Check/Uncheck all check box using java script

Html code

< input type="checkbox" class="pTable" id="Checkbox1" onclick="checkedAllnonsurgical(this);" runat="server" > Select All

Java script

< script type="text/javascript" language="javascript" >
function checkedAll(e) {
var aa = document.forms[0];
var checked = e.checked
for (var i = 0; i < aa.elements.length; i++) {
var ValId = aa.elements[i].id;
if (ValId.indexOf('chkp') != -1) {
aa.elements[i].checked = checked;
}
}

}
< script >