JSFiddle - React, Tailwind, and code Playground

HTML

<div id="Left">123123</div>

CSS

div { float: left; border: solid black 1px; width: 10%; }

JavaScript

var element = document.getElementById('Left');
var style = getComputedStyle(element, null);
var initial_width = style.width;
var initial_pixels = parseFloat(initial_width);
element.style.width = initial_pixels + 240 + "px";