feat(switchScan): add switchScan() operator

[martinsik]

• Add the operator to Rx
• It must have a -spec.ts tests file covering the canonical corner cases, with marble diagram tests
• If possible, write a asDiagram test case too, for PNG diagram generation purposes
• The spec file should have a type definition test at the end of the spec to verify type definition for various use cases
• The operator must be documented in JSDoc style in the implementation file, including also the PNG marble diagram image
• The operator should be listed in doc/operators.md in a category of operators
• The operator should also be documented. See Documentation Guidelines.
• It should also be inserted in the operator decision tree file doc/decision-tree-widget/tree.yml
  You may need to update MIGRATION.md if the operator differs from the corresponding one in RxJS v4

Description:

This PR adds switchScan operator as described here #2931 and reopened by @benlesh some time ago. I was told on Slack that it might get refused but I'm ok with that (no harsh feelings :)).

It's internally just using switchMap and tap because I first started implementing it myself but then realised I'm doing almost the same thing as switchMap is doing already so I just used combination of those two operators (I think it's been recommended to rather reuse existing operators than creating them from scratch). Tests are a combination of existing tests for scan and mergeScan.

I made it to work similarly to scan because that's more close to Array.reduce() even though mergeScan behaves a little differently. I tried to describe it here #4348 (comment) and here #4441 (the way it handles the first emission from source without seed and that it passes index to the accumulator function).

I also didn't add this operator to rxjs-compat because it didn't exist previously.

Related issue (if exists):

Closes #2931

[coveralls]

Pull Request Test Coverage Report for Build 8447

• 14 of 14 (100.0%) changed or added relevant lines in 3 files are covered.
• 2 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.03%) to 96.816%

---

Files with Coverage Reduction	New Missed Lines	%
src/internal/Observer.ts	1	75.0%
src/internal/Subscriber.ts	1	81.48%

Totals
Change from base Build 8442:	-0.03%
Covered Lines:	5808
Relevant Lines:	5999

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💛 - Coveralls

[benlesh]

Very interesting, @martinsik... I know this is something @trxcllnt wanted to add back in the day. I've marked this for core team discussion. Thank you!

[cartant]

This isn't a complete review. I'm just pointing out some things I've noticed whilst perusing the changes. I'll have a closer look at them later.

[benlesh]

Apologies for the delayed review. Let's get this in!

[martinsik]

@benlesh @cartant Hey, I updated this PR with everything you mentioned above.

[BioPhoton]

I stumble over this open PR and see that all requested changes are implemented.
The failed build on node v9 is related to some non-code related things which I guess could be ignored.
@benlesh may I kindly ask you for a status update to this PR= :)

[tonivj5]

Any update on this PR? 👍

[martinsik]

Rebased to master and fixed some types in spec.

[cartant]

Thanks. I'll re-review it.

[benlesh]

Oh man... this old thing. Sigh. Casualty of burnout mostly. I'll mark it as an agenda item and see if the team still wants to land it, or if it would be better in rxjs-misc.

cc @cartant

Taken over.

[benlesh]

Resurrected! 🧟

[cartant]

LGTM, just a bunch of nitpicks.

[cartant]

thought: This looks like one of those tests that purports to test something that's synchronous, but isn't really synchronous-within-subscribe. Nothing to do here, this is just an observation related to the sync-within-subscribe marble syntax that we talked about.

[cartant]

question: @niklas-wortmann With the addition of a new operator, is there some config that needs to updated so that a marble diagram for it is generated?

[benlesh]

Added a TODO around this.