JSFiddle - React, Tailwind, and code Playground

HTML

<div id="left">
    Menu
</div>
<div style="text-align:center;">
<div id="right">
    Content
</div>
</div>

CSS

#left {
    background-color: green; /*for demonstration purposes*/
    float:left;
}

#right{
    background-color: blue;
    width: 800px;
}
}

JavaScript

$(document).ready(function(){
    var width = $(document).width();
    var rightDivWidth = $("#right").width();
    var leftDivWidth = width - rightDivWidth;
    $("#left").css("width", leftDivWidth);
});