原生JavaScript实现返回顶部的通用方法

function backTop(btnId) {
  var btn = document.getElementById(btnId);
  var d = document.documentElement;
  var b = document.body;
  window.onscroll = set;
  btn.style.display = "none";
  btn.onclick = function() {
    btn.style.display = "none";
    window.onscroll = null;
    this.timer = setInterval(function() {
      d.scrollTop -= Math.ceil((d.scrollTop + b.scrollTop) * 0.1);
      b.scrollTop -= Math.ceil((d.scrollTop + b.scrollTop) * 0.1);
      if ((d.scrollTop + b.scrollTop) == 0) clearInterval(btn.timer, window.onscroll = set);
    },
    10);
  };
  function set() {
    btn.style.display = (d.scrollTop + b.scrollTop > 100) ? 'block': "none"
  }
};
backTop('goTop');