JSFiddle - React, Tailwind, and code Playground

HTML

<div class="fs-id2 a"></div>
<div class="fs-id3 b"></div>
<div class="fs-id4 c"></div>

JavaScript

$("[class^=fs-id]").each(function(){
      var num = this.className.split(' ')[0].match(/fs-id(\d)/)[1];
      alert(num);
});