spec/asCallbackSpec.js
function asCheckedCallback(promise, cb, useOwnMethod) {
  let called = false;
  if (useOwnMethod) {
    promise.asCallback(after);
  } else {
    asCallback(promise, after);
  }

  function after() {
    expect(called).toBeFalsy();
    called = true;
    return cb.apply(this, arguments);
  }
}
function asCheckedCallback(promise, cb, useOwnMethod) {
  let called = false;
  if (useOwnMethod) {
    promise.asCallback(after);
  } else {
    asCallback(promise, after);
  }

  function after() {
    expect(called).toBeFalsy();
    called = true;
    return cb.apply(this, arguments);
  }
}
Wrap the asCallback function with a check that ensures the callback is never called twice.