<script type="text/javascript">
    function validation() {
        var inputs = document.querySelectorAll("[class^=class_]");
        var hash = {};
        var checked = 1;
        for(var i = 0; i < inputs.length; i++) {
            var counter = 0;
            if(inputs[i].type == "checkbox" && inputs[i].offsetParent!== null) {
                if (hash[inputs[i].classList]) {
                    counter = hash[inputs[i].classList];
                }
                if (inputs[i].checked) {
                    counter = counter + 1;
                }
                hash[inputs[i].classList] = counter;
            }
        }
        Object.keys(hash).forEach(function (key) {
            var value = hash[key]
            if(value == 0) {
                checked = 0
            }
        })
        if(checked == 0) {
            alert("You must check at least one checkbox.");
            return false;
        }
    }
    object.addEventListener("submit", validation());
</script>