The obvious solution is to use a Web Worker,

  var code = "onmessage = function (evt) {evt.data.sort(); postMessage(evt.data)}";

  function asyncSort(data, cb) {
    var worker = new Worker(URL.createObjectURL(new Blob([code])));
    worker.onmessage = function (evt) { cb(evt.data); };
    worker.postMessage(data);
  }

Example,

  asyncSort([3, 2, 1], function (res) { console.log(res); });

Prints,

  "[1, 2, 3]"