title: Add return type information in a fat arrow method
body: <p>I searched in all possible ways but I can't figure out how to combine return type annotation and fat arrow syntax. </p>
  
  <pre class="lang-js prettyprint-override"><code>class BasicCalculator{
      value:number;
      constructor(value:number=0){
          this.value=value;
      }
      add= (operand:number)=&gt;{ // CAVEAT how to add return type???
          this.value+=operand;
          return this;
      }
      subtract= (operand:number)=&gt;{
          this.value-=operand;
          return this;
      }
      multiply= (operand:number)=&gt;{
          this.value*=operand;
          return this;
      }
      divide= (operand:number)=&gt;{
          this.value/=operand;
          return this;
      }
  }
  </code></pre>
  
  
  
  <p>I tried:</p>
  
  <pre class="lang-js prettyprint-override"><code>add= (operand:number)=&gt;number { ....
  </code></pre>
  
  
  
  <p>but it doesn't work.</p>