004.对象属性深度只读

实现一个泛型 DeepReadonly<T>，它将对象的每个参数及其子对象递归地设为只读

您可以假设在此挑战中我们仅处理对象。不考虑数组、函数、类等。但是，您仍然可以通过覆盖尽可能多的不同案例来挑战自己

例如：

type X = { 
  x: { 
    a: 1
    b: 'hi'
  }
  y: 'hey'
}
 
type Expected = { 
  readonly x: { 
    readonly a: 1
    readonly b: 'hi'
  }
  readonly y: 'hey' 
}
 
type Todo = DeepReadonly<X> // should be same as `Expected`

答案

// 判断一个类型是否有键
type HasKey<T> = T extends object ? (keyof T extends never ? false : true) : false
 
type DeepReadonly<T> = {
  readonly [K in keyof T]: HasKey<T[K]> extends true
    ? DeepReadonly<T[K]>
    : T[K]
}

要考虑覆盖更多的类型，可以参考 Vue 源码：

type Primitive = string | number | boolean | bigint | symbol | undefined | null
type Builtin = Primitive | Function | Date | Error | RegExp
type DeepReadonly<T> = T extends Builtin
  ? T
  : T extends Map<infer K, infer V>
    ? ReadonlyMap<DeepReadonly<K>, DeepReadonly<V>>
    : T extends ReadonlyMap<infer K, infer V>
      ? ReadonlyMap<DeepReadonly<K>, DeepReadonly<V>>
      : T extends WeakMap<infer K, infer V>
        ? WeakMap<DeepReadonly<K>, DeepReadonly<V>>
        : T extends Set<infer U>
          ? ReadonlySet<DeepReadonly<U>>
          : T extends ReadonlySet<infer U>
            ? ReadonlySet<DeepReadonly<U>>
            : T extends WeakSet<infer U>
              ? WeakSet<DeepReadonly<U>>
              : T extends Promise<infer U>
                ? Promise<DeepReadonly<U>>
                : T extends {}
                  ? { readonly [K in keyof T]: DeepReadonly<T[K]> }
                  : Readonly<T>