no-useless-undefined - should not be fixable

As it stands, the rule corrects console.log(undefined) to console.log(). These have different runtime behaviour, so it isn’t safe to autofix. The fixer should be downgraded to a suggestion.

[mmkal]

Fair enough - I suppose there is a threshold of what an edge case is. Relevant xkcd:

I still think this rule is making too many assumptions about the runtime implementation of the function. It relies on the function being called not doing something strange, which to me is dangerous.

[mmkal]

Fair enough that we can’t expect to succeed in never making runtime behaviour changes. But in this case, I think there’s a pretty reasonable chance that automatically deleting a parameter will have a real side-effect.

Which is related to another reason this shouldn’t auto-fix IMO: unlike most autofix rules like prettier/prettier, indent, no-semi, etc., if I write doSomething(undefined), it’s fairly likely that I did it on purpose, specifically because I didn’t want doSomething() for whatever reason. The rule is autofixing to the more obvious overload. The fact that I wrote doSomething(undefined) is a signal that doSomething is a weird function that behaves differently based on the number of args - at least the probability is much higher than for the average function call.

So while it’s an edge case for the set of all function calls that undefined is “useful”, when this rule is actually triggered, it’s much more likely to be significant. It’s a good thing that the linter is flagging unusual and potentially dangerous code, but it’s a bad thing that it’s blindly fixing it, potentially without the developer noticing. In my case, I updated this package in a medium sized repo, and there were a bunch of lint changes. Included were a small number of false positive fixes for this rule that got silently changed by yarn lint --fix. They might have slipped in unnoticed if there wasn’t proper test coverage. I would have preferred to handle them manually, since then I could have given proper thought to whether some refactoring/redesign was needed.

Anyway, I’ll shut up now - it’s not that big a deal 🙃