Closures

• Ask: which variables can the inner function still see after the outer one returns?

Model answer. A closure is a function together with the variables from the scope where it was created. The inner function keeps access to those variables even after the outer function has returned.

function makeCounter() {
  let count = 0;              // private: nothing outside can touch it
  return {
    next: () => ++count,
    reset: () => { count = 0; },
  };
}
const c = makeCounter();
c.next(); c.next();   // 2

Practical uses:

• Private state, as above (data hiding without classes).
• Function factories: const add5 = makeAdder(5).
• Callbacks and event handlers that remember configuration.
• Memoisation caches.

A bug they can cause: closures capture variables, not values. With var in a loop, every callback shares one variable and sees its final value. With let, each iteration gets a fresh binding. Closures can also keep large objects alive longer than expected if a long-lived callback references them.