Keyof 类型运算符

keyof运算符获取对象类型，并生成其key的字符串或数字文字并集.将keyof类型与映射类型结合使用时尤其有用

keyof (A & B) 形式的类型被转换为 keyof A | keyof B.

type A = { a: string };
type B = { b: string };
type T1 = keyof (A & B); // "a" | "b"
type T2<T> = keyof (T & B); // keyof T | "b"
type T3<U> = keyof (A & U); // "a" | keyof U
type T4<T, U> = keyof (T & U); // keyof T | keyof U
type T5 = T2<A>; // "a" | "b"
type T6 = T3<B>; // "a" | "b"
type T7 = T4<A, B>; // "a" | "b"

type Point = { x: number; y: number };
type P = keyof Point;
    

如果该类型具有字符串或数字索引签名，则keyof将返回这些类型：

type Arrayish = { [n: number]: unknown };
type A = keyof Arrayish;

// type A = number

type Mapish = { [k: string]: boolean };
type M = keyof Mapish;
// type M = string | number
这是因为JavaScript对象键始终被强制转换为字符串，因此obj[0]始终与obj [“ 0”]相同。

获取对象的key

const a = 1

type Eg1 =  {
  readonly name: string,
  age: number,
}
// T1的类型实则是name | age
type T1 = keyof Eg1

const b:T1= 'name'


class Eg2 {
  private name: string;
  public readonly age: number;
  protected home: string;
  constructor() {
      this.name = ''
      this.age = 10
      this.home = ''
  }
}
// T2实则被约束为 age
// 而name和home不是公有属性，所以不能被keyof获取到
type T2 = keyof Eg2

const c: T2 = 'age'
const c1: T2 = 'name'