`RegExpMatchArray` index 0 incorrectly possibly `undefined` when `noUncheckedIndexedAccess` is enabled

[DetachHead]

Bug Report

🔎 Search Terms

RegExpMatchArray noUncheckedIndexedAccess

🕗 Version & Regression Information

4.2.0-dev.20210112

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about noUncheckedIndexedAccess

⏯ Playground Link

Playground link with relevant code

💻 Code

const foo = "".match(/a/)
if (foo !== null) {
    const bar: string = foo[0] //Type 'string | undefined' is not assignable to type 'string'
}

🙁 Actual behavior

index 0 is of type string|undefined

interface RegExpExecArray extends Array<string> {
    index: number;
    input: string;
}

🙂 Expected behavior

index 0 should be of type string, since RegExpMatchArray will always have at least one value in it as when no matches are found, it returns null instead. see https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/match#return_value

therefore we can be confident that if a RegExpMatchArray is returned, it will always have a length of at least 1

should be something like this:

type RegExpMatchArray = [string] & string[] & {
    index?: number;
    input?: string;
}

[RyanCavanaugh]

I tried to fix this by adding 0: string to RegExpMatchArray, but it induces a different error in existing code that seems problematic:

    (''.match(/ /) || []).map(s => s.toLowerCase());

We'd need a fix that didn't create new other problems, IMO

[utkarshkukreti]

@RyanCavanaugh sorry I don't understand the problem with adding 0: string and that code snippet. Could you expand a bit?

[DetachHead]

@utkarshkukreti from what i understand the compiler fails to infer the type since the type of the value on the right ([]) isn't assignable to the type of the value on the left ([string]).

here's a minified example https://www.typescriptlang.org/play?#code/MYewdgzgLgBAZiEMC8MDaByDBdGBDCGaAJwEswBzNbAHwFcwATAUznOcYFgAoH0SWACM8xFOiy4C6EuQq0GLNmA48+4aDGEAvMQiQB+femxqBMAI50AHmOGjDx1d20A6ALZ4ADgAo8KAHwwAN4AvgCUMAD0kQDuIMQA1hAANJrMwHh0EMzGMKSEfgBueAA2pIxEUGSU1DyWVu5evgHB4VGRjCDMEGAYsHGJaRlZOdR5hGAgsEWl5dJVsthAA

[utkarshkukreti]

@DetachHead thanks, I get it now. I hope someone figures out a solution, noUncheckedIndexedAccess is a game changer for type safety!

[giancarlosisasi]

is there a solution for this? or maybe it's the intended behavior?

[bob-difronzo]

Testing the examples from @RyanCavanaugh and @DetachHead in TypeScript 4.3.5 and above, both seem to work now:

• 4.3.5
• 4.6.2
• Nightly

I'm not sure if it would still be appropriate to add 0: string to RegExpMatchArray and RegExpExecArray, or if there is a better solution that could be implemented now.

[DetachHead]

#49682