comma-dangle: should not insert comma in arrow function return expressions

[noelforte]

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Describe the bug

Possibly related: #158 / #160

When writing arrow functions with an implicit return, sometimes it's useful to drop onto the next line to get more space to write the expression. Prettier uses this pattern a lot:

const nums = [1, 2, 3];

const remappedArray = nums.map(
  (el) => `${el} plus 5 is ${el + 5}`
);

However when using a callback function that also happens to be an arrow function with an implicit return statement, such as when working with arrays, patterns like:

(el) => `${el} plus 5 is ${el + 5}`

get formatted to:

(el) => `${el} plus 5 is ${el + 5}`,

While valid, it is somewhat confusing because a line ending with a comma is not allowed in a return statement, but is allowed in an expression via the comma operator...but neither of those cases are happening here becuase Array.map takes 2 arguments, the callback and thisArg. So, in this case JavaScript is interpreting the above as:

... nums.map( 
  ((el) => `${el} plus 5 is ${el + 5}`),
) ...

Since the comma operator doesn't seem to be as well-known of a feature and the correct way to use it in an arrow function is actually () => (exp1, exp2, ...), perhaps the best way to handle this would be to not insert dangling commas in places where their meaning is ambiguous.

Happy to submit a PR with failing tests, however Vitest isn't something I'm as familiar with so I don't know how much help I could be. Still willing to give it a shot though!

Reproduction

https://stackblitz.com/edit/github-2cdzyb?file=src%2Findex.ts&view=editor

Contributes

• I am willing to submit a PR to fix this issue
• I am willing to submit a PR with failing tests