Bill to Address same as Ship to

Question

I'm new to JavaScripting and I'm trying to create a PDF form that has a check box that when checked will auto fill information.  Specifically, when the user fills out the bill to address they can click the check box "same as billing" and it will populate the ship to address.  Any advice would be greatly appreciated.   
My data fields are as follows:
BTName
Billaddress1
Billaddress2
BillCity
BillState
BillZIP
SAMEAS (this is the check box)
STName
ShipAddress1
ShipAddress2
ShipCity
ShipState
ShipZIP
My JavaScript that I added to the check box is as follows:
// check the state of the button
if(this.getField('SAMEAS').value == '1') {
// checked
// copy requestor information
// get each requestor field object's value and set the sponsor field's value
this.getField('BTName').value = this.getField('STName').value;
this.getField('BillAddress1').value = this.getField('ShipAddress1').value;
this.getField('BillAddress2').value = this.getField('ShipAddress2').value;
this.getField('BillCity').value = this.getField('ShipCity').value;
this.getField('BillState').value = this.getField('ShipState').value;
this.getField('BillZIP').value = this.getField('ShipZip').value;

Answer 1

Sorry for not mentioning the problem...I was actually not getting anything to happen.  I do have the trigger to be mouse up, and run JavaScript.  Now with the updated script when I check the box it clears all the bill to address info and nothing appears in the shipping fields?  Thanks so much for the quick response.  
I just updated the script to the following:
// check the state of the button
if(event.target.value !== 'OFF') {
// checked
// copy requestor information
// get each requestor field object's value and set the sponsor field's value
this.getField('BTName').value = this.getField('STName').valueAsString;
this.getField('BillAddress1').value = this.getField('ShipAddress1').valueAsString;
this.getField('BillAddress2').value = this.getField('ShipAddress2').valueAsString;
this.getField('BillCity').value = this.getField('ShipCity').valueAsString;
this.getField('BillState').value = this.getField('ShipState').valueAsString;
this.getField('BillZIP').value = this.getField('ShipZip').valueAsString;