//頁面加載后觸發(fā)
window.onload = function(){
  var btn = document.getElementById('btn');
  var timer = null;
  var isTop = true;
  //獲取頁面可視區(qū)高度
  var clientHeight = document.documentElement.clientHeight;

  
  //滾動條滾動時觸發(fā)
  window.onscroll = function() {
  //顯示回到頂部按鈕
    var osTop = document.documentElement.scrollTop || document.body.scrollTop;
    if (osTop >= clientHeight) {
      btn.style.display = "block";
    } else {
      btn.style.display = "none";
    };
  //回到頂部過程中用戶滾動滾動條，停止定時器
    if (!isTop) {
      clearInterval(timer);
    };
    isTop = false;

  };

  btn.onclick = function() {
    //設(shè)置定時器
    timer = setInterval(function(){
      //獲取滾動條距離頂部高度
      var osTop = document.documentElement.scrollTop || document.body.scrollTop;
      var ispeed = Math.floor(-osTop / 7);
      
      document.documentElement.scrollTop = document.body.scrollTop = osTop+ispeed;
      //到達頂部，清除定時器
      if (osTop == 0) {
        clearInterval(timer);
      };
      isTop = true;
      
    },30);
  };
};