JSFiddle - React, Tailwind, and code Playground

HTML

<div id="container" style="background:#CCC;"></div>

JavaScript

var videoAspect= 600/400;/* normalWidth/normalHeight*/

var $window = $(window), windowW= $window.width(), windowH= $window.height();
var windowAspect= windowW/windowH;
var sizes={ }
if( windowAspect > videoAspect){
   /* wider so set height 100% and adjust width*/
    sizes.height=windowH;
    sizes.width = windowH * videoAspect;
}else{
    sizes.width=windowW;
    sizes.height = windowW / videoAspect; 
}

$('#container').css( sizes);