Javascript concats instead of adding

I have this Javascript code. The addition line doesnt add. It concats. For example, i have 2 values: 100.00 and 10.00 . It comes out as 10.00100.00

I googled and "+" is the right thing to use. Not sure why it's not adding:

<script type="text/javascript">
     var amount = 0.00;
     function prices(price) {


         var tempAmount = document.getElementById('<%=x_amount.ClientID %>');
         alert(tempAmount.value);
         amount = price + tempAmount.value;
         alert(amount);

         tempAmount.value = amount;
        // alert(amount.value);
   


     }
  </script>

Select allOpen in new window