Closure — Functions That Remember

function makeCounter(start = 0) {
  let count = start;  // private to this call
  return {
    increment() { return ++count; },
    decrement() { return --count; },
    value()     { return count; }
  };
}

const a = makeCounter(10);
const b = makeCounter(0);

a.increment();  // 11
a.increment();  // 12
b.increment();  // 1   — independent state