JSFiddle - React, Tailwind, and code Playground

HTML

<div id="sample">sample<div id="last">last</div></div>

CSS

#sample {
  width: 9999px;
  border: solid 1px red;
  background-color:#fee;
}

#last {
  float: right;
}

JavaScript

(function(){ 
    var elem = document.getElementById(prompt("idを入力してください"));

    resize();

    function resize() { 
       if (!elem) {
          alert('elem us null');
      } else {
         elem.style.width = window.innerWidth + 'px'; 
      }
    } 
})();