const and let

Use const for constants and let for variables. Avoid var entirely in modern code.

const PI = 3.14159;    // Constant
let count = 0;         // Variable
count = 1;             // ✓ Allowed
PI = 3;                // ✗ Error

Arrow Functions

// Traditional
const add = function(a, b) { return a + b; };

// ES6 Arrow
const add = (a, b) => a + b;
const greet = name => `Hello, ${name}`;

Destructuring

// Objects
const { name, age } = user;

// Arrays
const [first, second, ...rest] = numbers;

// Parameters
function greet({ name, greeting = "Hello" }) {
  return `${greeting}, ${name}`;
}

Spread and Rest

// Spread
const merged = [...arr1, ...arr2];
const copy = { ...obj, newProp: value };

// Rest
function sum(...numbers) {
  return numbers.reduce((a, b) => a + b, 0);
}

Conclusion

ES6+ features make JavaScript cleaner and more powerful. Master these fundamentals for modern development.