title: Function inside a class not triggering
body: <p>I'm experimenting with different ways of implementing Typescript classes and I am having a hard time figuring out how to get my function to work properly.</p>
  
  <p>So far this is what I have:</p>
  
  <p><strong>calc-func.ts :</strong></p>
  
  <pre><code>export function calcFunc( a: number, b: number ){ return a*b; }
  </code></pre>
  
  <p><strong>callc-class :</strong></p>
  
  <pre><code>import { calcFunc } from './calc-func';
  
  export class CalcClass implements calcFunc {
    NumA: number = null;
    NumB: number = null;
    NumC: number = calcFunc( this.NumA, this.NumB );
  }
  </code></pre>
  
  <p><strong>app.component :</strong></p>
  
  <pre><code>export class App implements OnInit {
  
      Calculation: CalcClass = new CalcClass();
  
      ngOnInit(){
        this.Calculation.NumA = 3;
        this.Calculation.NumB = 7;
     }
  }
  </code></pre>
  
  <p><strong>app.component (template) :</strong></p>
  
  <pre><code> &lt;h2&gt;number = {{Calculation| json}}&lt;/h2&gt;
  </code></pre>
  
  <p><code>NumC</code> is showing up as 0 instead of 21 even though <code>NumA</code> is showing up as 3 and 'NumB' shows up as 7 as I'm aiming for.</p>
  
  <p>What else do I need to do to make this work?</p>