Extracting numerical data from "data-" in html

Question

In the HTML below, I want to simply divide two numbers and return the result to the page. The javascript accomplishes this, however, the variable A (GAL2_G2rAimTonsPerHr) is updated every 10 seconds or so from our PI server (historian). How can I set the variable A to equal the changing value? Is there a way I can store it in a temporary variable to be used as a divisor in the script?

< td align="left" style="font-size: 12px" width="60px"                     
 < span class='PIData' data-tag='GAL2_G2rAimTonsPerHr' data-on_pi_change='truncReading' >< /span>
< /td>

<SCRIPT LANGUAGE="JavaScript">
  function divide(n1,n2)
  {
    ans = n1/n2;
    document.write(" "+ans+"<BR>");
    return ans;
  }

  var a = 'GAL2_G2rAimTonsPerHr'????;

  var b = 2;

  divide(a,b);                          

</SCRIPT> 

Answer 1

You've said the actual element will get updated by PI-Plug. If so, and it's really a number, then:

var value = parseInt($(".PIData[data-tag]").attr("data-tag"), 10);

The .PIData[data-tag] part is a CSS-style selector for an element with class="PIData" and a data-tag attribute. Then the rest is getting the value of that attribute and parsing it into a decimal number (as it will be a string, e.g. "300" rather than 300).

Or instead of attr you can use data and leave off the data- part of the attribute name:

var value = parseInt($(".PIData[data-tag]").data("tag"), 10);

I don't do that because I don't like the fact that the data function is asymmetrical (it reads from data-* attributes but doesn't write to them), but it would work fine for what you're doing.

A good read through the jQuery API documentat would be a good idea. It literally only takes about an hour, and it repays you very, very quickly indeed.