Closure là gì? Cho ví dụ thực tế.

Closure là hàm 'nhớ' được biến từ lexical scope bên ngoài, ngay cả sau khi hàm ngoài đã return.

function makeCounter() {
  let count = 0;
  return () => ++count;
}
const c = makeCounter();
c(); // 1
c(); // 2

Hàm trả về vẫn truy cập count.

Ứng dụng thực tế: private variables (encapsulation), factory functions, event handlers giữ state, debounce/throttle, React hooks (useState bên trong dùng closure).

A closure is a function that 'remembers' variables from its outer lexical scope, even after the outer function has returned.

function makeCounter() {
  let count = 0;
  return () => ++count;
}
const c = makeCounter();
c(); // 1
c(); // 2

The returned function still accesses count.

Practical uses: private variables (encapsulation), factory functions, event handlers holding state, debounce/throttle, React hooks (useState internally uses closures).