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function-signature.md

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Function Signature

A function signature is a type declaration. It tells the compiler:

  • how many input parameters the functions accepts (if any),
  • what type each of those input parameters are, and
  • what type of data (if any) the function returns.

For example:

// `SmartConstructor` is a function signature
type SmartConstructor<IN, OUT> = (
    input: IN,
    { onError }: OnErrorOptions,
    ...fnOptions: FunctionalOption<OUT>[]
) => OUT;

Function signatures are an essential part of a language's type system. Without them, we cannot safely pass functions as parameters, because the compiler can't make sure that we're passing compatible functions in.