原生js的触底加载

•  function 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;
          }
  
          function getWindowHeight() {
              var windowHeight = 0;
              if (document.compatMode == "CSS1Compat") {
                  windowHeight = document.documentElement.clientHeight;
              } else {
                  windowHeight = document.body.clientHeight;
              }
              return windowHeight;
          }
  
          function 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;
          }
          window.onscroll = function() {
              if (getScrollHeight() <= getWindowHeight() + getDocumentTop()) {
                  console.log('bottom');
                  pageSize++
                  getList('0', pageSize)
              }
          }