JavaScript 实现返回顶部按钮的完整案例

.backtotop {
    position: fixed;
    bottom: 80px;
    right: 80px;
    width: 80px;
    height: 80px;
    background-color: #ccc;
    font-size: 20px;
    text-align: center;
    padding-top: 12px;
    box-sizing: border-box;
    cursor: pointer;
    color: #000;
    /* 先隐藏按钮 */
    display: none;
  }

<a href="javascript:;" class="backtotop" id="backtotop">返回<br>顶部</a>

a {
    text-decoration: none;
  }

  body {
    height: 5000px;
  }

  .backtotop {
    position: fixed;
    bottom: 80px;
    right: 80px;
    width: 80px;
    height: 80px;
    background-color: #ccc;
    font-size: 20px;
    text-align: center;
    padding-top: 12px;
    box-sizing: border-box;
    cursor: pointer;
    color: #000;
    /* 先隐藏按钮 */
    display: none;
}

<script>
(function(){
  //获得元素
  var backtotop = document.getElementById('backtotop');

  var timer;
  backtotop.onclick = function(){
    //设表先关，防止定时器冲突
    clearInterval(timer);

    //设置定时器
    timer = setInterval(function(){

      // 更改根元素的scrollTop元素值
      //兼容性问题
      var top = document.documentElement.scrollTop || document.body.scrollTop;
      top = top - 80;
      document.documentElement.scrollTop = top;
      document.body.scrollTop = top;

      //判断
      if(top <= 0) {
        //关闭定时器
        clearInterval(timer);
      }
    },20);
  };

  //监听页面滚动
  window.onscroll = function() {
    //得到卷动值
    var scrollTop = document.documentElement.scrollTop || document.body.scrollTop || window.scrollY;

    //当页面没有卷动时，返回顶部 按钮就隐藏
    if(scrollTop == 0) {
      backtotop.style.display = 'none';
    }else {
        backtotop.style.display = 'block';
    }
  };
})();

<script>