title: Typescript: Object literal may only specify known properties
body: <p>I have a code:</p>
  
  <pre><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[] &amp; NewPops[]  =&gt; {
      return [{
          name: 'Oleg',
          age: 25,
          nick: 'yyy',
          girl: 'Test',
          friends: [{
              name: 'Roman',
              age: 22,
              nick: 'bob',
              girl: 'Test',
          }]
      }];
  }
  </code></pre>
  
  <p>I want to add a new interface parameter <strong>girl</strong> to the library interface. In this code, I have an Error on line 31</p>
  
  <p>TYPESCRIPT VERSION 3.5.3</p>
  
  <pre><code>Type '{ name: string; age: number; nick: string; girl: string; friends: { girl: string; }[]; }[]' is not assignable to type 'Friends[] &amp; 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'.
  </code></pre>
  
  <p>Playground link: <a href="http://www.typescriptlang.org/play/?ts=3.5.1#code/FDD0oAgGQSwIwE4EMEE8IAcEHsMGdgYA7AFwFMEAzJAYzIgAUK9sikAbAESRKQgG9gEYRDYBbMgC4IeEgmIBzANxCRSBVNEBXMXAoqRomDQDW02fKLLVwyvLJEAJngD80gGL2neANoBdFQBfEGJyKlp6TxgHZwgyAA9yb0ZmVg5uXgEIYLBIAEkIAHckUggSbAgkR0dRMkLMHHxCUgpqOggAOTqGRoBxGAR2ARsIBQH2czlFAxE7aO83Tu7cXwDgHNDWiKXChhW4xJi8HZ7cfsGsnJpWWTKyW4BeCAAKAEoPL2d-CAAyE5XvhAHgA%20YaGBBkEhaBBECA%20QSGQziTQAcgA8uwyAoUQAaEaGdSaABMAFY8YiRERjGYICjUPTcfiRGNBtIUQAVe4kRkU2yfPDSeFMinItkAJWwYhKPN5BI00iJRPJssp1LZcGwcBlKtG4zZnNk2sRgT8IxNQSAA" rel="nofollow noreferrer">playground</a></p>