RegExpIndicesArray - Named group indices can be undefined

[TomStrepsil]

🔎 Search Terms

RegExpIndicesArray, Regex indices

🕗 Version & Regression Information

• This is the behaviour in every version I tried, and I reviewed the FAQ for entries about Regular Expressions

⏯ Playground Link

https://www.typescriptlang.org/play/?target=9&ts=6.0.0-dev.20260322#code/PTAEGcCcGNgWwIYEsB2A6ALuAUN6B7FcDUAc0nwFcAHUAXlGAQAoB+AHjiQBMA+AIwCUrWNzQBTAB7jozAOQJocwQEI0qbkmjjwa8lWq6A2gCIu3EwF0A3LgJESGqfTIUaRgAw3sQA

💻 Code

const group = /a(?<mid>b)?c/d.exec('ac')!.indices!.groups!["mid"];
const index = group[0];

The typing assumes that group indices are of type [number, number], but these can also be undefined

🙁 Actual behavior

typeof group === "undefined"

hence, the type system should have complained when attempting to access via index.

🙂 Expected behavior

'group' is possibly 'undefined'

...should be reported by the type system.

Additional information about the issue

#61079 fixed the same issue for the outer RegExpIndicesArray itself, but not the indices array on the matched groups

[RyanCavanaugh]

I think this falls under #32098

[TomStrepsil]

I think this falls under #32098

It'd be great if this could be fixed as part of that, but that feels like a much bigger change - i.e. type safety around the keys of named capture groups themselves.

The example above doesn't really change the reality of keyed groups, the example:

/a(?<mid>b)?c/d.exec('ac').indices.groups;

returns:

{ mid: undefined }

Having strongly-typed keys seems unrelated to the fact the value is undefined rather than [number, number], if the group failed to match.