JavaScript

const name = 'Aditya';        // constant
let age = 17;                   // reassignable
// var is legacy  avoid it

// Data Types
'string'   // String
42         // Number
true       // Boolean
null       // Null
undefined  // Undefined
{}         // Object
[]         // Array

// Traditional
function greet(name) {
  return `Hello, ${name}!`;
}

// Arrow function
const greet = (name) => `Hello, ${name}!`;

// Default parameters
const add = (a, b = 0) => a + b;

const nums = [1, 2, 3, 4, 5];

nums.map(n => n * 2);           // [2, 4, 6, 8, 10]
nums.filter(n => n > 3);        // [4, 5]
nums.reduce((sum, n) => sum + n, 0); // 15
nums.find(n => n === 3);        // 3
nums.forEach(n => console.log(n));
nums.includes(3);               // true
nums.some(n => n > 4);          // true
nums.every(n => n > 0);         // true

// Spread & destructuring
const copy = [...nums];
const [first, ...rest] = nums;

// Select elements
const el = document.querySelector('.my-class');
const all = document.querySelectorAll('.items');

// Modify
el.textContent = 'New text';
el.innerHTML = '<strong>Bold</strong>';
el.classList.add('active');
el.classList.toggle('hidden');
el.style.color = 'red';

// Events
el.addEventListener('click', (e) => {
  console.log('Clicked!', e.target);
});

// Async / Await
async function getUser() {
  const response = await fetch('https://api.example.com/user');
  const data = await response.json();
  return data;
}

// With error handling
async function fetchData(url) {
  try {
    const res = await fetch(url);
    if (!res.ok) throw new Error(`HTTP ${res.status}`);
    return await res.json();
  } catch (err) {
    console.error('Fetch failed:', err);
  }
}