Explain closures with a practical example.

A closure is a function that retains access to its lexical scope even after the outer function has returned.

function makeCounter() {
  let count = 0;
  return {
    increment: () => ++count,
    decrement: () => --count,
    value:     () => count
  };
}
const counter = makeCounter();
counter.increment(); // 1
counter.increment(); // 2

Real-world uses: data encapsulation, factory functions, event handlers with state, memoization, partial application.