Allow comparing special/tagged objects

[mrcljx]

I wanted to add a custom comparator to handle Temporal.Instant for https://fullcalendar.io/docs#toc to call equals but was surprised that areObjectsEqual was never called. The reason is that the TC39 proposal requires [Symbol.toStringTag] to be set, which in turn makes fast-equals to not call any comparator but instead return false without any way to override this behavior.

I initially thought about adding a areUnknownTaggedObjectsEqual callback instead, but I think this would make it harder to do non-breaking additions in the future. For example if Temporal gets approved and this library wants to add support for it directly, one can't warn/fallback to the user's behavior if they defined it. So instead the config allows passing a tag-comparator mapping.

Let me know if you'd rather want to go a different route.

[planttheidea]

Apologies for not getting to this sooner, this flew under my radar.

I like the general concept; I was unaware that setting a custom Symbol.toStringTag value would impact the Object.prototype.toString.call() output, so this would definitely fill a gap. Hopefully you are still interested in this, and assuming so I'll provide some feedback on the PR. If I don't hear back in a reasonable timeframe, I'll move forward with the change (this may slide under your radar like it did mine: :P ).

[planttheidea]

Most of the comments here are to stimulate discussion, but I'd prefer to flesh them out before approving.

[planttheidea]

Recommendation: Change to a shared callback areUnhandledObjectsEqual which handles all unhandled tags.

In the description you mention that you wanted to go with an object of keys to avoid potential refactor pitfalls with version changes, but in reality the refactor work would still be the same. There really isn't a scale issue here since practical usage doesn't involve having a ton of these custom tags for comparison, and it would also allow for an escape hatch if someone wanted to include known-but-unhandled object types in their comparison, such as Promise, WeakMap / WeakSet, etc.

[planttheidea]

Issue: Since the use of Symbol.toStringTag updates the output of Object.prototype.toString.call(), it can be used directly.

[planttheidea]

Request: I know this is a bit more concise, but I request that the explicit if check is done before this.

if (unknownTagComparator) {
  return unknownTagComparator(a, b, state);
}

// Massive comment explaining ultimate fallback
...
return false;

It keeps the code comments applicable, and also allows for smaller diffs in the future if it was preferred to be bumped in priority instead of the ultimate fallback.