老师再视频中演示的时候，并没有解释为什么要这样做。

  function extend<T extends object,U extends object>(first:T,second:U):T&U{
    let result = {} as T & U

    for(let id in first){
    // 这里
      result[id] = first[id] as any
    }

    for(let id in second){
      if(!result.hasOwnProperty(id)){
      // 这里
        result[id] = second[id] as any
      }
    }
    return result
  }

我觉得这里result 被 断言为 T&U类型，那就说明result包含类型T和U上的所有属性并且类型会保持一致，所以我们没理由要加上as any 啊，但是不加这句又会像视频中演示的那样报错:

Type 'T[Extract<keyof T, string>]' is not assignable to type '(T & U)[Extract<keyof T, string>]'.
  Type 'T' is not assignable to type 'T & U'.
    Type 'object' is not assignable to type 'T & U'.
      Type 'object' is not assignable to type 'T'.
        'object' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint 'object'.
        //...

希望老师能解答一下