索引访问类型

我们可以使用索引访问类型来查找另一种类型的特定属性:

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

索引类型本身就是一种类型,所以我们可以完全使用联合、keyof 或其他类型:

type Person = { age: number; name: string; alive: boolean };
type I1 = Person["age" | "name"];

type I2 = Person[keyof Person];

type AliveOrName = "alive" | "name";
type I3 = Person[AliveOrName];

如果您尝试索引不存在的属性,您甚至会看到错误:

type Person = { age: number; name: string; alive: boolean };
type I1 = Person["alve"];

使用任意类型进行索引的另一个示例是使用 number 来获取数组元素的类型。我们可以将它与 typeof 结合起来,以方便地捕获数组字面量的元素类型:

const MyArray = [
  { name: "Alice", age: 15 },
  { name: "Bob", age: 23 },
  { name: "Eve", age: 38 },
];

type Person = typeof MyArray[number];
type Age = typeof MyArray[number]["age"];
// Or
type Age2 = Person["age"];

您只能在索引时使用类型,这意味着您不能使用 const 来进行变量引用:

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

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