can't return a date from a firebase cloud function?

Question

I'm trying to simply return a date from a firebase function:

import * as functions from 'firebase-functions';

const date = functions.https.onCall(() => {
  return {
    date: new Date(),
    iso: new Date().toISOString()
  };
});
export default date;

But here's the result I'm getting (using firebase functions:shell):

RESPONSE RECEIVED FROM FUNCTION: 200, {
  "result": {
    "date": {},
    "iso": "2018-12-08T18:00:20.794Z"
  }
}

Note that the Date() object is being serialized as an empty object which seems wrong? I would have expected at least a .toString() or something of the Date instance...

Does this mean I have to explicitly avoid returning Date instances? I can write a custom serializer which I wrap around my functions to deeply convert Date instances to strings via .toISODate() etc but that seems like I must be missing something!

THanks.

Answer 1

If you have a date object d, you should either:

1. Send its unix epoch time in milliseconds with d.getTime(), or
2. Send it JSON serialized form with d.toJSON().

I suggest #1, as it's easier to interop with different systems. Every system understands dates in unix epoch time, and that number doesn't require parsing.