'in' does not remove undefined from indexed value (with noUncheckedIndexedAccess)

[Sainan]

🔎 Search Terms

in operator, noUncheckedIndexedAccess

🕗 Version & Regression Information

Tested 5.8.2 and 5.9.0-dev.20250310

⏯ Playground Link

https://www.typescriptlang.org/play/?noUncheckedIndexedAccess=true&ts=5.8.2#code/MYewdgzgLgBAhgJwQLhmArgWwEYFMEDaAujALwwECMANDAEy0DMRA3AFCiSwCWAJgB6oMOfGXrtuAMxgAKPvxjcw8JAEoYAbzYwdMTtBiC0WPAjGJC81tt36QAG1wA6eyADmM-qvYBfNkA

💻 Code

With an array:

const arr: number[] = [1, 2, 3];
const idx: number = 2;
if (idx in arr) {
 const x: number = arr[idx];
 console.log(x);
}

With a map/object:

const map: Record<string, number> = { a: 1 };
const key: string = "a";
if (key in map) {
 const x: number = map[key];
 console.log(x);
}

🙁 Actual behavior

Type 'number | undefined' is not assignable to type 'number'.
 Type 'undefined' is not assignable to type 'number'.

🙂 Expected behavior

Expected 'in' operator to guarantee that the result of indexing is not undefined in this branch.

Additional information about the issue

No response

[MartinJohns]

Duplicate of #54241.

[Andarist]

There is some inconsistency here since #57847 though: TS playground

[cole-abbeduto-particle]

Mateusz Burzyński (@Andarist) Would this fix this issue too?

// TYPE NARROWING CORRECTLY
const key = 'data';
if (key in obj) {
	const v = obj[key];
} else {
	const v = obj[key];
}

// DUMB
const other = { key: 'data' };
if (other.key in obj) {
	const v = obj[other.key];
} else {
	const v = obj[other.key];
}