index.html

<!doctype html>
<html lang="en-us">

<button class="mybutton1">Run myFunction</button>
</html>

 <script async type="text/javascript" src="main.js"></script>
<script type='text/javascript'>

function finished() {
  alert('finished');
}

 function back(text){
    alert(Pointer_stringify(text));
}

window.addEventListener('message', function (e) {
    
        //e.source.postMessage(e.data + " message");
        alert (e.data + ' message');
  });

document.querySelector('.mybutton1').addEventListener('click', function()
{
  alert('before async');

  var pointer = Runtime.addFunction(back);
  Module.ccall ('async', null, 'pointer', [pointer], { async: true });
  //Runtime.removeFunction(pointer);// Do not call removeFunction becuase after 20 sec. js cannot fint callback. into main.cpp - function onMessage.
  alert('after async');
});


 </script>