Find number of total checkboxes in form or page Javascript

var formobj = document.getElementById('FORM_ID');
var counter = 0;
for (var j = 0; j < formobj.elements.length; j++)
{
    if (formobj.elements[j].type == "checkbox")
    {
        if (formobj.elements[j].checked)
        {
            counter++;
        }
    } 

}

alert('Total Checked = ' + counter);

This javascript is using to find number of total checkboxes in form or page and also 

user can find from all checkboxes how many is checked. In this 'FORM_ID' is the id 

of <form>.