js 监听滚动条事件

/*监听滚动条，显示或者隐藏某元素*/
window.onscroll = function(){ 
    var t = document.documentElement.scrollTop || document.body.scrollTop;  
    var subBox = document.getElementById( "sus-box" );//要显示或者隐藏的元素
    if( t >= 300 ) { 
        subBox.style.display = "block"; 
    } else { 
        subBox.style.display = "none"; 
    } 
}