JavaScript Interview Question: What Is a Closure?

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

function makeCounter() {
  let count = 0;               // captured by the inner function
  return () => ++count;
}

const next = makeCounter();
next(); // 1
next(); // 2  - 'count' survived after makeCounter returned

Why it matters

• Private state: count can't be changed from outside.
• Function factories: const add10 = makeAdder(10).
• Callbacks and event handlers that remember their context.
• React hooks: every render's handlers close over that render's props and state.

Follow-up: the loop question

for (var i = 0; i < 3; i++) setTimeout(() => console.log(i)); // 3 3 3
for (let i = 0; i < 3; i++) setTimeout(() => console.log(i)); // 0 1 2

With var there's one shared i, so all callbacks see its final value. let creates a new binding for each iteration, so each closure captures its own value.

Bonus point

Closures keep captured variables in memory, so a long-lived closure over a large object can cause a memory leak if it's never released (for example, an event listener that's never removed).