无法推断 属性 类型，即使它是从记录定义扩展的

Question

我很困惑为什么 typescript 在 TParent 扩展 Record<TParentProperty, TChild> 时无法推断 destination[destinationProperty] 是 TChild 应该允许它推断 属性 的类型是 TChild.

class Person  {
  favoriteDog: Dog | undefined;
}

class Dog  {
  name: string;
}

  function mapSingle<
    TChild extends object | undefined,
    TParent extends Record<TParentProperty, TParentPropertyType>,
    TParentProperty extends Extract<keyof TParent, string>,
    TParentPropertyType extends TChild,
  >(
    destination: TParent,
    destinationProperty: TParentProperty,
    source: TChild,
  ) {
    destination[destinationProperty] = source; // Error Line
  }

Type 'TChild' is not assignable to type 'TParent[TParentProperty]'.

Type 'object | undefined' is not assignable to type 'TParent[TParentProperty]'.

Type 'undefined' is not assignable to type 'TParent[TParentProperty]'.(2322)

Typescript Playground Example

Answer 1

这里有点复杂，像这样的东西可以工作：

class Person  {
  favoriteDog: Dog | undefined;
}

class Dog  {
  name: string;
}

function mapSingle<
  Parent,
  ParentProp extends keyof Parent,
  Child extends Parent[ParentProp],
>(
  destination: Parent,
  destinationProperty: ParentProp,
  source: Child,
) {
  destination[destinationProperty] = source;
}

mapSingle(new Person(), "favoriteDog", new Dog())
mapSingle(new Person(), "favoriteDog", undefined)

这是最简单的版本；你有一个 Parent，属性 ParentProp，以及 属性 Child 的类型。不需要额外的通用参数！

Playground