<div class="fixedDiv">123213</div>
<div class="fixedDiv fixedDiv_second">312312</div>

body {
  height: 10000px;
}

#fixedDiv {
  height: 100px;
}

.fixedDiv_second {
  font-size: 30px;
}

#red {
  background: red;
}

#blue {
  background: blue;
}

.fixed-top {
  position: fixed;
  top: 0;
}

.fixed-bottom {
  position: fixed;
  bottom: 0;
  background: green;
}

var oldScrollY = 0;
var divs = [].slice.call(document.querySelectorAll(".fixedDiv"));

 window.onscroll = function() {
   var scrolled = window.pageYOffset || document.documentElement.scrollTop;
   var dY = scrolled - oldScrollY;
  
   if ( dY > 0 ) {
     divs.forEach(function (div) {
       div.classList.remove('fixed-top');
       div.classList.add('fixed');
       div.classList.add('fixed-bottom');
     });
  } else {
    divs.forEach(function (div) {
      console.log(div);
      div.classList.remove('fixed-bottom');
      div.classList.add('fixed');
       div.classList.add('fixed-top');
     });
   }

  oldScrollY = scrolled;
}