Async Cross-Domain Ajax helper without jQuery and uses Promises.

xRequest.js

"use strict";

/**
 * Cross-Domain Ajax helper.
 * @author Amir Khawaja <khawaja.amir@gmail.com>
 * @license MIT
 * @constructor
 */
var XRequest = function () {
  var self = this;
  self.headers = [];
};

/**
 * Make the Ajax request.
 * @param {string} method - The HTTP method to use.
 * @param {string} url - The URL.
 * @param {string} [payload] - The data to send in the request.
 */
XRequest.prototype.send = function (method, url, payload) {
  var self = this;
  return new Promise(function (resolve, reject) {
    var xhr = new XMLHttpRequest();
    xhr.withCredentials = true;
    xhr.onload = function () {
      if (xhr.status === 200) {
        if (xhr.responseText !== void 0 && xhr.responseText.length > 0) {
          resolve(xhr.responseText);
        } else {
          resolve(null);
        }
      } else if (xhr.status === 403) {
        reject(JSON.stringify({
          success: false, status: xhr.status, response: ['403 Forbidden - ' + url]
        }));
      } else if (self.xhr.status === 404) {
        reject(JSON.stringify({
          success: false, status: xhr.status, response: ['404 Not Found - ' + url]
        }));
      } else {
        reject(JSON.stringify({
          success: false, status: xhr.status, response: [xhr.response]
        }));
      }
    };
    xhr.onerror = function () {
      reject(JSON.stringify({
        success: false, status: null, response: 'An error occurred while trying to make the request'
      }));
    };

    // Set the HTTP headers.
    if (self.headers.length > 0) {
      for (var i in self.headers) {
        var key = Object.keys(self.headers[i])[0];
        self.xhr.setRequestHeader(key, self.headers[i][key]);
      }
    }

    // Open the connection.
    xhr.open(method.toLocaleUpperCase(), url);

    // Make the request.
    switch (method.toLocaleLowerCase()) {
      case 'get':
      case 'delete':
        xhr.send();
        break;
      case 'post':
      case 'put':
        xhr.send(payload);
        break;
      default:
        xhr.send();
        break;
    }
  });
};

/**
 * Set HTTP header.
 * @param {string} key - The name of the header.
 * @param {string} value - The value of the header.
 */
XRequest.prototype.setHeader = function (key, value) {
  var self = this;
  var obj = {};
  obj[key] = value;
  self.headers.push(obj);
};