JSFiddle - React, Tailwind, and code Playground

HTML

<div id="sideBar">asd</div>

CSS

#sideBar {
  float: left;
  width: 27px;
 
    
}

JavaScript

var width = ( 100 * parseFloat($("#sideBar").css('width')) / parseFloat($("#sideBar").parent().css('width')) ) + '%';

alert(width);