Problem with getting radio button value

Hello, im having a bit problem getting the radio button value, here's the code

<SCRIPT LANGUAGE="JavaScript"><!--

function changeForm(what) {
    for (var i=0; i<what.options.length; i++) {
        if (what.options[i].selected) {
            if (document.all)         document.all[what.options[i].value].style.visibility="visible";
            else if (document.layers) document.layers[what.options[i].value].visibility = "visible";
        }
        else {
            if (document.all)         document.all[what.options[i].value].style.visibility="hidden";
            else if (document.layers) document.layers[what.options[i].value].visibility = "hidden";
        }
    }
}

//--></SCRIPT>

<FORM name="orderform" id="orderform">
Form1: <input type="radio" value="form1" name="persontype" onChange="changeForm(this.document.orderform.persontype[0].checked)"> 
Form2: <input type="radio" value="form2" name="persontype" onChange="changeForm(this.document.orderform.persontype[0].checked)">
</FORM>


<DIV STYLE="position: absolute">
</DIV>

<DIV ID="form1" style="position: absolute; top: 100; left: 100; visibility: visible;">
<FORM>
Form 1 - <INPUT TYPE="TEXT" NAME="myName1" VALUE="myValue1">
</FORM>
</DIV>

<DIV ID="form2" style="position: absolute; top: 100; left: 100; visibility: hidden;">
<FORM>
Form 2 - <INPUT TYPE="TEXT" NAME="myName2" VALUE="myValue2">
</FORM>
</DIV>

<DIV ID="form3" style="position: absolute; top: 100; left: 100; visibility: hidden;">
<FORM>
Form 3 - <INPUT TYPE="TEXT" NAME="myName3" VALUE="myValue3">
</FORM>
</DIV>

Code (markup):

this.document.orderform.persontype[0].checked doesnt seem to work or maybe i have forgotten something.

 

1. use onlick and pass selected radio button value using 'value' property
<FORM name="orderform" id="orderform">
Form1: <input type="radio" value="form1" name="persontype" onclick="changeForm(this.value)">
Form2: <input type="radio" value="form2" name="persontype" onclick="changeForm(this.value)">
</FORM>

2. use changeForm function idea below
function changeForm(what)
{
if(what == 'form1')
{
alert('show form1, hide form2');
}

if(what == 'form2')
{
alert('show form2, hide form1');
}

}