title: Typescript keyof return array of strings
body: <p>Let's say I have a class: </p>
  
  <pre><code>class Test { 
    propA;
    propB;
    propC;
  }
  </code></pre>
  
  <p>I want to create a method that returns an array of strings and type it to be only the keys existing in the Test class, how can I do this with the <code>keyof</code> feature? </p>
  
  <pre><code> class Test { 
        propA;
        propB;
        propC;
  
     getSomeKeys() : keyof Test[] {
       return ['propA', 'propC']
     }
   }
  </code></pre>