JSFiddle - React, Tailwind, and code Playground

HTML

<input type="button" id="start" value="Отправить">

JavaScript

var btn = document.getElementById('start');

btn.addEventListener('click', function(){
  btn.value = "start"; //проблема тут
  
  var xhr = new XMLHttpRequest();
  xhr.open('POST', 'test.php', false);
  xhr.onload = function(e){
    if(xhr.readyState == 4 && xhr.status == 200){
      btn.value = "ready";
    }
  };
  xhr.send();
});