Can't index an object literal despite 'in'

[Sainan]

🔎 Search Terms

index, object, literal

🕗 Version & Regression Information

Tried 5.8.2 and 5.9.0-dev.20250323

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.8.2#code/MYewdgzgLgBAJgQygmBeGBvAUDGAzEEALhgEYAaHGAIwQCcSAmLAXwG4stRJYBLMAA4BXKCWh1+AczQwARARCyOvPDAAU-YXzDwkCAJSYq3aDABuCADZCApiTBCAttRt0ZiZAG1NIgLodcExBLGwA6SxBJNQtrG30OFiwgA

💻 Code

const data = {
  foo: 1,
  bar: 2
};

const input: string = "foo";
if (input in data) {
  const value: number = data[input];
  console.log(value);
}

🙁 Actual behavior

Element implicitly has an 'any' type because expression of type 'string' can't be used to index type '{ foo: number; bar: number; }'.
  No index signature with a parameter of type 'string' was found on type '{ foo: number; bar: number; }'.

🙂 Expected behavior

Indexing succeeds because in operator narrowed input to "foo" | "bar"

Additional information about the issue

I know that in the given example, data could simply be given a type of Record<string, number> but this is not an option when using JSON file imports.

[MartinJohns]

Duplicate of #34867.

[jcalz]

And #43284

[typescript-bot]

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