1、重学TS-第一题

type User = {
  id: number;
  kind: string;
};

function makeCustomer<T extends User>(u: T): T {
  // Error（TS 编译器版本：v4.4.2）
  // Type '{ id: number; kind: string; }' is not assignable to type 'T'.
  // '{ id: number; kind: string; }' is assignable to the constraint of type 'T', 
  // but 'T' could be instantiated with a different subtype of constraint 'User'.
  return {
    id: u.id,
    kind: 'customer'
  }
}

以上代码会报错，为什么呢？

• 1、T是User的子类型，T拥有的属性是比User多的
• 2、返回值的类型是User， 子类型可以赋值给父类型，但是父类型不可以赋值给子类型

解法1: 将返回值修改为子类型

type User = {
  id: number;
  kind: string;
};

function makeCustomer<T extends User>(u: T): T {
  // Error（TS 编译器版本：v4.4.2）
  // Type '{ id: number; kind: string; }' is not assignable to type 'T'.
  // '{ id: number; kind: string; }' is assignable to the constraint of type 'T', 
  // but 'T' could be instantiated with a different subtype of constraint 'User'.
  return {
    ...u,
    id: u.id,
    kind: 'customer'
  }
}

解法2: 返回值定义为父类型

type User = {
  id: number;
  kind: string;
};

function makeCustomer<T extends User>(u: T): User {
  // Error（TS 编译器版本：v4.4.2）
  // Type '{ id: number; kind: string; }' is not assignable to type 'T'.
  // '{ id: number; kind: string; }' is assignable to the constraint of type 'T', 
  // but 'T' could be instantiated with a different subtype of constraint 'User'.
  return {
    id: u.id,
    kind: 'customer'
  }
}

============================================

type User = {
  id: number;
  kind: string;
};
type ReturnMake<T extends User, U> = {
  [K in keyof U as K extends keyof T ? K : never]: U[K];
};
function makeCustomer<T extends User>(u: T): ReturnMake<T, User> {
  // Error（TS 编译器版本：v4.4.2）
  // Type '{ id: number; kind: string; }' is not assignable to type 'T'.
  // '{ id: number; kind: string; }' is assignable to the constraint of type 'T', 
  // but 'T' could be instantiated with a different subtype of constraint 'User'.
  return {
    id: u.id,
    kind: 'customer'
  } as T
}

解法3: 强制类型断言

type User = {
  id: number;
  kind: string;
};

function makeCustomer<T extends User>(u: T): T {
  // Error（TS 编译器版本：v4.4.2）
  // Type '{ id: number; kind: string; }' is not assignable to type 'T'.
  // '{ id: number; kind: string; }' is assignable to the constraint of type 'T', 
  // but 'T' could be instantiated with a different subtype of constraint 'User'.
  return {
    id: u.id,
    kind: 'customer'
  } as T
}

type ReturnMake<T extends User, U> = {
    []
}

总结： 父类型不能赋值给子类型，子类型可以赋值给父类型。