noUncheckedIndexedAccess does not narrow properly with "prop" in obj

[andreialecu]

Bug Report

Apologies if this is a duplicate, but I wasn't able to find anything.

🔎 Search Terms

noUncheckedIndexedAccess property in object

⏯ Playground Link

Playground link

💻 Code

declare function invariant(condition: boolean): asserts condition;

function foo(obj: Readonly<Record<string, string>>) {
  invariant(obj && "test" in obj);
  // this one below does narrow to `string`
  // invariant(!!obj["test"]);

  const x = obj["test"] // string | undefined but should be string
}

🙁 Actual behavior

x is typed as string | undefined

🙂 Expected behavior

x should be typed as string

[andreialecu]

Seems related to #41848

[andrewbranch]

This isn’t really related to noUncheckedIndexedAccess—in narrowing doesn’t ever take undefined out of the domain of a property: https://www.typescriptlang.org/play?noUncheckedIndexedAccess=true&ts=4.3.0-beta#code/CYUwxgNghgTiAEA3W8D2AjAVgLngb3gBcQBnQgflzJgEsA7Ac3gF8BuAKHZoDN4AKAETEyA+PTRYAlPnbx4YVHTLwAHvAC8EzAG0hpQgIC6HOQqWF4ATw1aAdMMIdm7IA

You should check for equality / non-equality with undefined if you want that behavior.

[typescript-bot]

This issue has been marked 'Working as Intended' and has seen no recent activity. It has been automatically closed for house-keeping purposes.

[andreialecu]

Not sure I understand. The type of obj cannot have undefined values in it as per its definition.

How does undefined fit into Readonly<Record<string, string>>?

[andrewbranch]

Ah, I see. When you set --noUncheckedIndexedAccess, the read type of property access on a Readonly<Record<string, string>> goes from string to string | undefined. But we don’t currently have the ability to model the difference between properties that are undefined and properties that are missing. If we were to introduce a missing type in the future, we would be able to implement the behavior you expect here—the read type from that index signature would be string | missing, and you could know that it’s not missing via an in test. But right now, because we lack the concept of missing, an in test on an object with an index signature does nothing.

[derolf]

With exactOptionalPropertyTypes, unchecked index access could map to
missing and not undefined.

Then key in guarded access should work equally on optionally declared properties and index signatures.

Example highlighting the obvious disparity:

https://www.typescriptlang.org/play?noUncheckedIndexedAccess=true&exactOptionalPropertyTypes=true&ts=4.4.2#code/CYUwxgNghgTiAEA3W8D2AjAVgLngJXFRmAB4BnAFxgEsA7AcwBp5KaGA+AbgChRJYEyGGiwAmXAG8KISgH5crOvQC+3btQBm8ABQAiaZV3w6IzAEp4E7vHgB6W-A1RqEMtfhhUtSvACe8BSoleABeUwA6AwoeVXUtPSijEwxMUQsrG3t4AHciAGs3G09vCj8AliCGUNNRSJkKbligA

[KaelWD]

This seems like a bug rather than a design limitation then.

[tskj]

I agree that this is neither a design limitation nor working as intended. This is simply a bug and @derolf's example proves it.