feat(breaking): update startWith so that it calls a function to get first value

[jorroll]

Feature Request

This is a re-opening of issue #5156 which was originally closed because, at the time, it was stated that breaking changes would not be considered to the RXJS core library. Seeing as RXJS has a breaking release being worked on (v7), I'm resurfacing this old discussion.

This is a request to change the startWith operator so it expects a function which is called to retrieve the first value.

I.e. update startWith interface from this:

startWith<T, D>(...array: Array<D | SchedulerLike>): OperatorFunction<T, T | D>

to this:

startWith<T, D>(...array: Array<(() => D) | SchedulerLike>): OperatorFunction<T, T | D>

See issue #5156 for previous discussion on this topic including this closing comment by @kwonoj.

Closing as the general intention has explained and we will not make changes into core at this moment.

[kwonoj]

Moved into discussions since issue is being used for core team's bug tracking purpose.

[cartant]

A question related to this has popped up before. There is already a general solution in the library for making things lazy: defer. See this comment. Which is basically the same comment that I made in the original issue.

[jorroll]

Yes, you suggested this workaround in #5156 and it is still the current workaround I use (and a much appreciated suggestion). While the ease of a workaround greatly reduces the need for improving the startWith api, I, personally, don't think it eliminates the need to improve the api. Given that breaking changes are already happening in v7, I resurfaced this discussion in the hope that an improved startWith can be included in v7. Obviously I understand if the maintainers feel differently tho.

[benlesh]

you can always use concat:

concat(
  defer(() => { /* get value here */ return of('value'); }),
  source
)

Or roll your own operator:

const startWithDeferred = <T, V>(factory: () => V) => (source: Observable<T>) => concat(defer(() => of(factory())), source);