Disallow iif

[Erbenos]

Perhaps use https://github.com/cartant/eslint-plugin-rxjs/blob/main/docs/rules/ban-observables.md to disallow iif. It may be confusing that both branches do get evaluated (as they are just params to a function) to some. While thats due to language itself, people seem to often miss it which causes unexpected behavior. defer can be used to work around that but that is even less known if I had to guess.

Some uses can be refactored easily:

From:

x$.pipe(
  switchMap((x) => iif(() => Boolean(x), of(1), of(3)))
)

To:

x$.pipe(
  switchMap((x) => Boolean(x) ? of(1) : of(3)),
)

Most useful usecase of iff is being able to pipe whatever would be subscribed to near the condition.

x$.pipe(
   switchMap((x) => iif(() => Boolean(x), of(10), of(20)).pipe(map((x) => x ** 2)))
)

As opposed to:

x$.pipe(
   switchMap((x) => iif(() => Boolean(x), of(10), of(20))),
   map((x) => x ** 2),
)

When you will want to catch errors within the switchMap, the difference becomes even more glaring. You can work around it, but iif would be more succint. Sadly, if we were to forbid iif, some code would expand quite quickly.