clients to receive messages when WebSocket is not available.

📋 What You'll Learn

Create a message store array called messages with initial messages

Create a variable called lastMessageId to track the last message ID sent

Implement a long polling route /poll that waits for new messages

Complete the server setup to listen on port 3000

💡 Why This Matters

🌍 Real World

Long polling is used in chat apps and real-time notifications when WebSocket is not supported or blocked by firewalls.

💼 Career

Understanding fallback techniques like long polling helps backend developers build reliable real-time features that work across different network conditions.

1

Create initial message store

Create an array called messages with these exact objects: { id: 1, text: 'Hello' } and { id: 2, text: 'Welcome' }.

Node.js

// Create an array called messages with two message objects
// Your code here

Need a hint?

Use const messages = [ ... ] to create the array with two objects.

2

Add lastMessageId variable

Create a variable called lastMessageId and set it to the id of the last message in the messages array.

Node.js

const messages = [
  { id: 1, text: 'Hello' },
  { id: 2, text: 'Welcome' }
];

// Create lastMessageId variable here
// Your code here

Need a hint?

Use messages[messages.length - 1].id to get the last message's id.

3

Implement long polling route

Using Express, create a GET route at /poll that accepts a query parameter lastId. Inside the route, use a setInterval to check every 1 second if there are new messages with id greater than lastId. When new messages exist, respond with JSON containing those messages and clear the interval.

Node.js

const messages = [
  { id: 1, text: 'Hello' },
  { id: 2, text: 'Welcome' }
];

let lastMessageId = messages[messages.length - 1].id;

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

// Create GET /poll route with long polling logic
// Your code here

Need a hint?

Use app.get('/poll', (req, res) => { ... }) and inside use setInterval to check for new messages.

4

Start the server

Add code to make the Express app listen on port 3000 and log 'Server running on port 3000' when it starts.

Node.js

const messages = [
  { id: 1, text: 'Hello' },
  { id: 2, text: 'Welcome' }
];

let lastMessageId = messages[messages.length - 1].id;

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

app.get('/poll', (req, res) => {
  const lastId = Number(req.query.lastId) || 0;
  const interval = setInterval(() => {
    const newMessages = messages.filter(m => m.id > lastId);
    if (newMessages.length > 0) {
      clearInterval(interval);
      res.json(newMessages);
    }
  }, 1000);
});

// Start server on port 3000
// Your code here

Need a hint?

Use app.listen(3000, () => { console.log('Server running on port 3000') }) to start the server.