JSFiddle - React, Tailwind, and code Playground

HTML

<div id="wrapper">
    <div id="sideBar"></div>
    <div id="mainContent"></div>
</div>

CSS

#wrapper{
    width:600px;
    height:400px;
    background:#f5f5f5;
}

#sideBar{
    width:200px;
    height:200px;
    background:red;
    display:block;
    float:left;
}
#mainContent{
    height:200px;
    background:blue;
    display:block;
    float:left;
}

JavaScript

var w = $('#wrapper').width();
var s = $('#sideBar').width();
$('#mainContent').width(w - s);