Closures in JavaScript

function makeCounter(start = 0) {
  let count = start;  // this variable lives in makeCounter's scope

  return {
    increment() { count++; },
    decrement() { count--; },
    value()     { return count; }
  };
  // makeCounter returns and its stack frame is gone
  // but count survives because the returned object closes over it
}

const counter = makeCounter(10);
counter.increment();
counter.increment();
counter.decrement();
console.log(counter.value()); // 11

// Two independent counters — each has its own count
const a = makeCounter(0);
const b = makeCounter(100);
a.increment();
a.increment();
b.increment();
console.log(a.value()); // 2
console.log(b.value()); // 101 — b has its own count

// The trap — using var
const fns = [];
for (var i = 0; i < 3; i++) {
  fns.push(() => console.log(i));
}
fns[0](); // 3 — not 0!
fns[1](); // 3
fns[2](); // 3
// Why? var has function scope, not block scope.
// All three closures share the SAME i variable.
// By the time they run, i is already 3.

// Fix 1: use let (block-scoped — a new binding per iteration)
const fns2 = [];
for (let i = 0; i < 3; i++) {
  fns2.push(() => console.log(i));
}
fns2[0](); // 0 ✓
fns2[1](); // 1 ✓
fns2[2](); // 2 ✓

// Fix 2: IIFE to capture the current value (pre-ES6)
const fns3 = [];
for (var i = 0; i < 3; i++) {
  fns3.push(((capturedI) => () => console.log(capturedI))(i));
}
fns3[0](); // 0 ✓

function memoize(fn) {
  const cache = new Map();  // closed over by the returned function

  return function(...args) {
    const key = JSON.stringify(args);
    if (cache.has(key)) {
      console.log(`Cache hit for ${key}`);
      return cache.get(key);
    }
    const result = fn(...args);
    cache.set(key, result);
    return result;
  };
}

const expensiveSqrt = memoize((n) => {
  console.log(`Computing sqrt(${n})...`);
  return Math.sqrt(n);
});

console.log(expensiveSqrt(16));  // Computing sqrt(16)... → 4
console.log(expensiveSqrt(16));  // Cache hit for [16]   → 4
console.log(expensiveSqrt(25));  // Computing sqrt(25)... → 5