获取页面高度，窗口高度，滚动条高度等参数值

最近在项目中想实现Div层垂直居中显示，可是无论怎么尝试都无法实现在IE中获得当前Page的高度。后来Google到下面的代码，发现原来是利用document.documentElement.clientHeight，真让人有点莫名其妙。

function getPageScroll(){
  var yScroll;
  if (self.pageYOffset) {
    yScroll = self.pageYOffset;
  } else if (document.documentElement && document.documentElement.scrollTop){   // Explorer 6 Strict
    yScroll = document.documentElement.scrollTop;
  } else if (document.body) {// all other Explorers
    yScroll = document.body.scrollTop;
  }

  arrayPageScroll = new Array('',yScroll)
  return arrayPageScroll;
}

function getPageSize(){  
  var xScroll, yScroll;  
  if (window.innerHeight && window.scrollMaxY) {  
    xScroll = document.body.scrollWidth;
    yScroll = window.innerHeight + window.scrollMaxY;
  } else if (document.body.scrollHeight > document.body.offsetHeight){ // all but Explorer Mac
    xScroll = document.body.scrollWidth;
    yScroll = document.body.scrollHeight;
  } else { // Explorer Mac...would also work in Explorer 6 Strict, Mozilla and Safari
    xScroll = document.body.offsetWidth;
    yScroll = document.body.offsetHeight;
  }

  var windowWidth, windowHeight;
  if (self.innerHeight) {  // all except Explorer
    windowWidth = self.innerWidth;
    windowHeight = self.innerHeight;
  } else if (document.documentElement && document.documentElement.clientHeight) { // Explorer 6 Strict Mode
    windowWidth = document.documentElement.clientWidth;
    windowHeight = document.documentElement.clientHeight;
  } else if (document.body) { // other Explorers
    windowWidth = document.body.clientWidth;
    windowHeight = document.body.clientHeight;
  }  
  
  // for small pages with total height less then height of the viewport
  if(yScroll < windowHeight){
    pageHeight = windowHeight;
  } else {
    pageHeight = yScroll;
  }

  if(xScroll < windowWidth){  
    pageWidth = windowWidth;
  } else {
    pageWidth = xScroll;
  }

  arrayPageSize = new Array(pageWidth,pageHeight,windowWidth,windowHeight)
  return arrayPageSize;
}