typescript教程上的这句话是不是错了

Question

这是官方教程的截图，很明显说的是{name: 'Alice', location: 'Seattle'}是{name: 'Alice'}的子类型。然而文档上怎么说呢,地址截图如下:
https://github.com/Microsoft/...

子类型不能有父类型所不具有的属性。
上面的教程是不是表述的有问题啊

Answer 1

对 3.11.5 excess properties 的理解问题。

A source type S is considered to have excess properties with respect to a target type T if

• S is a fresh object literal type, as defined below, and

• S has one or more properties that aren't expected in T.

所以假如是这样才算有多余的属性

x = {name: "Alice", location: "Seattle"}

x = y; 正确的原因

S is an object type, an intersection type, an enum type, or the Number, Boolean, or String primitive type, T is an object type, and for each member M in T, one of the following is true:

• M is a property and S has an apparent property N where

  • M and N have the same name,

  • the type of N is a subtype of that of M,

  • M and N are both public, M and N are both private and originate in the same declaration, M and N are both protected and originate in the same declaration, or M is protected and N is declared in a class derived from the class in which M is declared.

y = x; 错误的原因

S is an object type, an intersection type, an enum type, or the Number, Boolean, or String primitive type, T is an object type, and for each member M in T, one of the following is true:

• M is a property and S has an apparent property N where

  • M and N have the same name,

  • the type of N is a subtype of that of M,

  • M and N are both public, M and N are both private and originate in the same declaration, M and N are both protected and originate in the same declaration, or M is protected and N is declared in a class derived from the class in which M is declared.

Answer 2

教程表述没有问题，“source function's return type be a subtype of the target type's return type”这句话，其实你可以缩减了看，就认为function's return type不存在，其实就是“source be a subtype of target”。

而，在TypeScript里，subtype的意义和正常理解并不是太相同。
这个要看你第二张图的红框后面一句，if S has no excess properties with respect to T，是指，如果S对于T而言，没有excess property，就算是它的subtype。

然而，什么是excess property呢？
参见这个文档的3.11.5：

A source type S is considered to have excess properties with respect to a target type T if
S is a fresh object literal type, as defined below, and
S has one or more properties that aren't expected in T.

简单而言，只要S中的项，合并到T里面去，不会被T的属性检查排斥，就不算有excess property。相反，如果被T的属性检查排斥，就算有excess property。

因此，在TypeScript中，subtype并不表示数据项少，而是指不存在合并之后会被排斥的属性，就被称为subtype。