jquery checkbox

var folderids = [];
            $.each( $("#list_periodical").find("input[name='folder']"), function(i, v){
            if( $(v).attr("checked") == "checked" ){
            folderids.push( $(v).val() );
            }
            });
           
            var sourceids = [];
            $.each( $("#list_periodical").find("input[name='source']"), function(i, v){
           
            if( $(v).attr("checked") == "checked" ){
            sourceids.push( $(v).val() );
            }
           
            });
           
            var typeids = [];
            $.each( $("#list_periodical").find("input[name='typeids']"), function(i, v){
            if( $(v).attr("checked") == "checked" ){
            typeids.push( $(v).val() );
            }
            });
           
            var weiboids = [];
            $.each( $("#list_periodical").find("input[name='weibo']"), function(i, v){
            if( $(v).attr("chekced") == "checked" ){
            weiboids.push( $(v).val() );
            }
            });


在四个checkbox都被勾选的情况下，且都有value，猜这4个数组中哪个数组始终为null