JSFiddle - React, Tailwind, and code Playground

HTML

<div style="width:100%">
    <div style="width:50%;background-color:red" class="left">Left Div</div>
    <div style="background-color:green">Right Div</div>
</div>

JavaScript

$(document).ready(function(){
    $('.left').css('width', $('.left').parent().width() - 100);
});