JSFiddle - React, Tailwind, and code Playground

JavaScript

// Get ID
var myID;
id = window.localStorage.getItem("id");
if (id==1) { myID = 1;    window.localStorage.setItem("id", 0); }
      else { myID = 0;    window.localStorage.setItem("id", 1); }

// Initialize statistics variables
var lastrun    = (new Date()).getTime();
var totaldelay = 0;
var count      = 0;
var checks     = 0;

document.documentElement.innerHTML = "ID: "+myID;

// Method that checks the round-trip time
function check() {
    window.setTimeout(check, 1); // Keep running
    value = window.localStorage.getItem("state");
    checks++;
    if (value==myID) return;
    window.localStorage.setItem("state", myID);
    now = new Date().getTime();
    totaldelay += now - lastrun;
    count++;
    lastrun = now;
    document.documentElement.innerHTML = "ID: "+myID+"<br/>"+
                                         "Number of checks: "+checks+"<br/>"+
                                         "Number of round-trips: "+count+"<br/>"+
                                         "Checks per round-trip: "+checks/count+"<br/>"+
                                         "Average round-trip time:"+totaldelay/count;
}

// Go!
window.setTimeout(check, 1000);