array-callback-return should report .flatMap()

[mdjermanovic]

Array.prototype.flatMap was added in ECMAScript 2019

What rule do you want to change?

array-callback-return

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

more

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

new default behavior

Please provide some example code that this change will affect:

/*eslint array-callback-return: "error"*/

foo.flatMap(x => {})

What does the rule currently do for this code?

No errors

What will the rule do after it's changed?

Error: Expected to return a value in arrow function.

Are you willing to submit a pull request to implement this change?

Yes.

Can this be a semver-minor default behavior change? It looks too 'small' for an option.

[platinumazure]

I'm worried about the potential breaking change impact.

Could we look for ecmaVersion >= 2019 in a semver-minor update, then maybe remove that check in the next major release?

[mdjermanovic]

Could we look for ecmaVersion >= 2019 in a semver-minor update, then maybe remove that check in the next major release?

I think it's a good idea.

Might be technically a bit more correct to check if an ES2019+ environment is activated rather than parser, but I don't think it's possible from within a rule at this moment.

[g-plane]

I don't think it's a breaking change, though it may break user's tests or CI.