Array.prototype.at returns intersection instead of union if one set is a subset of the other

[KrzysztofKarol]

🔎 Search Terms

"Array.prototype.at", "Array.at", "item located at the specified index", "intersection", "union", "union type", "subset"

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about everything (it's very interesting) but I could not find anything related.

⏯ Playground Link

https://www.typescriptlang.org/play/?target=99&ts=5.7.0-dev.20241026#code/C4TwDgpgBAggTnAhiGUC8UDeVEC4oCMUAvgNoC6A3ALABQoksCyAQullAEb4BMJFNWnQAmEAMYAbRHGhiA9gDsAzsBwIC+eEhRQAPk20tBdectVj20uAVIAGKnQD0jqK6gA9APxOXAdTlwANZKJooqUMKW6gB0iMAAFLYAlILObh7etACWAGZQ8cJJWHSuAEScpVBZChFQnhHRnFD4paI5iACuEqoAbogSHRClgq5p6eMZJVBjE25NWUpQPHTEdD5QACJyEIsKcqoA7gGBdAzQWsgAouzYEPhEZA704OfMIABiN1B3hJRQObx+E8ROIpDIoKZwlYeJo3td9BcPsZaJDVABzKJwHh2J4zSYosKqAAWmJ4sQSyVSLjcXjouXyRKKmCmpRylWqUBJ9SJ0TyLTanW6UD6AyGI2m1NmrlptFGkqleQWUA6CkCewOChWdCAA

💻 Code

type ArrayA = { a: 1 }[];
type ArrayB = { b: 2 }[];

declare const arr1: ArrayA | ArrayB;

const c = arr1[0];
//    ^?
// Works
const d = arr1.at(0);
//    ^?
if (d) {
  "b" in d ? d.b : "default value";
  //           ^?
  //           b is 2
}

// Does not work
type ArrayE = { e: 1 }[];
type ArrayF = { e: 1; f: 2 }[];

declare const arr2: ArrayE | ArrayF;

const g = arr2[0];
//    ^?
// const g: { e: 1 } | { e: 1; f: 2 }
const h = arr2.at(0);
//    ^?
// const h: { e: 1 } | undefined
if (h) {
  "f" in h ? h.f : "default value";
  //           ^?
  //           f is unknown
}

🙁 Actual behavior

In the example, const h type is only an intersection of those two types. Therefore, f property is unknown.

🙂 Expected behavior

const h should a union type of those two types and f property should be 2.

Additional information about the issue

No response

[MartinJohns]

Look up "subtype reduction". It's working as intended / a design limitation.

[jcalz]

It’s not an intersection, either. An intersection would indeed be incorrect. Maybe you mean the keys are an intersection but that’s quite different.

effectively a duplicate of #55201 and other issues linked within