JS:Get all SelectBox Options for adding hidden field

lb = document.form.listbox;
 hiddenfield = document.form.hidden;
 if(hiddenfield.value.length > 0) hiddenfield.value="";

 for (i=0;i < lb.length;i++) {
    if (!isNaN(lb.options[i].value)) {
    if(hiddenfield.value.length > 0)
    hiddenfield.value = hiddenfield.value + "," + lb.options[i].value;
    else
    hiddenfield.value = hiddenfield.value + lb.options[i].value;
    }
 }