feat(mergeScan): Add index to the accumulator function

[martinsik]

Description:
This PR adds index parameter to the accumulator function used by mergeScan just like in most other operators.

of('a', 'b', 'c', 'd').pipe(
  mergeScan((acc, x, index) => {
    console.log(index); // prints: 0, 1, 2, 3
    return of(x);
  }, 0)
).subscribe();

Related issue (if exists):
Closes #4441

[coveralls]

Pull Request Test Coverage Report for Build 8001

• 4 of 4 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 96.91%

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Totals
Change from base Build 7999:	0.0%
Covered Lines:	5770
Relevant Lines:	5954

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💛 - Coveralls

[cartant]

LGTM, but would you be able to re-write the test to use marble diagram?

[benlesh]

@cartant I think the test is fine given it's just asserting the index is being passed to the function.

[benlesh]

this new test is unnecessary. :)

[cartant]

I'm pretty sure this test will fail if the index is removed from the signature, so it guards against a regression, IMO.

Nah. You're right. The spec will fail first. Duh.

[martinsik]

I removed this test. I was looking at how is this thing tested elsewhere and it seemed to be the same usecase as here https://github.com/ReactiveX/rxjs/blob/master/spec-dtslint/operators/map-spec.ts#L12-L14 or here https://github.com/ReactiveX/rxjs/blob/master/spec-dtslint/operators/findIndex-spec.ts#L8-L10

[cartant]

Personally, I'd favour the dtslint typing tests being written without regard to what's tested in the specs - even if that means there are some redundant tests.

[benlesh]

Can you please move expected inside of equal here? It will make the test moderately more readable, IMO.

[rxjs-bot]

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[benlesh]

Thank you, @martinsik!