JSFiddle - React, Tailwind, and code Playground

HTML

<div class="hihi"></div>
<div class="hihi"></div>

CSS

.hihi {
  border: 10px solid #f00;
}

.mimi {
  border: 10px solid #0489B1;
}

JavaScript

var elements = document.getElementsByClassName("hihi");
for(var i=elements.length;i--;){
  elements[i].className = "mimi";
}