how to get scrollTop

We learn an event: page scrolling effect 

window.onscroll = function() { page scroll statement }

Google Chrome and no DTD <DOCTYPE> declared: 

  document.body.scrollTop; 

Firefox and other browsers  

  document.documentElement.scrollTop;

 Both ie9+ and latest browsers know

  window.pageXOffset;     pageYOffset  （scrollTop）

 Compatibility writing:

 var scrollTop = window.pageYOffset || document.documentElement.scrollTop || document.body.scrollTop || 0;