Uses XMLHttpRequest to make an AJAX get request.

getRequest.js

var getRequest = function(url, callback) {
  var xhr = new XMLHttpRequest();
  
  xhr.onreadystatechange = function() {
    if (xhr.readyState === XMLHttpRequest.DONE)
      callback(xhr.status === 200 ? xhr.responseText : null);
  };
    
  xhr.open('GET', url, true);
  xhr.send();
};