A closure is a function bundled together with references to the variables from its surrounding (lexical) scope. Because of closures, an inner function keeps access to its outer function's variables even after the outer function has returned.
() {
count = ;
{
: ++count,
: count,
};
}
a = ();
b = ();
a.(); a.();
b.();
count is not accessible from outside — only inc/get can see it. That's data privacy built from a closure. And each call to makeCounter() creates a fresh, isolated scope.
for (var i = 0; i < 3; i++) {
setTimeout(() => console.log(i), 0); // prints 3, 3, 3 — all share one `i`
}
for (let i = 0; i < 3; i++) {
setTimeout(() => console.log(i), 0); // prints 0, 1, 2 — `let` is per-iteration
}
With var, every callback closes over the same i (which is 3 by the time they run). let creates a new binding each iteration.
Closures power module privacy, memoization, currying, event handlers/callbacks that remember state, and are exactly how React hooks (useState) persist values between renders.