鼠標滾輪事件（瀏覽器兼容性寫法）

1.這種寫法，看得好蒙圈，之后再仔細閱讀。

    //鼠標的滾動事件 兼容各主流瀏覽器
    /** Event handler for mouse wheel event. 
         *鼠標滾動事件 
         */  
         var wheel = function(event) {  
            var delta = 0;  
            if (!event) /* For IE. */  
                event = window.event;  
            if (event.wheelDelta) { /* IE/Opera. */  
                delta = event.wheelDelta / 120;  
            } else if (event.detail) {  
                /** Mozilla case. */  
                /** In Mozilla, sign of delta is different than in IE. 
                 * Also, delta is multiple of 3. 
                 */  
                delta = -event.detail / 3;  
            }  
            /** If delta is nonzero, handle it. 
             * Basically, delta is now positive if wheel was scrolled up, 
             * and negative, if wheel was scrolled down. 
             */  
            if (delta)  
                handle(delta);  
            /** Prevent default actions caused by mouse wheel. 
             * That might be ugly, but we handle scrolls somehow 
             * anyway, so don't bother here.. 
             */  
            if (event.preventDefault)  
                event.preventDefault();  
            event.returnValue = false;  
        }  
  
        /** Initialization code.  
         * If you use your own event management code, change it as required. 
         */  
        if (window.addEventListener) {  
            /** DOMMouseScroll is for mozilla. */  
            window.addEventListener('DOMMouseScroll', wheel, false);  
        }  
        /** IE/Opera. */  
        window.onmousewheel = document.onmousewheel = wheel;  
  
        /** This is high-level function. 
         * It must react to delta being more/less than zero. 
         */  
        var handle = function(delta) {  
            var random_num = Math.floor((Math.random() * 100) + 50);  
            if (delta < 0) {  
                //  alert("鼠標滑輪向下滾動：" + delta + "次！"); // 1  
                return;  
            } else {  
                //  alert("鼠標滑輪向上滾動：" + delta + "次！"); // -1  
                return;  
            }  
        }