JSFiddle - React, Tailwind, and code Playground

HTML

<div id="sync">
</div>

<div id="async">
</div>

JavaScript

function syncSleep(ms){
    var end = new Date().getTime() + ms;
    var start = new Date().getTime();
  
    while (start < end) {
      start = new Date().getTime();
    }
}

function p() {
  return new Promise(function(resolve) {
     syncSleep(5000);
     resolve("syncSleep done!");
  });
}

p().then( function(s) {
  var div = document.getElementById('async');
  div.innerHTML = s;
} );

var div = document.getElementById('sync');
div.innerHTML = "This should appear right away! (but it doesn't)";