js鼠标滚轮事件如何做兼容

兼容谷歌，火狐，IE

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jquery-mousewheel

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window.onload=function()
    {
        var oBox=document.getElementById('box');
        
        wheel(oBox,function(down){
            alert(down);
        });
    }
    
    function wheel(obj,wheelFn)
    {
        window.navigator.userAgent.indexOf('Firefox')!=-1 ? obj.addEventListener('DOMMouseScroll',fn,false) : addEvent(obj,'mousewheel',fn);
        
        function fn(ev)
        {
            var oEvent=ev||event;
            var down=oEvent.wheelDelta ? down=oEvent.wheelDelta<0 : down=oEvent.detail>0;
            ev.preventDefault&&ev.preventDefault();
            wheelFn&&wheelFn(down);
            return false;
        }
    }
    
    function addEvent(obj,sEv,fn)
    {
         obj.addEventListener ? obj.addEventListener(sEv,fn,false) : obj.attachEvent('on'+sEv,fn);
    }