Simplest possible Ajax implementation

simple-ajax.js

function ajax(url, callback) {
  var xhr;
  xhr = new XMLHttpRequest();
  xhr.open('GET', url, true);
  xhr.onreadystatechange = function () {
    if (4 === xhr.readyState && 400 > xhr.status) {
      callback(xhr);
    }
  };
  xhr.send(null);
}