Browser vs Node.js

// ✅ Works in BROWSER only

// Access HTML elements
const button = document.getElementById('myButton');
button.addEventListener('click', () => {
  alert('Button clicked!');
});

// Change page styles
document.body.style.backgroundColor = 'lightblue';

// Get user location
navigator.geolocation.getCurrentPosition((position) => {
  console.log('Latitude:', position.coords.latitude);
  console.log('Longitude:', position.coords.longitude);
});

// Store data in browser
localStorage.setItem('username', 'John');
const username = localStorage.getItem('username');

// Make HTTP request to API
fetch('https://api.example.com/data')
  .then(response => response.json())
  .then(data => console.log(data));

// ❌ This will ERROR in Node.js
// ReferenceError: document is not defined

// ✅ Works in NODE.JS only

// Read a file from computer
const fs = require('fs');
const fileContent = fs.readFileSync('data.txt', 'utf8');
console.log(fileContent);

// Write to a file
fs.writeFileSync('output.txt', 'Hello from Node.js!');

// Create a simple HTTP server
const http = require('http');
const server = http.createServer((req, res) => {
  res.writeHead(200, { 'Content-Type': 'text/plain' });
  res.end('Hello World from Node.js server!');
});
server.listen(3000, () => {
  console.log('Server running on http://localhost:3000');
});

// Access system information
const os = require('os');
console.log('Platform:', os.platform());
console.log('CPU cores:', os.cpus().length);

// Run in terminal (not browser!)
// $ node server.js

// ❌ This will ERROR in Browser
// ReferenceError: require is not defined

// ✅ Works in BOTH Browser and Node.js

// Variables and data structures
const numbers = [1, 2, 3, 4, 5];
const doubled = numbers.map(n => n * 2);
console.log(doubled); // [2, 4, 6, 8, 10]

// Functions
function calculateTotal(items) {
  return items.reduce((sum, item) => sum + item.price, 0);
}

// Objects and classes
class Person {
  constructor(name, age) {
    this.name = name;
    this.age = age;
  }
  
  greet() {
    return `Hello, I'm ${this.name}`;
  }
}

// Promises and async/await
async function fetchData() {
  // Works with different fetch implementations
  const response = await fetch('https://api.example.com/data');
  return response.json();
}

// Array methods, string operations, math
const text = 'hello world';
const uppercase = text.toUpperCase();
const randomNum = Math.random();

// JSON operations
const user = { name: 'Alice', age: 25 };
const json = JSON.stringify(user);
const parsed = JSON.parse(json);