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A function that takes another function as input or returns one as output.
A function that operates on other functions — either by taking them as arguments (map, filter, reduce) or by returning a new function (factory, decorator). Higher-order functions enable composition and abstraction.
In type theory, a function of type (A → B) → C or A → (B → C). JavaScript's first-class functions make every function potentially higher-order. Currying transforms a multi-argument function into a chain of unary higher-order functions.