打字稿：对象文字只能指定已知属性

Question

I have a code:

// Library interfaces
interface PersonalData {
    name: string;
    age: number;
    nick: string;
    friends?: Friends[];
}

interface Friends extends PersonalData { }

// I want to add new props girls in friend interface
interface NewPropGirl {
    girl: string;
    friends?: NewPops[];
}

interface NewPops extends NewPropGirl { }

const test = (): Friends[] & NewPops[]  => {
    return [{
        name: 'Oleg',
        age: 25,
        nick: 'yyy',
        girl: 'Test',
        friends: [{
            name: 'Roman',
            age: 22,
            nick: 'bob',
            girl: 'Test',
        }]
    }];
}

I want to add a new interface parameter girl to the library interface. In this code, I have an Error on line 31

TYPESCRIPT VERSION 3.5.3

Type '{ name: string; age: number; nick: string; girl: string; friends: { girl: string; }[]; }[]' is not assignable to type 'Friends[] & NewPops[]'.
  Type '{ name: string; age: number; nick: string; girl: string; friends: { girl: string; }[]; }[]' is not assignable to type 'Friends[]'.
    Type '{ name: string; age: number; nick: string; girl: string; friends: { girl: string; }[]; }' is not assignable to type 'Friends'.
      Object literal may only specify known properties, and 'girl' does not exist in type 'Friends'.

Playground link: playground

Answer 1

I have extended PersonalData to NewPropGirl to let your result hold an extra field and made that as the return type

interface PersonalData {
  name: string;
  age: number;
  nick: string;
  friends ? : Friends[];
}

interface Friends extends PersonalData {}

// I want to add new props
interface NewPropGirl extends PersonalData {
  girl: string;
  friends ? : NewPops[];
}

interface NewPops extends NewPropGirl {}

const test = (): NewPops[] => {
  return [{
    name: 'Oleg',
    age: 25,
    nick: 'yyy',
    girl: 'Test',
    friends: [{
      name: 'Roman',
      age: 22,
      nick: 'bob',
      girl: 'Test',
    }]
  }];
}

Answer 2

interface PersonalData {
    name: string;
    age: number;
    nick: string;
    friends?: Friends[];
}

interface Friends extends PersonalData { }

// I want to add new props
interface NewPropGirl extends PersonalData {
    girl: string;
    friends?: NewPops[];
}

interface NewPops extends NewPropGirl { }

const test = (): Friends[] | NewPops[]  => {
    return [{
        name: 'Oleg',
        age: 25,
        nick: 'yyy',
        girl: 'Test',
        friends: [{
            name: 'Roman',
            age: 22,
            nick: 'bob',
            girl:"Bella"
        }]
    }];
}

See if it fit your needs. Playground