JSFiddle - React, Tailwind, and code Playground

HTML

<button id="btnChange">Change width</button>
<br>
<div class="mydiv">test</div>

CSS

.mydiv {
    width:50px;
    background-color:red;
}

JavaScript

var b = document.getElementById("btnChange");
b.onclick = function(event) {
    var divs =document.getElementsByTagName("div");
    for(var i=0;i<=divs.length;i++) {
        if(divs[i].className == "mydiv") {
            divs[i].style.width = "100px";            
        }
    }
};