What is a closure and what are common use cases?

A closure is a function that retains access to its outer (lexical) scope's variables even after the outer function has returned. This happens because functions in JavaScript form a closure over the variables in scope at the time of their creation. IIFE (Immediately Invoked Function Expression): (function() { /* private scope */ })(); — creates a private scope in pre-ES6 code. Counter example: function makeCounter() { let count = 0; return () => ++count; } const counter = makeCounter(); — each call to counter() increments the private count variable. Module pattern: Using closures to create private state with public methods. Common pitfall: closures in loops — all callback functions share the same variable reference. Fix with let (block-scoped per iteration) or IIFE to capture the value. LWC class properties are not closures, but event handlers using arrow functions close over the class instance.