Basic AJAX loading

gistfile1.js

  /**
   * Does an AJAX load of the specified URL
   *
   * @param {String} url        URL to load
   * @param {Object} data       Hash of data to send in querystring
   * @param {Function} callback Function to call once request returns
   */
  function load(url, data, callback) {
    if (data) {
      var params = Object.keys(data)
        .map(function(key) {
          return key + '=' + encodeURIComponent(data[key]);
        })
        .join('&');
        url += '?' + params;
    }
    var xhr = new XMLHttpRequest();
    xhr.open('get', url, true);
    xhr.onreadystatechange = function() {
      if (xhr.readyState === 4) {
        // Try parse as JSON
        var data;
        try {
          data = JSON.parse(xhr.responseText);
        } catch (ex) {
          data = null;
        }

        // Check for success
        if (xhr.status != 200 || !data || !data.success) {
          var message = (data && data.message) || xhr.responseText;
          alert('Something broke with the request: ' + message);
          return;
        }

        callback(data);
      }
    }
    xhr.send();
    return xhr;
  }