JS scroll to anchor

js.js

//requires helpers.js
function zScrollToAnimate(el,from,to,time){
  var start=new Date().getTime();
  var timer=setInterval(function(){
    var step=Math.min(1,(new Date().getTime()-start)/time);
    el.scrollTop=from+step*(to-from);
    //el.scrollTo(0,from+step*(to-from));
    if(step==1){clearInterval(timer);}
  },25);
}
/*function scrollToSection(event){
  event.preventDefault ? event.preventDefault():event.returnValue=false;
  if(typeof event.target!='undefined'){targetElement=event.target;}
  else{targetElement=window.event.srcElement;}
  var windowScrollPosition=document.body.scrollTop || document.documentElement.scrollTop;
  var scrollTo=windowScrollPosition+zQA(event.target.getAttribute('href'))[0].getBoundingClientRect().top-76;
  zScrollToAnimate(window,windowScrollPosition,scrollTo,250);
}*/
if(zQ('.js-scrollToAnchor')){
    for(i=0;i<zQA('.js-scrollToAnchor').length;i++){
      zBindEvent(zQA('.js-scrollToAnchor')[i],"click",function(e){
        var currentEl=zGetCurrentTargetElement(e),
          anchor=zQ(currentEl.getAttribute('href')),
          container=body,
          containerScrollPosition=container.scrollTop,
          offset=zQ('.site-header').getBoundingClientRect().height;
        scrollToAnchor(e,container,containerScrollPosition,anchor,offset,250);
      })
    }
  }
function scrollToAnchor(e,container,containerScrollPosition,anchor,buffer,speed){
  e.preventDefault ? e.preventDefault():e.returnValue=false;
  var scrollTo=containerScrollPosition+anchor.getBoundingClientRect().top-buffer;
  zScrollToAnimate(container,containerScrollPosition,scrollTo,speed);
}