"simulates" an async function and passes either `res` directly or the result of `res()` to the callback function, see https://gist.github.com/toddzebert/cee6d8b0475275be0f48d7dfef142bd3

asyncSim.js

// requires https://gist.github.com/toddzebert/b4465de41b0f8d317d7a685604983c36
function asyncSim (res, data, onDone) {
  var result = (typeof res === "function") ? res(data) : res;
  setTimeout(function () { onDone(result); }
  , randInt(400, 200));
};