Easily run a function in a background worker, out of the main UI thread.

offthread.js

function offthread(fn, args, cb) {
  var blob = new Blob(
    [
      'var fn = ' + fn.toString() + ';',
      'var ret = fn.call(null, ...' + JSON.stringify(args) + ');',
      'postMessage(ret);'
    ],
    { type: 'text/javascript' }
  );

  var worker = new Worker(window.URL.createObjectURL(blob));
  worker.onmessage = function (e) {
    cb && cb(e.data);
    worker.terminate();
  };
}

var start = Date.now();
offthread(
  function (nIterations) {
    var n = 0;
    for (var i = 0; i < nIterations; i++) {
      n++;
    }
    return n;
  },
  [1e9],
  function (n) {
    console.log('Completed', n, 'iterations offthread in', Date.now() - start, 'ms.');
  }
);