Rule discussion: Arrow function should not return assignment

[tom-sherman]

What version of standard?

v12.0.1

What operating system, Node.js, and npm version?

Node.js

What did you expect to happen?

When trying to sum the sizes of an array of Sets I tried to write

sets.reduce((acc, set) => acc += set.size, 0)

What actually happened?
Standard shows the error

Arrow function should not return assignment. (no-return-assign)

I don't think this rule is helpful in this scenario, I can't use Array.reduce at all in this scenario AFAIK. I understand why you don't want to allow the following code, but this Array.reduce use case seems more legitimate

const foo = x => x = 2
// or
function foo (x) {
  return x = 2
}
// The author in either of these examples probably meant to return a comparison

If the above is is intentional, please can someone explain what code smell I'm creating here?

If not, I'd like to suggest changing the rule to except-parens^[1] option provided by Eslint or remove it completely.

The except-parens option would allow for the following style which I like because it clearly signals the author's intent to return an assignment.

sets.reduce((acc, set) => (acc += set.size), 0)

[LinusU]

I actually think is a quite good rule to have since it's quite uncommon to want to return an assignment value.

This is currently enforced regardless of wether it is an arrow function or not btw, so return a += 1 is invalid. The proper way to signal that it was the intention, is to wrap it in a parenthesis: return (a += 1), which is valid.

In your code it seems like there is no need to modify acc since that is only a local copy to your function, is there any reason to not write it like this?

sets.reduce((acc, set) => acc + set.size, 0)

[tom-sherman]

@LinusU For some reason I had to mutate the accumulator, thanks for the tip 🤦‍♂️

Also, looks like the except-parens is already turned on so this ticket is just completely bogus, please close!

[LinusU]

No stress 👌