实现 ReturnType

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不使用 `ReturnType` 实现 TypeScript 的 `ReturnType` 泛型。

不使用 ReturnType 实现 TypeScript 的 ReturnType<T> 泛型。

例如：

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const fn = (v: boolean) => {
    if (v)
        return 1
    else
        return 2
}

type a = MyReturnType<typeof fn> // 应推导出 "1 | 2"

解答

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

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

type fnType = MyReturnType<typeof fn>;
type fnType1 = MyReturnType<typeof fn1>;

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

type fnType2 = MyReturnType<() => ComplexObject>;