TypeScript：需要类似于ReturnType <…>的东西，但第一个函数参数的类型

Question

Is it possible in TypeScript to declare something like ReturnType<...> that does not retrieve the type of the return value but the type of the first argument instead?

type SingleArgFunction<A, R> = (arg: A) => R

// wrong => how to implement the following line correctly?
type FirstArgType<T extends SingleArgFunction<infer A, any>> = A 

const numberToString: SingleArgFunction<number, string> =
    (x: number) => String(x)

type R = ReturnType<typeof numberToString> // R => string

type F = FirstArgType<typeof numberToString> // F => number

Answer 1

看起来这可以满足您的要求：

type FirstArgumentType<T> = T extends (arg1: infer U, ...args: any[]) => any ? U : never;