A function that takes a function as an argument, returns a function, or both. This is possible because functions are first-class citizens in JavaScript.

Yes. A function that returns another function is a higher-order function (a function factory). The returned function often closes over the outer function's variables, creating a closure.

They enable abstraction (hide details), reusability (write one function, use it with many callbacks), composition (chain small functions into pipelines), and functional programming (pure functions, immutability).