fix: Make no-unused-var function the same way in for..of loops as for..in loops

[alexbassy]

Prerequisites checklist

• I have read the contributing guidelines.

What is the purpose of this pull request? (put an "X" next to an item)

[ ] Documentation update
[ ] Bug fix (template)
[ ] New rule (template)
[x] Changes an existing rule (template)
[ ] Add autofix to a rule
[ ] Add a CLI option
[ ] Add something to the core
[ ] Other, please explain:

What changes did you make? (Give an overview)

Tweaked the no-unused-vars rule to treat for..of loops and for..in loops the same

Sorry for the labels 🙈 not sure if this is a bug or a feature

What rule do you want to change?

no-unused-vars`

Does this change cause the rule to produce more or fewer warnings?

Fewer

How will the change be implemented? (New option, new default behavior, etc.)?

for..of loops will be treated the same way as for..in loops

Please provide some example code that this change will affect:

// without change
for (const name in object) { return true } // valid
for (const name of iterable) { return true } // invalid, `name` is unused

// with change
for (const name of iterable) { return true } // valid

What does the rule currently do for this code?

It will cause show a no-unused-vars error on the variable declaration in the for..of loop

What will the rule do after it's changed?

It will not show an error

Is there anything you'd like reviewers to focus on?

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• ✅ login: alexbassy / name: Alex Bass (b2f6b81)

[mdjermanovic]

Hi @alexbassy, thanks for the PR!

for (const name in object) { return true } // valid
for (const name of iterable) { return true } // invalid, `name` is unused

According to the discussion in the original issue #2342 to which this behavior relates, it seems that this pattern should have been allowed for for-of loops as well (in particular #2342 (comment)).

On the other hand, a drawback of allowing more patterns is always that some cases where the variable was intended to be used could be missed, and this pattern with for-of loops can usually be replaced with:

const [name] = iterable
if (typeof name !== "undefined") return true

Granted, this isn't fully equivalent (in particular, it doesn't account for the possibility that the first element of the sequence is undefined), so my opinion on this is neutral. I wouldn't mind including for-of if other team members agree that this is a bug (oversight in implementing accepted change #2342).

[alexbassy]

Thank you @mdjermanovic 🎈 for some reason, the for..of behaviour wasn’t added despite Ilya’s comment. I was also wondering why this doesn’t allow using the variable pattern ^_, which I think might address the problem you’re describing?

Happy to go with your suggestion and wait for some more feedback 🤝

[mdjermanovic]

I was also wondering why this doesn’t allow using the variable pattern ^_, which I think might address the problem you’re describing?

I've just tried with varsIgnorePattern: "^_", and it seems to work (there are no errors reported): Online Demo

(maybe I didn't understand, did you mean that varsIgnorePattern doesn't allow these cases?)

[nzakas]

I think it makes sense to match the behavior of for-of to that of for-in. 👍

[mdjermanovic]

LGTM, thanks for contributing!

[mdjermanovic]

Some checks didn't run. I'll close-reopen to see if that will trigger the missing checks.

[nzakas]

@alexbassy we'd like to pay you for this contribution, but I couldn't find an email address for you. Can you please contact us at contact (at) eslint (dot) org? Thanks.