Why is `MagicRegExpMatchArray` typed with `string | undefined`?

[Michael Schaufelberger]

Hi

First, thank you so much for this library. I love it so far. I came across a typing issue while trying it out, and thought I'd report it.

🐛 The bug

const regex = createRegExp(
  anyOf('A', 'B', 'C')
    .groupedAs('opponent')
    .and(' ')
    .and(anyOf('X', 'Y', 'Z').groupedAs('self'))
);

// Results in: /(?<opponent>A|B|C) (?<self>X|Y|Z)/

console.log('A Y'.match(regex)?.groups); // { opponent: 'A', self: 'Y' }
console.log('B '.match(regex)?.groups); // undefined
console.log('C Z'.match(regex)?.groups); // { opponent: 'C', self: 'Z' }

The type of each groups is Record<"opponent" | "self", string | undefined>.

Why is the type in
https://github.com/danielroe/magic-regexp/blob/50ac0caa55e6e3fe2c7397297d8a1533190c5012/src/core/types/magic-regexp.ts#L35

defined as string | undefined?

How can the value of the group be undefined while still matching the reg exp?

🛠️ To reproduce

https://stackblitz.com/edit/github-ppjiny?file=index.ts

🌈 Expected behaviour

The type of each groups should be Record<"opponent" | "self", string>.

ℹ️ Additional context

No response

[David Tai]

Hi @michaelschufi,

I think the behavior you're observing is actually working as intended.

We didn't touch any runtime logic (helpers are transform to pure RegExp), so it behave exactly as native RegExp at runtime. The issue here is that /(?<opponent>A|B|C) (?<self>X|Y|Z)/ required both groups opponent and self to be matched, you can actually try with native RegExp:

console.log('B '.match(/(?<opponent>A|B|C) (?<self>X|Y|Z)/)?.groups); 
// return `undefined`

maybe what you want/expect is making the self part optional, you can try:

const regex = createRegExp(
  anyOf('A', 'B', 'C')
    .groupedAs('opponent')
    .and(' ')
    .and(anyOf('X', 'Y', 'Z').groupedAs('self').optionally())
);

this resolve to /(?<opponent>A|B|C) (?<self>X|Y|Z)?/, this will still return undefined if matching 'B ', as the self groups is (or inside) a optional quantifier:

console.log('B '.match(/(?<opponent>A|B|C) (?<self>X|Y|Z)?/)?.groups) 
// return `{ opponent: 'B', self: undefined }`

this is why the type of groups is Record<"opponent" | "self", string | undefined>, as we can still match a certain RegExp but not all of the groups having captured values.

Please let me know if you have any further questions or concerns. Thank you!

[Michael Schaufelberger]

Hi @didavid61202

Thank you for the fast response.

The first part, I completely agree with. Since groups will always be undefined if the regexp doesn't match.

Regarding the second case, you hit the nail on the head. I expect the value part of the type to be string | undefined if and only if I mark at least one group as optional. Otherwise all the groups will always have a value, correct?

Edit: In the meantime I've started to write an enhancement request issue regarding the type resolution for the groups. It would be awesome if the type of each group could be inferred based on the anyOf parameters.

[David Tai]

Hi @michaelschufi,

Yes, you are right. If all the groups are not optional, the match result should either be null or matched array with all groups containing value. I think same goes for capturing groups too.

I've benn working on a new type inferrencing for a while, I've put more detail at #235, hopefully will resolve this and many other cases 👍