6. MyReturnType

[astak16]

题目

题目链接：MyReturnType

实现 ReturnType 返回函数的返回类型

import type { Equal, Expect } from "@type-challenges/utils";

type cases = [
  Expect<Equal<string, MyReturnType<() => string>>>,
  Expect<Equal<123, MyReturnType<() => 123>>>,
  Expect<Equal<ComplexObject, MyReturnType<() => ComplexObject>>>,
  Expect<Equal<Promise<boolean>, MyReturnType<() => Promise<boolean>>>>,
  Expect<Equal<() => "foo", MyReturnType<() => () => "foo">>>,
  Expect<Equal<1 | 2, MyReturnType<typeof fn>>>,
  Expect<Equal<1 | 2, MyReturnType<typeof fn1>>>
];

type ComplexObject = {
  a: [12, "foo"];
  bar: "hello";
  prev(): number;
};

const fn = (v: boolean) => (v ? 1 : 2);
const fn1 = (v: boolean, w: any) => (v ? 1 : 2);

答案

方法一

type MyReturnType<T extends (...args: any) => any> = 
  T extends (...args: any) => infer R ? R : never;

方法二

type MyReturnType<T> = T extends (...args: any) => infer R ? R : never;

解析

方法一和方法二是一样的， MyReturnType 泛型 T 越不约束都行，因为后面都要进行约束，也就是说方法一更严谨一点。

基础知识

1. ReturnType