Deferred

deferred.js

function Deferred() {
  var deferred = {
    resolve: null,
    reject: null
  };
  deferred.promise = new Promise((resolve, reject) => {
    // Save resolve/reject to deferred, for later usage.
    deferred.resolve = resolve;
    deferred.reject = reject;
  });

  return deferred;
}

Here's another one:

function AsyncSpy() {
  const deferred = Deferred();
  return _.extend(sinon.spy(() => deferred.promise), deferred); 
}

const asyncSpy = AsyncSpy();
asyncSpy('foo')
  .then(() => console.log('spy resolved');

assert(asyncSpy.calledWith('foo'))

asyncSpy.resolve();
// 'spy resolved'