JSFiddle - React, Tailwind, and code Playground

HTML

<iframe name=i></iframe>

JavaScript

function spracuj(e) {
    alert("Event:"+e.newValue);
}
var i = frames.i;
i.document.onstorage = spracuj;
i.document.onstoragecommit = spracuj;
  if (window.addEventListener) {
    i.addEventListener('storage',spracuj,false);
    }
    else if (window.attachEvent) {
        i.attachEvent('onstorage',spracuj);
    }


var val = localStorage.getItem("test");
if (val==null){
  var a = prompt("Zadej hodnotu");
  localStorage.setItem("test", a);
} else {
  alert("Ulozena hodnota: "+val);
    localStorage.removeItem("test");
}