How to count checked ckeckbox using onclick even ref by id?

How to count checked ckeckbox using onclick even ref by id ?

http://ift.tt/15inozr

Html is

<div id="yyy">
<input type="checkbox" id="check_id" onclick="xxx()" checked>
<input type="checkbox" id="check_id" onclick="xxx()">
<input type="checkbox" id="check_id" onclick="xxx()">
<input type="checkbox" id="check_id" onclick="xxx()" checked>
<input type="checkbox" id="check_id" onclick="xxx()">
<div>

and javascript is

function xxx()
{
    var zzz = document.getElementById("check_id").length;
    alert(zzz);    
}