javascript - 密封/协变对象和 $Exact + $ReadOnly 之间有什么区别？

Question

有什么区别:

type MovieType = {|
  +blob?: string,
  +name: string,
  +url?: string
|};

和

type MovieType = $Exact<$ReadOnly<{
  blob?: string,
  name: string,
  url?: string
}>>;

?

我想知道 flowtype正在根据对象的定义方式或前者是否只是后者的语法糖来区别对待对象。

Answer 1

这两个对象类型应该是等价的。

$ReadOnly<T> 使所有属性协变:

$ReadOnly is a type that represents the read-only version of a given object type T. A read-only object type is an object type whose keys are all read-only.

This means that the following 2 types are equivalent:

type ReadOnlyObj = {
  +key: any,  // read-only field, marked by the `+` annotation
};

type ReadOnlyObj = $ReadOnly<{
  key: any,
}>;

$Exact<T> 接受一个不精确的对象并使其精确:

$Exact<{name: string}> is a synonym for {| name: string |} as in the Object documentation.