JSFiddle - React, Tailwind, and code Playground

HTML

<div>
    <p></p>
    <p></p>
    <p class="target"></p>
    <p></p>
</div>

JavaScript

var ps = document.getElementsByTagName("p");
var num = 0;
for(var i=0; i<ps.length; i++) {
 var p = ps[i];
 if(p.className == 'target') {
 num = i+1;
 break;
 }
}
alert(num);