auto tab between jsf form fields

Hi all I am having some trouble getting this to work.


I am getting an error Elem is null or not an object.

here is my java script:

function autotab(original,destination)
{

if (document.all) // if IE
{

if (event.keyCode == 9 || //tab
event.keyCode == 16 || //backtab
event.keyCode >= 37 && event.keyCode <= 40 || //arrow keys
event.keyCode == 45 || //insert
event.keyCode == 46 || //delete
event.shiftKey) //shift
{ return } // no autotab on tab/backtab/insert/delete/shift
}


if
(original.getAttribute&&original.value.length==original.getAttribute("maxlength"))
elem = document.getElementById(destination);
elem.focus()
}
</SCRIPT>


and here is my jsf code that is invoking the function:


<h:inputText
styleClass="inputText" id="ssn1"
size="3"
onkeyup="autotab(this, 'ssn2');"
maxlength="3">
<f:validateLength
maximum="3"></f:validateLength>
<hx:validateConstraint regex="^[0-9]+$"
/>
</h:inputText>-

<h:inputText styleClass="inputText" id="ssn2"
size="2" onkeyup="autotab(this,'ssn3');"
maxlength="2">
<f:validateLength
maximum="2"></f:validateLength>
<hx:validateConstraint
regex="^[0-9]+$"/>
</h:inputText>-

<h:inputText styleClass="inputText" id="ssn3"
size="4" maxlength="4">
<f:validateLength
maximum="4"></f:validateLength>
<hx:validateConstraint regex="^[0-9]+$"
/>
</h:inputText>


Thanks.