Flow: regex String .match flow type wrong

[derektli]

Currently it's defined as ?Array<string> for return type.
However, the returned object is something like
["12:12a", "a", index: 0, input: "12:12a"]
https://github.com/facebook/flow/blob/v0.41.0/lib/core.js#L286

[SanichKotikov]

Hi. Is there any way to override this in local project?

[samwgoldman]

String.prototype.match will return null if there is no match. For example, "abc".match(/foo/) evaluates to null.

[gabelevi]

@samwgoldman - this is the array extending thing we discussed last week.

Someone tried to improve it in https://github.com/facebook/flow/commit/9611080b787711a392b8d5548241ecb4ed784e6b but I had to revert in https://github.com/facebook/flow/commit/3107fb3394cce82abd3c964e992d1178da33fb2d 😢

[SanichKotikov]

I suppose it must be something like this:

declare class RegExpMatchArray extends Array<string> {
    index?: number;
    input?: string;
}

and

match(regexp: string | RegExp): RegExpMatchArray | null;

[samwgoldman]

oh, I misunderstood the original post. sorry about that.

[derektli]

so maybe re-open this issue?
thanks @samwgoldman!

[julienw]

I'd use:
```
declare class RegExpMatchArray extends Array {
index: number;
input: string;
}

and for `String.match`:

match(needle: string | RegExp): Array | RegExpMatchArray | null;

and for `RegExp.prototype`:

exec(haystack: string): RegExpMatchArray | null;
```

[julienw]

@qm3ster your fundamental issue is that Flow doesn't understand properly the filter step and so doesn't rule out null results. See here

I believe this is a different issue than this one.

[julienw]

I'm surprised, as from my testing I thought Flow let's you do whatever you want when converting from string arrays to string tuples. See this for example.

[akleiber]

This is correct if the input is a RegExp:

> "STRING".match('S')
[ 'S', index: 0, input: 'STRING' ]
> "STRING".match(/.{1,3}/g)
[ 'STR', 'ING' ]

Butt currently flow says the returned value is undefined :-( Why would this be undefined?

https://flow.org/try/#0PQKgBAAgZgNg9gdzCYAoVUCuA7AxgFwEs5swBnABxkPwElsAVAJ0KoFN8ywAKQ7CzPgBc5fC2wBzAJQiAgkyYBDAJ4AeMmL4SAfGADeqMGFwkNYTe05gAvGD4D8AOgC2i-LgAW3YI70BGABoAZgBfYGl0I0IoHgsYDi5rJLBsTBgYKX1DI3MPJkQUtiQAUQU4Jm4AA1xFbGw4fHIqGjAAEj17QRC7bHw4cxZLMkqpbJDIsCYOTCZSOITUEKA

[madbence]

?T means T | null | undefined, you only handled the null case in your example (ie. use == instead of === to fix it).

[michaelhogg]

Also see:

• Duplicate issue #2450 – Native String#match declaration incorrect
• PR #3007 – Fixes String#match and RegExp#exec declarations
• PR #3359 – Add new lib def for String#match() result

[KevinGrandon]

Seems like the landscape has changed a bit in the last year. Opened a PR to try something like this: https://github.com/facebook/flow/pull/6790

[KevinGrandon]

I believe that this can now be closed due to #6790. @asolove @mrkev Could you help to close this issue?