In the absence of CORS support the Hidden Iframe technique is the only way to submit a big amount of data (over 2kb that jsonp can handle).

hidden-iframe.js

/**
 * @fileOverview Hidden iFrame implementation for browsers without CORS.
 *
 * Usage:
 * var iframe = new Iframe();
 * iframe.submit(url, {data: yourData}, function(err) {});
 *
 * The server will get a POST request on the defined URL with the data
 * JSON encoded in the body attribute named "data".
 */

var noop = function() {};

/**
 * Hidden iFrame implementation for browsers without CORS.
 *
 * @constructor
 */
var Iframe = module.exports = function() {
  /** @type {String} The target action of the form submission */
  this.url = '';
  /** @type {Object} Contains the invoke options */
  this.opts = {};
  /** @type {Function} User defined callback */
  this.cb = noop;

  /** @type {?Document.iframe} iframe document instance */
  this.iframe = null;
  /** @type {?Document.form} form document instance */
  this.form = null;
  /** @type {String} The name / id of the iframe */
  this.iframeName = 'hidden_iframe_submit';

  /** @type {?Document.iframe} The iframe element on the DOM */
  this.iframeEl = null;

  this._iframeLoadBinded = this._onIframeLoad.bind(this);
};

/**
 * Submit data using a hidden iframe.
 *
 * @param {string} url The url to submit to.
 * @param {Object} opts Options for the submission.
 *   @param {*} data Data to send.
 * @param {Function} cb Nodejs Callback.
 */
Iframe.prototype.submit = function(url, opts, cb) {
  this.url = url;
  this.cb = cb;
  this.opts = opts;

  this._createIframe();

  this._createForm();

  document.body.appendChild(this.form);
  document.body.appendChild(this.iframe);

  this.iframeEl = document.getElementById(this.iframeName);

  if (this.iframeEl.addEventListener) {
    this.iframeEl.addEventListener('load', this._iframeLoadBinded, true);
  }
  if (this.iframeEl.attachEvent) {
    this.iframeEl.attachEvent('onload', this._iframeLoadBinded);
  }

  this.form.submit();
};

/**
 * Triggers when iframe emits the onload event.
 *
 * @private
 */
Iframe.prototype._onIframeLoad = function() {
  // Add event...
  if (this.iframeEl.detachEvent) {
    this.iframeEl.detachEvent('onload', this._iframeLoadBinded);
  } else {
    this.iframeEl.removeEventListener('load', this._iframeLoadBinded, false);
  }

  this.cb(null);
};

/**
 * Create the iframe object.
 *
 * @private
 */
Iframe.prototype._createIframe = function() {
  var iframe = this.iframe = document.createElement('iframe');
  iframe.style.display = 'none';
  iframe.setAttribute('name', this.iframeName);
  iframe.setAttribute('id', this.iframeName);
  iframe.setAttribute('width', '0');
  iframe.setAttribute('height', '0');
  iframe.setAttribute('border', '0');
  iframe.setAttribute('src', 'javascript:false;');
};

/**
 * Create the form to submit.
 *
 * @private
 */
Iframe.prototype._createForm = function() {
  var form = this.form = document.createElement('form');
  form.style.display = 'none';
  form.setAttribute('target', this.iframeName);
  form.setAttribute('action', this.url);
  form.setAttribute('method', 'post');
  form.setAttribute('enctype', 'application/x-www-form-urlencoded');
  form.setAttribute('encoding', 'application/x-www-form-urlencoded');

  var input = document.createElement('input');
  input.style.display = 'none';
  input.setAttribute('type', 'text');
  input.setAttribute('name', 'data');

  var data = this._jsonData();
  input.setAttribute('value', data);

  form.appendChild(input);
};

/**
 * JSON Stringify the incoming data.
 *
 * @return {string} JSON Encoded string.
 * @private
 */
Iframe.prototype._jsonData = function() {
  var data = {};
  try {
    data = JSON.stringify(this.opts.data);
  } catch(ex) {
    // no action
  }
  return data;
};