Hi, what is the exact problem that you are facing? Calling the FormUtil.getField(fieldId) will return you the required field element. It is a JQuery element, so you can call .val() to get the value, and val(value) to set the value, as per http://api.jquery.com/val/

I think you need to attach event listener to detect changes to the fields you mentioned. Here's quick coding you can start with.

<script type = "text/javascript" >

function updateField(){
	var amount = $(FormUtil.getField("total_prop")).val() - $(FormUtil.getField("total_bonus")).val();

	$(FormUtil.getField("grand_total")).val(amount);

}

$(function(){
 
$(FormUtil.getField("total_prop")).change(function(){
	updateField();
});
 
$(FormUtil.getField("total_bonus")).change(function(){
	updateField();
});
 
});
 
 
</script>

Cheers

Hi Friends, thanks for your answer,

I think that Your code is very weel for my requirement, but in the variable "amount" i get the value NaN, i did decide to evaluate the content of the fields "total_pro" and "total_bonus" witjth te function isNaN () and surprisingly both throw out true As value, what do you think about this?

This is the definitive code

<h3>Nota: Todo abono a la factura será revisado por el departamento de finanzas. .</h3>
<!-- Javascript JQuery to substring string -->

<script type = "text/javascript" >

var field1 = isNaN(FormUtil.getField("total_abonado").val());

function updateField(){
var amount = $(FormUtil.getField("total_prop")).val() - $(FormUtil.getField("total_abonado")).val();

$(FormUtil.getField("porpagar")).val(amount);

}

$(function(){

$(FormUtil.getField("total_prop")).change(function(){
updateField();
});

$(FormUtil.getField("total_abonado")).change(function(){
updateField();
});

});

// To Debug
window.alert(field1);  // This show "TRUE"
// window.alert(a);
window.alert(amount); //This show NaN"
// window.alert(amounts);
// window.alert(5 + 6);
// console.log("x");
// document.write(5 + 6);

//});

</script>

thanks 

fabian