JSFiddle - React, Tailwind, and code Playground

HTML

<div id = "panel"></div>
<button id = "cambiar">Cambiar</button>

CSS

#panel{
    width: 141px;
    border: .1em #549 solid;
    height: 4em;
    transition: .5s;
}

JavaScript

var panel = document.getElementById("panel"),
    boton = document.getElementById("cambiar");

boton.addEventListener("click", function(){
    panel.style.width = panel.style.width == "500px" ? "141px" : "500px";
}, false);