索引存取類型

我們可以使用索引存取型別來查詢另一種型別上的特定屬性

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

索引型別本身是一種型別，因此我們可以使用聯集、keyof 或其他型別

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

如果您嘗試索引不存在的屬性，甚至會看到錯誤

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

使用 number 來取得陣列元素型別的索引的另一個範例。我們可以將它與 typeof 結合使用，以方便擷取陣列文字的元素型別

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，這表示您不能使用 const 來建立變數參考

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

不過，你可以使用類型別名來進行類似的重構

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