Breaking: check unnamed default export in func-names (fixes #12194)

[golopot]

fixes #12194

What is the purpose of this pull request? (put an "X" next to item)
[X] Bug fix (template)

What changes did you make? (Give an overview)
Start report code like export default function() {} when always or as-needed.

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

[platinumazure]

Looks pretty good. Just left one question. 😄

[platinumazure]

How is this valid? The option is "never" but the function has a name.

[mdjermanovic]

That would require more checks, foo is here a declaration and can be used from within the module. And even if that's not the case, I believe this would break some builds

[golopot]

The reasoning is that, under option never, function f() is allowed, export function f() is allowed, so likewise export default function f() should be allowed.

I understand in some perspective this should be reported, so this case is controvesial. Not reporting in this case would make both sides happy.

[mdjermanovic]

Just to note that this PR at the moment also changes how the rule works with function expressions when the option is as-needed.

Also, all 3 changes would be 'false negatives' if this is seen as a bug, and the message should certainly start with Update:

[kaicataldo]

How do we want to proceed here? It looks like this was marked as a breaking change rather than a semver-minor bug fix, but it's not entirely clear why.

[kaicataldo]

@mdjermanovic Can we merge this?

[mdjermanovic]

An overview of the new behavior after this PR:

/*eslint func-names: ["error", "always"]*/

export default function () {} // error (*)

export default function foo() {} // ok

export default (function (){}); // error

export default (function foo(){}); // ok

/*eslint func-names: ["error", "as-needed"]*/

export default function () {} // error (*)

export default function foo() {} // ok

export default (function (){}); // error (*)

export default (function foo(){}); // ok

/*eslint func-names: ["error", "never"]*/

export default function () {} // ok

export default function foo () {} // ok!

export default (function (){}); // ok

export default (function foo(){}); // error

(*) marks changes in behavior made in this PR.

There are three (*), caused by these two different changes in logic:

• export default function () {} will be treated as function expressions. It was ignored before since it's a function declaration.
• Anonymous functions in export default will be treated as functions that don't get automatically assigned name, thus they'll be reported when the option is as-needed. They were valid before since they get assigned the "default" name (intentionally valid, there is an explicit condition for that and an example in the original issue #7235).

Also, export default function foo () {} with "never" is an intentional exception to the new logic. It might be consistent to require removing the name, but users probably wouldn't want that.

Questions:

• Do we have consensus on both changes.
• Should we update the documentation (aside from the new examples). At the moment, it's explicitly about function expressions only. Also, export default does assign a name.

[btmills]

In today's TSC meeting, we added this to the v7.0.0 release!

[mdjermanovic]

The code LGTM!

Should we add a short note that as-needed requires names for functions in export default, somewhere in the documentation (maybe in the ### as-needed section)?

The description for the option is now inaccurate:

"as-needed" requires function expressions to have a name, if the name cannot be assigned automatically in an ES6 environment

so it might be good to clarify the behavior, as this in an exception?

Maybe also a short note that always and as-needed options also apply to function declarations in export default?

[mdjermanovic]

LGTM, thanks!

[kaicataldo]

Thanks for contributing to ESLint!