What are closures in JavaScript? Give a practical use case.

A closure is a function bundled together with its lexical environment — the variables from the scope where it was declared. The inner function keeps access to outer variables even after the outer function has finished executing.

function createCounter() {
  let count = 0
  return function () {
    count++
    return count
  }
}

const counter = createCounter()
counter() // 1
counter() // 2

Why it matters in interviews

• Data privacy — hide state without classes (module pattern, factory functions).
• Partial application / currying — capture fixed arguments for later calls.
• Event handlers & callbacks — retain context from the enclosing scope.

Closures are not a special syntax; they fall out of JavaScript's lexical scoping rules whenever a function references variables from an outer scope.