`finnish` doesn't work for functions without an explicit return type

[JasonWeinzierl]

This will get flagged by the finnish rule:

function getNumber(): Observable<number> {
	return of(42);
}

but this will not get flagged:

function getNumber() {
	return of(42);
}

This is likely due to couldReturnType not being able to handle implicit return types.

tsTypeNode = tsNode.type ?? tsNode.body;

If type is null (no explicit return type) and body is a Block instead of something like a CallExpression, then the type checker will not be able to determine the type of tsTypeNode and so couldReturnType will always return false. This is currently impacts the finnish, no-finnish, and no-misused-observables rules.

Implicit return types don't have test coverage in the finnish test suite, so this bug was never discovered (or ignored?).

[Dmytro27Ind]

I’ve also encountered this problem where implicit return types in functions aren't flagged correctly by the finnish rule.

As a temporary workaround, I added the following rule to enforce explicit return types:

"@typescript-eslint/explicit-function-return-type": "error"

This ensures that all functions have explicit return types, which indirectly resolves the issue by preventing cases where couldReturnType fails to handle implicit types.

While this workaround helps in the interim, it would be great to see a fix for the root cause in the finnish rule itself.

[JasonWeinzierl]

Agreed I also have been using explicit-function-return-type + finnish with the old library, and so I never noticed this bug until starting this fork. I would like to get this bug fixed before v1 is released, because no-misused-observables now has the same problem.

[JasonWeinzierl]

This is fixed in version 0.7.2!