title: TS Type is missing the following properties
body: <p>I'm using Angular 7.x.x with TS in version 3.2.4.</p>
  
  <p>I have two TS interfaces. One ist extending the other one:</p>
  
  <p>This is the main interface:</p>
  
  <pre><code>export interface Result {
        var1: string;
        var2: number;
        var3: boolean;
  } 
  </code></pre>
  
  <p>The second one just adds a property:</p>
  
  <pre><code>export interface ResultPlus extends Result {
        var4: boolean;
  }
  </code></pre>
  
  <p>Now I have a service returning <code>Observable&lt;Result[]&gt;.</code></p>
  
  <p>In my component I subscribe on this service:</p>
  
  <pre><code>dataArray: ResultPlus[] = [];    
  
  getResults(): void {
        this.service.getResults()
            .subscribe(data =&gt; {
              **this.dataArray** = (data as unknown as ResultPlus);
            });
       }
  </code></pre>
  
  <p>(There are no * in the code)</p>
  
  <p>Now the <strong>this.dataArray</strong> (bold above - **) is underlined in red and it says: </p>
  
  <p><em>error TS2740: Type 'ResultPlus' is missing the following properties from type 'ResultPlus[]': length, pop, push, concat, and 26 more.</em></p>
  
  <p>What am I doing wrong?</p>
  
  <p>Thank you in advance!</p>