prefer-flat-map warns about and auto-fixes the wrong order of flat()/map() calls

[arild-haugstad]

According to MDN --- https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/flatMap --- and my quick tests in NodeJS and Firefox, flatMap() is equivalent with running flat() on the output of map(), not with running map() on the output of flat() which the rule warns about and rewrites from...

E.g. console.log([[1, 2], [3, 4]].flat().map((x) => x * 2)); outputs [2, 4, 6, 8], but is "autofixed" to console.log([[1, 2], [3, 4]].flatMap((x) => x * 2)); which outputs [NaN, NaN].

[freaktechnik]

Hm, I remember reversing that order multiple times due to being confused about what flatMap actually did. Thanks for pointing this out.

[arild-haugstad]

I assume that the use case for flatMap() would be for transformations where some of the elements in the input array may become several elements in the output array; where flatMap() is both faster and conceptually "cleaner" that map() followed by flat() in that it doesn't construct some nonsense intermediate array --- that's significantly more useful than the other way around ;-)