RegExpIndicesArray type definition incomplete

[stefnotch]

🔎 Search Terms

RegExpIndicesArray, Regex indices

🕗 Version & Regression Information

• This was added in PR Add RegExp indices and hasIndices #52085

⏯ Playground Link

https://www.typescriptlang.org/play/?target=99#code/DYUwLgBAlgdgJlAxiAzhAvBAFAeiwLwEoAfLAQxKwCNLFCc5CA6EADxESwCIyrEvCAfiawEyFAG4AUFABm2UUlQQAhOkwBXeCFmwQjCAG8pEUxFCQyGaPCUoA2gEYAuhIg4cEezA0BbKiAATgA0ED7+Qc4QZIiIAPaBCDAA5hBgcRAAKgCeAA6oiIFQuWAmZnLkGOoQWnA6egbGZs0Q8TAocaBMwHHJ3Dn5KIXFkChk2WhgABZQaIhkMDBxkFNkufkwAtLNAL4QIMAoIEZlLa1x7Z0g3b3khNtmO1JPQA

💻 Code

let indices = (/(z)|(a)|(b)|(c)/d).exec("abc")?.indices;
if (indices !== undefined) {
    let a = indices[1]; // [number, number] according to Typescript
                               // also try out indices[2]. That one has a value.
    if(a === undefined) {
        console.log("Typescript says this cannot happen");
    } else {
        console.log(a);
    }
}

🙁 Actual behavior

The elements of the RegExpIndicesArray can be undefined, if a capturing group did not match anything at all.

This is demonstrated by running the code above, and by the spec
https://tc39.es/ecma262/multipage/text-processing.html#sec-regexpbuiltinexec
(34. b. ii. Append undefined to indices.)

🙂 Expected behavior

I expected the Typescript type definitions to tell me that the elements of the indices array can be undefined.

Additional information about the issue

Am creating a PR with a fix

[jakebailey]

FWIW this "works" as you expect if you use noUncheckedIndexAccess, but the spec does seem to imply that it can explicitly place undefined in an array, not that it's out of bounds?

[jcalz]

Like #49228 and issues linked from there? Not considered worth the breakage in those other issues, from the looks of it

[stefnotch]

FWIW this "works" as you expect if you use noUncheckedIndexAccess, but the spec does seem to imply that it can explicitly place undefined in an array, not that it's out of bounds?

Yes, the spec guarantees the behaviour of getting undefined elements in the array. Like [[0, 1], undefined, [0, 1], undefined, undefined] . This is also implemented that way on both Firefox and Chrome.
Even if I were to write seemingly bulletproof code by checking the array length, I would still access undefineds.

noUncheckedIndexAccess does not handle all cases. For example, doing .forEach still results in the same issue.
https://www.typescriptlang.org/play/?noUncheckedIndexedAccess=true&target=99#code/DYUwLgBAlgdgJlAxiAzhAvBAFAeiwLwEoAfLAQxKwCNLFCc5CA6EADxESwCIyrEvCAfiawEyFAG4AUFABm2UUlQQAhOkwBXeCFmwQjCAG8pEU9HhKUTWQHsATgFEyiABbUMAPgiIbMFDdAmYBsAc2pCQmkzCBMzUEgyDHMxVABtAEYAXQkIHBwIVJgNAFsqEDsAGggi0vLMiGcfOwQYEIgwGwgAFQBPAAdURDsoPrBY0zlyDHUILTgdPQNjaOifPwCQINDuXoGUIZHIFDIetDAXKDREMhgYG0gXMj6BmAEoswBfCBBgFBAjcYrNb+QLBMIUd6mD5SaFAA