Covariant length on ReadOnlyArray<.>

[popham]

$ReadOnlyArray shouldn't allow length setting because it can mutate the array.

[vkurchatkin]

I personally would prefer to have length covariant even on mutable arrays

[popham]

@vkurchatkin Sure, although I recall reading something about Flow striving toward "JavaScript as it is." Constraining length on Array seems more in line with "JavaScript as it ought to be." I personally agree with you because I prefer false negatives to false positives (#4246 (comment) was a similar instance of "JavaScript as it ought to be" in tension with "JavaScript as it is").

Can I get a "shall" directive from the Flow hive at FB? I was too lazy to look it up on the spec, but I noticed that the Node interpreter admits length assignments on strings and ignores them:

const a = "asdf"; //a.length===4
a.length = 1; //a.length===4
a.length = 7; //a.length===4

If length assignment on Array is forbidden, then it seems reasonable that it should be forbidden on String also.

[vkurchatkin]

length mutation on arrays is definetely observable and might even be considered useful. On strings it should do nothing, so that is a no-brainier

[popham]

Agreed. The PR is good as-is.

[nmote]

Looks reasonable to me. I'd like to see some tests to ensure that (a) length mutations are in fact banned on $ReadOnlyArrays, and that (b) length mutations are still allowed on Arrays.

[popham]

Sure. This closes #4265 BTW.

[facebook-github-bot]

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