Javascript - นับจำนวน Checkbox ที่ถูก Check

Javascript - นับจำนวน Checkbox ที่ถูก Check

    function checkboxAction() {
        var chk1 = document.getElementById('<%=CheckBox1.ClientID%>');
        var chk2 = document.getElementById('<%=CheckBox2.ClientID%>');
        var chk3 = document.getElementById('<%=CheckBox3.ClientID%>');
        var lab1 = document.getElementById('<%=Label1.ClientID%>');

        if (chk1.checked)
            a1 = 1;
        else
           a1 = 0;      /// lab1.innerHTML = parseInt(lab1.innerHTML) + 1

        if (chk2.checked)
            a2 = 1;
        else
            a2 = 0;

        if (chk3.checked)
            a3 = 1;
        else
            a3 = 0;
        lab1.innerHTML = a1 + a2 + a3;

    }