JSFiddle - React, Tailwind, and code Playground

HTML

<section id="box"></section>
<p>
<section id="box2"></section>

CSS

#box, #box2 {
    height: 10px;
    width: 5px;
    background-color: lightBlue;
}
#box2 {
    width: 4px;
}

JavaScript

var savedwidth = document.getElementById("box").offsetWidth;
alert("box 1 width " + savedwidth);
savedwidth = document.getElementById("box2").offsetWidth;
alert("box 2 width " + savedwidth);