Bug: RegExpMatchArray is missing properties `index` and `input`

[balupton]

TypeScript Version: 3.7.2

Search Terms: RegExpMatchArray, input, index, length, regexp, regex, result objects

Code

const match = 'a'.match(/a/)
if ( match ) console.log(match.input.includes('a'), match.index.toFixed(2))

Expected behavior: It to work.

Actual behavior: Type checks fails because RegExpMatchArray does not define index and input

Playground Link: Playground Link

Related Issues: none

For reference of the index and input properties, see:

RegExp result objects have some non-numerical properties already, which named capture groups may overlap with, namely length, index and input.

https://github.com/tc39/proposal-regexp-named-groups

[gvelchuru]

When I run this in the playground, I get "true 0.00" in the console, which is expected behavior.

[balupton]

Correct, the code will run, but the typescript type checking is invalid because it warns Object is possibly 'undefined'.(2532) because TypeScript's RegExpMatchArray type currently does not define index and input, as seen by red squiggly lines in the playground.

The match method returns either null or an array that includes length, index, input, and optionally groups.

Which is to say, that if the match result is not null, then its index and input properties should be set.

[nmain]

The error is correct. Typescript does define RegExpMatchArray.input and RegExpMatchArray.index, but warns that they might be undefined, because they might be undefined:

const m = 'a'.match(/a/g)
if (m) {
    console.log(m.input === undefined) // prints true
    console.log(m.index === undefined) // prints true
}

[balupton]

Interesting. Is there any further documentation on their behaviour anywhere?

[nmain]

spec

In short, if the regexp has the global flag set, then String.prototype.match won't return input or index.

[balupton]

Could we update the type definition then to have input and index set if the global flag is unset? The introspection could be done using https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp/global

[RyanCavanaugh]

Regex flags aren't propagated into the resulting type. I believe this is the only one that has any effect, so it'd be a tough sell to add a new feature to make these properties get detected

[typescript-bot]

This issue has been marked as 'Question' and has seen no recent activity. It has been automatically closed for house-keeping purposes. If you're still waiting on a response, questions are usually better suited to stackoverflow.