judofyr/foo.html

## foo.html
<script>parent.postMessage("Hello world", "*")</script>


## main.html
<script src="s.js"></script>
<body>

<form action="foo.html" name="f">
</form>

<script>
formproxy(f, function(data) {
  console.log(data);
});
</script>

</body>


## s.js
(function() {
  var id = 0;
  var pending = [];

  function createFrame() {
    var frame = document.createElement('iframe');
    frame.src = 'about:blank';
    frame.name = '_formproxy' + (formproxy.id++);
    frame.setAttribute('style', 'position:absolute;width:1px;height:1px;left:-200px');
    document.body.appendChild(frame);
    return frame;
  }

  window.addEventListener('message', function(evt) {
    for (var i = 0; i < pending.length; i++) {
      var req = pending[i];
      if (req.form.contentWindow === evt.otherWindow) {
        if (req.cb) req.cb(evt.data);
        req.frame.parentElement.removeChild(req.frame);
        pending.splice(i, 1);
        break;
      }
    }
  }, false);

  window.formproxy = function(form, cb) {
    var frame = createFrame();
    form.target = frame.name;
    pending.push({form: form, cb: cb, frame: frame});
    form.submit();
  };
})();
	<script src="s.js"></script>
	<body>

	<form action="foo.html" name="f">
	</form>

	<script>
	formproxy(f, function(data) {
	console.log(data);
	});
	</script>

	</body>
	(function() {
	var id = 0;
	var pending = [];

	function createFrame() {
	var frame = document.createElement('iframe');
	frame.src = 'about:blank';
	frame.name = '_formproxy' + (formproxy.id++);
	frame.setAttribute('style', 'position:absolute;width:1px;height:1px;left:-200px');
	document.body.appendChild(frame);
	return frame;
	}

	window.addEventListener('message', function(evt) {
	for (var i = 0; i < pending.length; i++) {
	var req = pending[i];
	if (req.form.contentWindow === evt.otherWindow) {
	if (req.cb) req.cb(evt.data);
	req.frame.parentElement.removeChild(req.frame);
	pending.splice(i, 1);
	break;
	}
	}
	}, false);

	window.formproxy = function(form, cb) {
	var frame = createFrame();
	form.target = frame.name;
	pending.push({form: form, cb: cb, frame: frame});
	form.submit();
	};
	})();