Pre-populate Loan Payment Schedule

<script type="text/javascript">
    function populate(){
        
        var loanAmount = parseFloat(FormUtil.getValue("loan_amount"));
        var loanInterest = parseFloat(FormUtil.getValue("loan_interest"));
        var loanMonthly = parseFloat(FormUtil.getValue("loan_monthly"));
        
        var field = FormUtil.getField("payment_schedule");
        var functionName = window[field.attr("id") + "_add"];
        if(typeof functionName === 'function') {
        
            var seq = 1;
            while(loanAmount>0){
                var balanceBefore = loanAmount;
                var interest = loanAmount * loanInterest/100 / 12;
                var principal = loanMonthly - interest;
                var balanceAfter = loanAmount - principal;
                loanAmount = balanceAfter;
                
                var currentDate = new Date();
                currentDate.setMonth(currentDate.getMonth() + seq);
                var paymentDate = currentDate.toISOString().slice(0, 10);
        
                if(interest > loanMonthly){
                    alert("Monthly repayment must be higher than interest of " + interest);
                    break;
                }
                
                if(balanceAfter<0){
                    loanMonthly += balanceAfter;
                    balanceAfter = 0;
                    principal = loanMonthly - interest;
                }
                
                var args = new Object();
                args['result'] = '{"payment_seq":"'+ seq +'","balance_before":"' + balanceBefore + '","balance_after":"","payment_date":"' + paymentDate + '","payment_amount":"' + loanMonthly + '","payment_interest":"' + interest + '","payment_principal":"' + principal + '","balance_after":"' + balanceAfter + '"}';
                functionName(args);
                
                seq++;
            }
        }
        $.unblockUI();
    }
</script>
    <button onClick="$.blockUI(); setTimeout('populate()',500); return false; "><strong style="font-size:130%">Populate</strong></button>