JSFiddle - React, Tailwind, and code Playground

HTML

<div id="main">
  <div>1</div>
  <div>2</div>
</div>

CSS

.foo1
{
    color:blue;    
}
.foo2
{
    color:red;    
}

JavaScript

var elem = document.getElementById("main");
var childDivs = elem.getElementsByTagName("div");
for (var i = 0; i < childDivs.length; i++) 
{        
    childDivs[i].className = 'foo' + (i+1).toString();
}