How to Use Closures in JavaScript

// Basic closure
function outer() {
  let count = 0;
  return function inner() {
    count++;
    return count;
  };
}

const counter = outer();
console.log(counter()); // 1
console.log(counter()); // 2
console.log(counter()); // 3

// Private variables
function createBankAccount(initialBalance) {
  let balance = initialBalance;
  
  return {
    deposit(amount) {
      balance += amount;
      return balance;
    },
    withdraw(amount) {
      if (amount > balance) {
        throw new Error("Insufficient funds");
      }
      balance -= amount;
      return balance;
    },
    getBalance() {
      return balance;
    }
  };
}

const account = createBankAccount(100);
account.deposit(50);
console.log(account.getBalance()); // 150
// console.log(account.balance); // undefined (private)

// Closure in loops
for (let i = 0; i < 3; i++) {
  setTimeout(() => console.log(i), 100);
}
// Output: 0, 1, 2 (let creates new scope each iteration)

// With var (old behavior)
for (var j = 0; j < 3; j++) {
  setTimeout(() => console.log(j), 100);
}
// Output: 3, 3, 3 (var is function-scoped)

// IIFE with closure
const module = (function() {
  let privateVar = "secret";
  
  return {
    getSecret() {
      return privateVar;
    },
    setSecret(value) {
      privateVar = value;
    }
  };
})();

console.log(module.getSecret());
module.setSecret("new secret");

// Memoization with closure
function memoize(fn) {
  const cache = {};
  return function(...args) {
    const key = JSON.stringify(args);
    if (cache[key]) {
      return cache[key];
    }
    const result = fn(...args);
    cache[key] = result;
    return result;
  };
}

const expensiveCalc = memoize((n) => {
  console.log("Calculating...");
  return n * 2;
});

console.log(expensiveCalc(5)); // Calculates
console.log(expensiveCalc(5)); // Cached

// Partial application
function multiply(a) {
  return function(b) {
    return a * b;
  };
}

const double = multiply(2);
const triple = multiply(3);
console.log(double(5)); // 10
console.log(triple(5)); // 15