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//获取滚动条当前的位置 
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;
}
//获取当前可视范围的高度 
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;
}
//获取文档完整的高度 
function getScrollHeight() {
    return Math.max(document.body.scrollHeight, document.documentElement.scrollHeight);
}
//滚动事件触发
window.onscroll = function() {
    if(getScrollTop() + getClientHeight() + 10 > getScrollHeight()) {
        alert("到底了")
    }
}