I can't seem to remember how to add two text box values together and display the answer dynamicaly in another control.

ASCX
<script language="javascript">
function mkdnLoss(){
var val1 = document.getElementById('txtPrice').value;
var val2 = document.getElementById('txtSoldPrice').value;
var lossamt = val1 - val2;
document.getElementById('txtLossAmount').value = lossamt;
}
</script>

.VB
txtSoldPrice.Attributes.Add("OnChange", "mkdnLoss();")

Help is appreciated!

Could it just be the case? Try....

txtSoldPrice.Attributes.Add("onChange", "mkdnLoss();")

Didn't work.

I also have tried writing the script in the page load like so

The client-side ID of a control is not the same as the server-side ID. When referring to a control in JS, you need to use its ClientID property. For example, in your ASCX you could use:

var myObject = document.getElementById("<%= txtPrice.ClientID %>");

This will give you the handle to the object as expected.

Nik

I cannot set the value of the textbox with this assignment?

function setGenSecMan(varGenSecMan){ 
   document.getElementById("").text = varGenSecMan; 
}

No. The document.getElementById() method requires a parameter which is the ID of an element on the page.

Nik

document.getElementById("<%= tbHidGenSec.ClientID %>").text = varGenSecMan;

When I copied and pasted from Visual Studio the delimited section was lost.  The above is the actual string.  I can also view the value within the function after I set it but it is either lost on postback or the value is never actually assigned to the control. 

alert(document.getElementById("<%= tbHidGenSec.ClientID %>").text); //value of 2 as assigned in function

Neither code behind or page show new value after postback

Dim strVar As String = tbHidGenSec.Text