What are closures in JavaScript?

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A closure is a function bundled together with references to the variables from its surrounding lexical scope. Even after the outer function has finished executing, an inner function keeps access to those outer variables because JavaScript resolves identifiers based on where the function was defined , not where it is called. Minimal example function counter() { let count = 0; return function () { count += 1; return count; }; } const tick = counter(); tick(); // 1 tick(); // 2 tick(); // 3 The inner function closes over count . Each call to counter() produces a brand-new binding, so two counters are fully independent. Why closures matter Data privacy — variables inside the enclosing scope are inaccessible from the outside, giving you private state without classes. Stateful callbacks — event handlers, timers, and async callbacks can remember the values they need to operate on. Partial application and currying — pre-supplying arguments is just a closure that captures them. Module pattern — returning an object of methods that share a private scope was the classic pre-ES6 way to build modules. A practical example function makeLogger(prefix) { return (message) => console.log(`[${prefix}] ${message}`); } const authLog = makeLogger("auth"); authLog("login success"); // [auth] login success The classic loop trap Closures inside a var loop all share the same binding, which is why the following logs 3 three times: for (var i = 0; i < 3; i++) { setTimeout(() => console.log(i), 0); } Switching to let fixes this: each iteration gets a fresh i , and each closure captures its own copy. Memory implications A closure keeps its captured variables alive as long as the closure itself is reachable. Holding on to closures longer than needed can unintentionally retain large objects, so dispose of event listeners and cached callbacks when you are done with them. The loop+setTimeout problem is the #1 closure interview question. Show the bug (var, all print 3), explain why (shared variable), and show two fixes (IIFE and let). The counter example for data privacy shows practical use.