Get selected check box through JavaScript

window.onload = function () {
        getChkItem();
    }
function getChkItem() {
var chk = "";
var arrChk = new Array('cbID_1','cb_2',cb_3,cb_4,cb_6);  // checkbox 1 ID & check 2
for(iCtr=0; iCtr < arrChk.length; iCtr++){
 var element = document.getElementById(arrChkRht[iCtrRht]);
 if (typeof(element) != 'undefined' && element != null) {  if(document.getElementById(arrChk[iCtr]).checked){
chk = chk  + arrChk[iCtr].split("_")[1] + "," ;            
                }
}
alert(chk.slice(0, -1)); }