2595 - PickByType

Answer TestCases

从 T 中，选择属性，满足其类型是 U

例如：

type OnlyBoolean = PickByType<
  {
    name: string
    count: number
    isReadonly: boolean
    isEnable: boolean
  },
  boolean
> // { isReadonly: boolean; isEnable: boolean; }

Solution

type PickByType<T, U> = {
  [P in keyof T as T[P] extends U ? P : never]: T[P]
}

P in keyof T as T[P] extends U ? P : never 看出两部分

A as B

A = P in keyof T 和 B = T[P] extends U ? P : never

A 遍历 T 中的属性，B 用来判断属性类型 T[P] 是否是 U, 若是，则返回 P, 否则返回 never