💅 Possibly a false positive in handling useArrowFunction

[rchl]

Environment information

v1.9.4

Rule name

useArrowFunction

Playground link

https://biomejs.dev/playground/?lintRules=all&code=aQBtAHAAbwByAHQAIAB0AHkAcABlACAAewAgAFAAbAB1AGcAaQBuACAAfQAgAGYAcgBvAG0AIAAnAEAAbgB1AHgAdAAvAHQAeQBwAGUAcwAnADsACgAKAGUAeABwAG8AcgB0ACAAZABlAGYAYQB1AGwAdAAgADwAUABsAHUAZwBpAG4APgAgAGYAdQBuAGMAdABpAG8AbgAoACkAIAB7AAoAfQA7AAoA&jsx=false

Expected result

When exported function uses typescript type assertion (I think that's what it's called but not sure, maybe it's just a type cast) then Biome seems to think that it's function expression and suggests converting it to an arrow function. I think it's a false positive.

Code of Conduct

• I agree to follow Biome's Code of Conduct

[Conaclos]

Actually I am not sure if we can consider this as a false positive.

Here is the rule docs:

This rule proposes turning all function expressions that are not generators (function*) and don't use this into arrow functions.

Why should we treat this case as a false positive ?

[rchl]

I believe this is not a function expression but a function declaration definition. And the linter error goes away when removing the type cast/assertion which I believe should have nothing to do with that rule.

[Conaclos]

I believe this is not a function expression but a function declaration. And the linter error goes away when removing the type cast/assertion which I believe should have nothing to do with that rule.

There are two kinds of default exports:

1. Default Function declaration exports;
2. Default expression exports.

Because you use a cast, you fell in the second one. You can check this in the TypeScript Playground:

export default <Plugin> function() {}

is converted to:

export default (function () {});

[rchl]

Looks like technically you are right.

In practice, let's think whether this rule makes sense in this particular case. It's a top-level, exported function, I don't really see the benefit in converting it to an arrow function.

For the reference here is the rule description: https://biomejs.dev/linter/rules/use-arrow-function/

Maybe this should be a special case that doesn't fall within this rule. I'm not sure what would that technically be.

[ematipico]

I think we should stay on the technical side for this one. Treating a function expression as it was a function declaration is technically wrong, and if people mistake this case, I am sure they will learn something new. You can check the CST of the playground you shared, or even the TypeScript one.

There are other ways to export a default function using an arrow function, users should be encouraged to use those ones.

[Conaclos]

Moreover, this cast syntax is considered as obsolete by the TypeScript team. Using the new cast syntax makes it clear that the function is a function expression.
I am closing the issue because I think we should keep things as they are here.