JSFiddle - React, Tailwind, and code Playground

HTML

<p class="o" id="a"></p>
<p class="o" id="b"></p>

JavaScript

var turnObjToArray = function(obj) {
  return [].map.call(obj, function(element) {
    return element;
  })
};

var a=document.getElementById("a");    
var b=document.getElementById("b");
console.log(a);
var pe=document.getElementsByClassName("o");
var listBox = turnObjToArray(pe);
console.log(listBox);
alert(listBox.indexOf(a));