firstValueFrom and lastValueFrom defaultValue type

[jakovljevic-mladen]

Hi all. I'd like to discuss the type of the defaultValue parameters from FirstValueFromConfig and LastValueFromConfig interfaces.

Based on this comment, there is a strong possibility that a new signal property will be added to these interfaces. Most probably, defaultValue will become optional so users don't have to provide it if they only want to provide an abortion signal, like this: firstValueFrom(source$, { signal: ac.signal });.

In that case, it won't be possible to allow promises to resolve with undefined instead of a promise erroring with an EmptyError if nothing has been sent through the stream. With current set up, it is possible to write this: firstValueFrom(source, { defaultValue: undefined }); and if source is EMPTY, it will resolve a promise with undefined.

I'd like to propose defaultValue to be renamed to default and to become an object with a value property. If firstValueFrom or lastValueFrom receive an object, they will emit the default value from a value property of that object.

So, firstValueFrom(EMPTY, { signal: ac.signal }); would reject a promise with EmptyError.

But, firstValueFrom(EMPTY, { default: {}, signal: ac.signal }); (or firstValueFrom(EMPTY, { default: { value: undefined }, signal: ac.signal });) would resolve a promise with undefined.

[cartant]

If/when signal is added to the configuration object, I think we can make defaultValue optional and can test for the absence of a defaultValue property in the configuration object using hasOwnProperty('defaultValue'). If defaultValue is absent and the source completes without emitting a next notification, the implementation can throw.

[jakovljevic-mladen]

Oh, yeah, I keep forgetting about this handful JS feature. Thanks Nicholas. I guess this can be closed (if that's possible with discussions).

[cartant]

I guess this can be closed

I don't think it's possible. 🙂

[kwonoj]

I think we can mark accepted answer and lock instead. Surprised there's no close in discussions.

[jakovljevic-mladen]

I can't seem to find accept button either. Only upvote...