title: Incrementing a decimal based on data attribute string value
body: <p>I have some jquery taking a string value from a data attribute, and then increasing the value based on the value of <code>value</code>. I am expecting it to increment the value each time and write the new total to the data attribute, however it does not work as expected.</p>
  
  <p><a href="http://jsfiddle.net/bluey/x54Kk/1/" rel="nofollow">JSfiddle</a></p>
  
  <p><strong>The JQ</strong></p>
  
  <pre><code>$("#btn").click( function() {
      discount = $("#thisid").data("discount_tendered");
      // alert(discount);
      value = $("#value").text();
      valueDec = parseFloat(value);
      // alert(value);
      newDiscountVal = discount += valueDec;
      // alert(newDiscountVal);
      $("#thisid").attr("data-discount_tendered", newDiscountVal);
  });
  </code></pre>