Type alias TypePropertys<T, KT>

选取T类型中值为KT类型的键

    class A
    {
        a = 1;
        f() { }
    }

    var a: TypePropertys<A, number>; //var a: Pick<A, "a">
    var a1: TypePropertys<A, Function>; //var a1: Pick<A, "f">