JSFiddle - React, Tailwind, and code Playground

HTML

<div id="menuEsquerda">asd<br>asdasdasdasda</br>asdsadasdadas</div>
    
<div id="barraFavoritos">asd asd asd asd sa</div>

CSS

* {
    padding:0px;
    margin:0px;
}
#menuEsquerda{
height: 100%;
width: auto;
background-color: #33627a;
position: fixed;
}
 
#barraFavoritos{
background-color: #dfdfdf;
width: 100%;
    position:absolute;
    top:0px;
    right:0px;
}

JavaScript

var esquerda = $("#menuEsquerda").width();

var site = $("body").width();

var favoritos = site-esquerda;

$("#barraFavoritos").css("width",favoritos);