function getScrollHeight() {
  var maxDisplacement=0;
  
  window.scrollTo(0,10000000);
  
  if( typeof self.pageYOffset!='undefined' )
    maxDisplacement=self.pageYOffset;
  else if( document.compatMode && document.compatMode != 'BackCompat' )
    maxDisplacement=document.documentElement.scrollTop ;
  else if( document.body && typeof(document.body.scrollTop)!='undefined' )
    maxDisplacement=document.body.scrollTop;

  window.scrollTo(0,0);
  return maxDisplacement;
}

window.onload=function(){
  var h = getScrollHeight();
  alert(h);
}

getScrollHeight()

JavaScript