TypeScript从类型创建类型之索引访问类型

我们可以使用索引访问类型（indexed access type）来查看一个类型上的属性的类型：

type Person = { age: number; name: string; alive: boolean };
type Age = Person["age"];
    // type Age = number

被索引的类型本身就是一种类型，所以我们可以使用联合类型，keyof或者其它的类型：

type I1 = Person["age" | "name"];
    // type I1 = string | number
 
type I2 = Person[keyof Person];
    // type I2 = string | number | boolean
 
type AliveOrName = "alive" | "name";
type I3 = Person[AliveOrName];
    // type I3 = string | boolean

如果你尝试去访问一个不存在的属性，那么会得到一个错误：

type I1 = Person["alve"];
// Property 'alve' does not exist on type 'Person'.

另一个例子是使用number类型的索引来获取数组的元素类型。我们可以结合keyof来方便的获得数组的元素的类型：

const MyArray = [
  { name: "Alice", age: 15 },
  { name: "Bob", age: 23 },
  { name: "Eve", age: 38 },
];
 
type Person = typeof MyArray[number];
       
type Person = {
    name: string;
    age: number;
}
type Age = typeof MyArray[number]["age"];
     
type Age = number
// Or
type Age2 = Person["age"];
      
type Age2 = number

你只能在类型上使用索引访问，意味着你不可以使用变量动态访问：

const key = "age";
type Age = Person[key];
// Type 'key' cannot be used as an index type.
// 'key' refers to a value, but is being used as a type here. Did you mean 'typeof key'?

但是你可以使用一个类型别名来实现这种方式：

type key = "age";
type Age = Person[key];

翻译自Index Access Types