Ts体操--myReadonly2

[zhuba-Ahhh]

实现一个通用MyReadonly2<T, K>，它带有两种类型的参数T和K。
K指定应设置为Readonly的T的属性集。如果未提供K，则应使所有属性都变为只读，就像普通的Readonly<T>一样。

type MyReadonly2<T, K extends keyof T = keyof T> = {
  readonly [P in K]: T[P];
} & {
  [P in keyof T as P extends K ? never : P]: T[P];
};


interface Todo {
  title: string
  description: string
  completed: boolean
}

const todo: MyReadonly2<Todo, 'title' | 'description'> = {
  title: "Hey",
  description: "foobar",
  completed: false,
}

todo.title = "Hello" // Error: cannot reassign a readonly property
todo.description = "barFoo" // Error: cannot reassign a readonly property
todo.completed = true // OK

[gswysy]

//实现1，使用内置Exclude
type MyReadonly2<T, K extends keyof T = keyof T> = {
  readonly [P in K]: T[P];
} & {
  [P in Exclude<keyof T, K>]: T[P];
};


//实现2 ，完全手写
type MyExclude<T, K> = T extends K ? never : T;

type MyReadonly2<T, K extends keyof T = keyof T> = {
  readonly [P in K]: T[P];
} & {
  [P in MyExclude<keyof T, K>]: T[P];
};