Multiple checkbox values into hidden field on click

I have a couple checkboxes, on clicking each I would like their value to go into a hidden field with jQuery seperated by commas, if they are all unchecked, the hidden field value would go back to -1. Below is what I have tried.

<input type="hidden" name="agreement_ids" value="-1">

<input type="checkbox" id="agreementChecker-1" name="agreeCheck-1" value="35" class="mandatory agreeChecker">
<input type="checkbox" id="agreementChecker-2" name="agreeCheck-2" value="45" class="mandatory agreeChecker">
<input type="checkbox" id="agreementChecker-3" name="agreeCheck-3" value="52" class="mandatory agreeChecker">

$('.agreeChecker').click(function(){

        if($(this).prop('checked')){

            var resultObj = $(this).val();
            var outputObj = $('input[name="agreement_ids"]');
            var stringToAppend = resultObj.val().length > 0 ? resultObj.val() + "," : "";
        resultObj .val( stringToAppend + outputObj.val() );

        }else{
            if($('.agreement-text').length > 0){
                $('input[name="agreement_ids"]').val('-1');//uncheck
            }else{
                $('input[name="agreement_ids"]').val('');
            }
        }
    });