通用函数的 Typescript ReturnType [英] Typescript ReturnType of generic function

问题描述

TypeScript 2.8 是一个非常有用的功能，可让您提取特定函数的返回类型.

The new ReturnType in TypeScript 2.8 is a really useful feature that lets you extract the return type of a particular function.

function foo(e: number): number {
    return e;
}

type fooReturn = ReturnType<typeof foo>; // number

但是，我无法在泛型函数的上下文中使用它.

However, I'm having trouble using it in the context of generic functions.

function foo<T>(e: T): T {
    return e;
}

type fooReturn = ReturnType<typeof foo>; // type fooReturn = {}

type fooReturn = ReturnType<typeof foo<number>>; // syntax error

type fooReturn = ReturnType<(typeof foo)<number>>; // syntax error

有没有办法提取泛型函数给定特定类型参数的返回类型?

Is there a way extract the return type that a generic function would have given particular type parameters?

推荐答案

TypeScript 编译器不会将 typeof foo 视为泛型类型.我会说这是编译器中的一个错误.

TypeScript compiler does not see typeof foo as generic type. I'd say it's a bug in the compiler.

然而，TypeScript 有可调用接口泛型没有任何问题，所以如果你引入一个与你的函数签名兼容的可调用接口，你可以像这样实现你自己的 ReturnType 等价物:

However, TypeScript has callable interfaces which can be generic without any problems, so if you introduce a callable interface compatible with the signature of your function, you can implement your own equivalent of ReturnType like this:

function foo<T>(x: T): T {
  return x;
}


interface Callable<R> {
  (...args: any[]): R;
}

type GenericReturnType<R, X> = X extends Callable<R> ? R : never;

type N = GenericReturnType<number, typeof foo>; // number

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