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

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

• javascript
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Share Improve this question Follow edited Dec 3, 2019 at 17:56 Oleg Ovcharenko asked Dec 3, 2019 at 17:47 Oleg OvcharenkoOleg Ovcharenko 53722 gold badges88 silver badges1919 bronze badges 3

• NewPropGirl should be derived from PersonelData. Or you have to mark all none existing properties on PersonalData as optional. – Eldar Commented Dec 3, 2019 at 17:53
• @Eldar I can't mark all none existing properties on PersonalData as optional because it is interface from node_modules – Oleg Ovcharenko Commented Dec 3, 2019 at 17:59
• Woah am a bit confused, but let me ask you something, you are putting cyclic dependency of interfaces, so from what i understand is your friends? variable could either be a PersonalData or NewPropGirl correct me if am wrong – pavan kumar Commented Dec 3, 2019 at 18:16

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2 Answers 2

Sorted by: Reset to default 2

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',
    }]
  }];
}

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"
        }]
    }];
}

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