Vanilla JS function to perform a GET XMLHttpRequest against a passed URL and execute the callback function with the returned results. On error, puts an error message to the console and returns null to the callback.

vanillaXhrGet.js

/*
 * Note: these days you should probably just use fetch:
 * https://developer.mozilla.org/en-US/docs/Web/API/Fetch_API
 * 
 * A convenience function, to handle the vanilla js xhr
 * request.
 * 
 * @param {string} url - the URI against which to make the request
 * @param callback - the callback function, which returns an obj,
 *        with two params, err (which is null of not error), and
 *        data, which returns the received content, for a successful
 *        GET request
 * 
 */
var xhrGet = function(url, callback(err, data)){
  var xhr = new XMLHttpRequest();
  xhr.onreadystatechange = function(data){
    if( xhr.readyState === XMLHttpResponse.DONE ){
      if( xhr.status === 200 ){
        callback(null,xhr.responseText);
      }else{
        throw new Error('Error with xhrGet: '+this);
        console.log('XHR error: '+xhr.status);
        callback(true,null);
      }
    }
  }
  xhr.open('GET', url);
  xhr.send();
}