实现 ReturnType

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

例如：

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

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

    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>;


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