How to Disable text box using Javascript for DDL events

script type="text/javascript"
function PAChange()

debugger
var ChangeSelect=document.getElementById("ddl_pachng").selectedIndex;
if((ChangeSelect==1) (ChangeSelect==2))
{
document.form1.txtpacd.disabled=false;
}
if(ChangeSelect==3)
{
document.form1.txtpacd.disabled=true;
}
}
script



////==================================call the above javascript inbody onload and dropdownlist change event

body onload="JavaScript:PAChange();"