实现不了啊亲，卡在这里好久了，哪里出错了呢~

<script type="text/javascript">
        
        function hasClass( obj, cls ){
            return obj.className.match(new RegExp("(\\s|^)" + cls + "(\\s|$)"));
        }

        function removeClass( obj, cls ){ 
        if( hasClass( obj, cls )){ 
            
            var reg = new RegExp("(\\s|^)" + cls + "(\\s|$)");
            obj.className = obj.className.replace(reg, "");
        }
    }
        function addClass( obj, cls ){
            if(!hasClass(obj,cls)){
                obj.className = " current";
            }
            // obj.className = " current";
        }

        //根据CLASSname获取元素
        // function getByClassName( obj, cls ){
        //     var elenemts = obj.getElementsByTageName("*");
        //     alert(elements);
        //     var result = [];
        //     for(var i = 0 ; i < elenemts.length; i++){
        //         if(elenemts[i].className == cls){
        //             result.push(elements[i]);
        //         }
        //     }
        //     return result;
            
        // }
		
        window.onload = function() {
        
            window.onscroll = function(){
            	
                var top = document.body.scrollTop || document.documentElement.scrollTop; 
               
               
          
                var menus = document.getElementById("menu").getElementsByTagName("a");
                
                var items = document.getElementById("content").getElementsByClassName("item");

                
                var currentId = "";
                
                for ( var i=0; i< items.length; i++ ){ 
                    var _item = items[i];
                    var _itemTop = _item.offsetTop;

                    // console.log(top);
                    // console.log(_itemTop-200);
                    if( top > _itemTop - 200){ 
                        currentId = _item.id;
                        // alert(currentId);

                    } else { 
                        break;
                    }  
               }         
               

                if(currentId){
                    //给正确A元素加CLASS
                    for(var i = 0; i < menus.length; i++){
                        var _menu = menus[i];
                         var _href = _menu.href.split("#");
                       // console.log(_href);

                        if(_href[_href.length-1] == currentId){
                        	 addClass(_menu,"current");
                           
                           
                        }
                        else{
                            removeClass(_menu,"current");
                           // alert(_menu.className);
                        }
                    }
                }
            }

        }
    </script>