prev()_next()使用报错

      //购物车数量的自增自减
	function qtyUpdate(kind,obj){
	    var count = 0;
	    if(kind == "up"){
	       	var tmp=obj.prev();
	       	count=$(tmp).val();
	       	count++;
	       	$(tmp).val(count);
	    }else if(kind == "down"){
	        if(count > 1) {
		        var ne=obj.next();
		        count=$(ne).val();
		        count--;
		        $(ne).val(count);
	        }
	    }
	}
    

---------------------------------------******************正解******************----------------------------------------

      //购物车数量的自增自减
   	function qtyUpdate(kind,obj){
		var count = 0;
		if(kind == "up"){
		    var tmp=$(obj).prev();
		    count=$(tmp).val();
		     count++;
		     $(tmp).val(count);
		}else if(kind == "down"){
	             var ne=$(obj).next();
		     count=$(ne).val();
		     if(count > 1) {
			count--;
			$(ne).val(count);
		     }
		}
	}