js中一些高度

//文档高度
getDocumentTop() {
  var scrollTop = 0,
    bodyScrollTop = 0,
    documentScrollTop = 0;

  if (document.body) {
    bodyScrollTop = document.body.scrollTop;
  }

  if (document.documentElement) {
    documentScrollTop = document.documentElement.scrollTop;
  }

  scrollTop =
    bodyScrollTop - documentScrollTop > 0
      ? bodyScrollTop
      : documentScrollTop;
  return scrollTop;
},
//可视窗口高度
getWindowHeight() {
  var windowHeight = 0;
  if (document.compatMode == "CSS1Compat") {
    windowHeight = document.documentElement.clientHeight;
  } else {
    windowHeight = document.body.clientHeight;
  }

  return windowHeight;
},
//滚动条滚动高度
getScrollHeight() {
  var scrollHeight = 0,
    bodyScrollHeight = 0,
    documentScrollHeight = 0;

  if (document.body) {
    bodyScrollHeight = document.body.scrollHeight;
  }

  if (document.documentElement) {
    documentScrollHeight = document.documentElement.scrollHeight;
  }

  scrollHeight =
    bodyScrollHeight - documentScrollHeight > 0
      ? bodyScrollHeight
      : documentScrollHeight;
  return scrollHeight;
}