索引訪問型別

我們可以使用索引訪問型別來查詢另一個型別上的特定屬性

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

索引型別本身就是一個型別，因此我們可以使用聯合型別、keyof 或其他完全不同的型別

ts
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 | booleanTry

如果你嘗試索引一個不存在的屬性，甚至會看到錯誤提示

ts
type I1 = Person["alve"];
Property 'alve' does not exist on type 'Person'.2339Property 'alve' does not exist on type 'Person'.Try

使用任意型別進行索引的另一個例子是使用 number 來獲取陣列元素的型別。我們可以將其與 typeof 結合使用，從而方便地捕獲陣列字面量的元素型別

ts
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 = numberTry

索引時只能使用型別，這意味著你不能使用 const 來建立變數引用

ts
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'?2538
2749Type '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'?Try

但是，你可以使用類型別名來實現類似的重構風格

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