JSFiddle - React, Tailwind, and code Playground
HTML
<div id="container" style="border: 1px solid lightblue; height: 100px; width: 400px;"></div>
JavaScript
var width = document.getElementById('container').style.width;
alert(width);
<div id="container" style="border: 1px solid lightblue; height: 100px; width: 400px;"></div>
var width = document.getElementById('container').style.width;
alert(width);