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AnyFunction<Args?, ReturnType?> matches function type with arguments type Args (any[] by default) and return type ReturnType (any by default)

It's handy to use it in generic constraint with default type Args and ReturnType

type FirstParameter<Type extends AnyFunction> = Head<Parameters<Type>>;

type StringValue = FirstParameter<typeof decodeURIComponent>;
//   ^? string

Args and ReturnType can be specified if necessary

type SumFunction = AnyFunction<[first: number, second: number], number>;

type NumberValue = FirstParameter<SumFunction>;
//   ^? number

TS Playground – https://tsplay.dev/mbEM2N