JS 滚轮事件实现固定在窗口某个位置的元素显现或隐藏

<div id="box"></box>

html,body{
   	height: 1000px;
}
#box{
	width: 100%;
	height: 200px;
	background-color: red;
	position: fixed;
	bottom: 0;
	display: none;
}

//获取元素
var box = document.getElementById('box');

//判断是否为火狐浏览器
//绑定滚轮事件
if (navigator.userAgent.indexOf('Firefox') != -1) {
    document.addEventListener('DOMMouseScroll', wheelFun, false);
} else {
    //非火狐
    document.onmousewheel = wheelFun;
}


function wheelFun(event) {
    //event兼容
    var event = event || window.event;
    //页面滚动上去的高度兼容
    var top = document.documentElement.scrollTop || document.body.scrollTop;
    
    //判断鼠标滚动方向
    if (event.detail) {
    	if(event.detail > 0) {
    		console.log('向下');
    	} else {
    		console.log('向上');
    	}
    } else if (event.wheelDelta) {
    	//非火狐
    	if (event.wheelDelta > 0) {
    		//console.log('向上');
    		top--;
    	} else {
    		//console.log('向下');
    		top++;
    	}
    
    //简单判断 改变box的样式
    if (top > 200) {
    	box.style.display = 'block';
    } else {
    	box.style.display = 'none';
    }