JavaScript Basics

// Variables, Scope & Closures
// Hoisting — declarations move to top of scope
console.log(a); // undefined (var is hoisted)
// console.log(b); // ReferenceError (let TDZ)
var a = 1;
let b = 2;

// Scope
function outer() {
  let x = 10; // function scope
  if (true) {
    let y = 20; // block scope
    var z = 30; // function scope (var ignores blocks!)
  }
  // y is NOT accessible here, z IS
}

// Closures — function retains access to outer scope
function createCounter() {
  let count = 0;
  return {
    increment: () => ++count,
    getCount: () => count,
  };
}
const counter = createCounter();
counter.increment(); // 1
counter.increment(); // 2

// IIFE — Immediately Invoked Function Expression
(function() {
  const private = "can't touch this";
})();

// Function Patterns
// Arrow functions — lexical 'this'
const add = (a, b) => a + b;
const greet = name => `Hello ${name}`;

// Default parameters
function createUser(name, role = "viewer") {
  return { name, role };
}

// Rest & Spread
function sum(...nums) { return nums.reduce((a, b) => a + b, 0); }
const merged = { ...obj1, ...obj2 };
const copied = [...array1, ...array2];

// Destructuring
const { name, age, role = "user" } = user;
const [first, second, ...rest] = array;

// Higher-order functions
const doubled = [1, 2, 3].map(n => n * 2);
const adults = users.filter(u => u.age >= 18);
const total = prices.reduce((sum, p) => sum + p, 0);