TS 类型报错：不能将类型“string”分配给类型“never”

Question

报错信息

(property) User.occupation: string
不能将类型“string”分配给类型“never”。ts(2322)

难道最后一个应该是被识别为了 PowerUser 类型，那么为什么这个类型的 role 和 occupation 都是 never 类型呢？

interface User {
  type: 'user';
  name: string;
  age: number;
  occupation: string;
}

interface Admin {
  type: 'admin';
  name: string;
  age: number;
  role: string;
}

type PowerUser = Omit<User & Admin, 'type'> & { type: 'powerUser' };

export type Person = User | Admin | PowerUser;

export const persons: Person[] = [
  {
    type: 'user',
    name: 'Max Mustermann',
    age: 25,
    occupation: 'Chimney sweep',
  },
  { type: 'admin', name: 'Jane Doe', age: 32, role: 'Administrator' },
  { type: 'user', name: 'Kate Müller', age: 23, occupation: 'Astronaut' },
  { type: 'admin', name: 'Bruce Willis', age: 64, role: 'World saver' },
  {
    type: 'powerUser',
    name: 'Nikki Stone',
    age: 45,
    role: 'Moderator',
    occupation: 'Cat groomer',
  },
];

function isAdmin(person: Person): person is Admin {
  return person.type === 'admin';
}

function isUser(person: Person): person is User {
  return person.type === 'user';
}

function isPowerUser(person: Person): person is PowerUser {
  return person.type === 'powerUser';
}

export function logPerson(person: Person) {
  let additionalInformation: string = '';
  if (isAdmin(person)) {
    additionalInformation = person.role;
  }
  if (isUser(person)) {
    additionalInformation = person.occupation;
  }
  if (isPowerUser(person)) {
    additionalInformation = `${person.role}, ${person.occupation}`;
  }
  console.log(`${person.name}, ${person.age}, ${additionalInformation}`);
}

console.log('Admins:');
persons.filter(isAdmin).forEach(logPerson);

console.log();

console.log('Users:');
persons.filter(isUser).forEach(logPerson);

console.log();

console.log('Power users:');
persons.filter(isPowerUser).forEach(logPerson);

// In case if you are stuck:
// https://www.typescriptlang.org/docs/handbook/utility-types.html

Answer 1

type:'powerUser'肯定被认为是PowerUser类型， 而PowerUser在你的写法中被判断为只有type和s两个属性(具体原因看交叉类型的定义).

另外吐槽一句，下次代码类的最好贴代码，否则对着敲很累 :)

Answer 2

interface User {
  type: 'user';
  name: string;
  age: number;
  occupation: string;
}

interface Admin {
  type: 'admin';
  name: string;
  age: number;
  role: string;
}

type PowerUser = Omit<User, 'type'> &
  Omit<Admin, 'type'> & { type: 'powerUser' };

export type Person = User | Admin | PowerUser;

export const persons: Person[] = [
  {
    type: 'user',
    name: 'Max Mustermann',
    age: 25,
    occupation: 'Chimney sweep',
  },
  { type: 'admin', name: 'Jane Doe', age: 32, role: 'Administrator' },
  { type: 'user', name: 'Kate Müller', age: 23, occupation: 'Astronaut' },
  { type: 'admin', name: 'Bruce Willis', age: 64, role: 'World saver' },
  {
    type: 'powerUser',
    name: 'Nikki Stone',
    age: 45,
    role: 'Moderator',
    occupation: 'Cat groomer',
  },
];