function cancellable(fn) {
  var cancelled = false;
  var wrapped = function() {
    if (!cancelled) {
      fn.apply(null, arguments)
    }
  };
  wrapped.cancel = function() {
    cancelled = true;
  };
  return wrapped;
}

var async = function (cb) {
  setTimeout(function () {
    cb(null);
  }, 1000);
}

var callback = cancellable(function () {
  console.log('yo, i called you back');
});

async(callback);
// callback.cancel();
// ^ call me before the timeout and i won't run