JS 原生JS 判断滚动条滑动到底部

window.addEventListener("scroll", function(event) {

    var scrollTop = document.documentElement.scrollTop || window.pageYOffset || document.body.scrollTop;

    console.log(scrollTop);

    if(document.documentElement.scrollHeight ==     document.documentElement.clientHeight + scrollTop ) {

    alert("到底部啦");

}

});