//页面加载后触发
window.onload = function(){
  var btn = document.getElementById('btn');
  var timer = null;
  var isTop = true;
  //获取页面可视区高度
  var clientHeight = document.documentElement.clientHeight;

  
  //滚动条滚动时触发
  window.onscroll = function() {
  //显示回到顶部按钮
    var osTop = document.documentElement.scrollTop || document.body.scrollTop;
    if (osTop >= clientHeight) {
      btn.style.display = "block";
    } else {
      btn.style.display = "none";
    };
  //回到顶部过程中用户滚动滚动条，停止定时器
    if (!isTop) {
      clearInterval(timer);
    };
    isTop = false;

  };

  btn.onclick = function() {
    //设置定时器
    timer = setInterval(function(){
      //获取滚动条距离顶部高度
      var osTop = document.documentElement.scrollTop || document.body.scrollTop;
      var ispeed = Math.floor(-osTop / 7);
      
      document.documentElement.scrollTop = document.body.scrollTop = osTop+ispeed;
      //到达顶部，清除定时器
      if (osTop == 0) {
        clearInterval(timer);
      };
      isTop = true;
      
    },30);
  };
};