gistfile1.js

var getJSON = function(url, successHandler, errorHandler) {
  var xhr = typeof XMLHttpRequest != 'undefined'
    ? new XMLHttpRequest()
    : new ActiveXObject('Microsoft.XMLHTTP');
  xhr.open('get', url, true);
  xhr.onreadystatechange = function() {
    var status;
    var data;
    if (xhr.readyState == 4) { // `DONE`
      status = xhr.status;
      if (status == 200) {
        data = JSON.parse(xhr.responseText);
        successHandler && successHandler(data);
      } else {
        errorHandler && errorHandler(status);
      }
    }
  };
  xhr.send();
};

getJSON('https://www.sugarlabs.org/tjs/package.json', function(data) {
  alert('Got this: ' + JSON.stringify(data));
}, function(status) {
  alert('Something went wrong.');
});