In my HTML view page, I have written this on the section

 <div class="stats-small__data">
   <span class="stats-small__percentage stats-small__percentage--" id="SatisPrecent"></span>
 </div>

Also, I created an ajax result to get value from the controller, and depending on that I want to add increase to the end of stats-small__percentage stats-small__percentage--

What I did was

 if (data.SatisAva > 50) {
   document.getElementById("SatisPrecent").className = "increase";
 } else {
   document.getElementById("SatisPrecent").className = "decrease";
 }

But it didn't work. Can any one help me on this please?