JSFiddle - React, Tailwind, and code Playground

HTML

<div class="fawas">
    this is my name fawas khan.
</div>

JavaScript

function getElementsByClass(tagType, className) {
    var elems = document.getElementsByTagName(tagType);
    var returns = [];
    for (var i in elems) {
        if ((' ' + elems[i].className + ' ').indexOf(' ' + className + ' ') > -1) {
            returns.push(elems[i]);
        }
    }
    return returns;
}

var dynamic = document.createElement("div");
dynamic.innerHTML = "this is my contact number.";

var elements = getElementsByClass("div", "fawas");
if (elements.length > 0) {
    // just change the first, as you did in your post
    elements[0].parentNode.insertBefore(dynamic, elements[0].nextSibling);
}