XHR

xhr.js

var xhr = function() {
  var xhr = new XMLHttpRequest();
  var mapper = function(data) {
    return Object.keys(data).map(function(k) {
      return encodeURIComponent(k) + '=' + encodeURIComponent(data[k])
    }).join('&');
  };
  return function(method, url, data, success, error, isJSON, headers) {
    xhr.onreadystatechange = function() {
      if (xhr.readyState === 4) {
        if (xhr.status === 200 && success) {
          success(isJSON? JSON.parse(xhr.responseText) : xhr.responseText);
        } else if (error) {
          error();
        }
      }
    };
    url += (data && method === 'GET') ? mapper(data) : '';
    xhr.open(method, url);
    if (headers) {
      for(header in headers) {
        xhr.setRequestHeader(header, headers[header]);
      };
    }
    // xhr.setRequestHeader("Content-Type", "application/json");
    xhr.send((data && method === 'POST') ? JSON.stringify(data) : '');
  };
}();

// xhr('GET', 'url', JSONheaders, function(data){console.log(JSON.parse(data))});