type ReturnType<T extends (...args: any) => any> = T extends (
...args: any
) => infer R
? R
: any
방법1.
const getString=()=>'a'
type FuncReturnType = ReturnType<typeof getString>
방법2.
const getString=()=>'a'
type GetStringF=typeof getString
type FuncReturnType = ReturnType<GetString>