add: ts indexed access

docs/ts-indexed-access-type.md

@@ -0,0 +1,75 @@
+---
+title: 索引访问类型
+date: 2024-04-15 22:09:42+8
+tags: [Typescript]
+layout: post
+cover:
+  image: /vp-blog/covers/ts.png
+---
+
+可以使用索引访问类型来查询另一个类型的指定属性：
+
+```typescript
+type Person = { age: number; name: string; alive: boolean };
+type Age = Person["age"];
+// type Age = number
+```
+
+索引类型是一个类型，所以我们可以使用联合类型：
+
+```typescript
+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
+```
+
+试图使用不存在的索引属性时会报错：
+
+```typescript
+type I4 = Person["alve"];
+// Error: Property 'alve' does not exist on type 'Person'.
+```
+
+使用 `number` 获取数组元素的类型。可以将其与 `typeof` 方便地结合使用：
+
+```typescript
+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
+```
+
+只能在索引时使用类型，这意味着不能使用变量：
+
+```typescript
+const key = "age";
+type Age = Person[key];
+// Error: 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'?
+```
+
+但是可以使用类型别名:
+
+```typescript
+type Key = "age";
+type Age = Person[Key];
+```