var cloneAsync = (function() {
  var queue = [];

  var worker = new Worker(URL.createObjectURL(new Blob(['onmessage=function(e){postMessage(e.data)}'], {
    'type': 'text/javascript'
  })));

  worker.addEventListener('message', function listener(e) {
    var callback = queue.shift().callback;
    if (queue.length) {
      this.postMessage(queue[0].value);
    }
    callback(e.data);
  });

  return function(value, callback) {
    if (!queue.length) {
      worker.postMessage(value);
    }
    queue.push({ 'value': value, 'callback': callback });
  };
}());

var object = { 'a': { 'b': [1, { 'c': 2 }] } };

cloneAsync(object, function(result) {
  console.log(result, object === result);
});