//獲取滾動條距離頂部位置function getScrollTop() { var scrollTop = 0; if (document.documentElement && document.documentElement.scrollTop) {  scrollTop = document.documentElement.scrollTop; } else if (document.body) {  scrollTop = document.body.scrollTop; } return scrollTop;}//獲取當前可視范圍的高度function getClientHeight() { var clientHeight = 0; if (document.body.clientHeight && document.documentElement.clientHeight) {  clientHeight = Math.min(document.body.clientHeight, document.documentElement.clientHeight); } else {  clientHeight = Math.max(document.body.clientHeight, document.documentElement.clientHeight); } return clientHeight;}//獲取文檔完整的高度function getScrollHeight() { return Math.max(document.body.scrollHeight, document.documentElement.scrollHeight);}//判斷滾動條是否達到底部getScrollTop() + getClientHeight() == getScrollHeight()