Element implicitly has an 'any' type because expression of type 'string' can't be used to index type 'Record<...>'

[predragnikolic]

Bug Report

Given the following code:

type Personality = "social" | "positive" | "enthusiastic"
type Result = {value: number}

type PersonalityRecord = Record<Personality, Result>

export function doStuff(personalityRecord: PersonalityRecord) {
  for (const personality in personalityRecord) {
    const result = personalityRecord[personality]
               //  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Error: Element implicitly has an 'any' type because expression of type 'string' can't be used to index type 'Record<Personality, Result>'. No index signature with a parameter of type 'string' was found on type 'Record<Personality, Result>'.
  }
}

🙁 Actual behavior

TypeScirpt is reporting an error.

Error: Element implicitly has an 'any' type because expression of type 'string' can't be used to index type 'Record<Personality, Result>'. No index signature with a parameter of type 'string' was found on type 'Record<Personality, Result>'.

Typescript thinks that personality is of type string, but I expect it to be of type Personality.

🙂 Expected behavior

Typescript should not report the error and the above code should just work.

Here is the CodeSandBox example: https://codesandbox.io/s/young-night-516gs?file=/src/index.ts:268-346

My current workaround is to use a cast const result = personalityRecord[personality as Personality], but I don't like that. Is there a more nicer way to solve this?

[fatcerberus]

This is for the same reason Object.keys() returns string[]: https://stackoverflow.com/questions/55012174/why-doesnt-object-keys-return-a-keyof-type-in-typescript

Your PersonalityRecord object isn't guaranteed (by the type system) to only contain Personality keys, and in turn those extra properties are not guaranteed to be of type Result.

[andrewbranch]

Duplicate of https://github.com/microsoft/TypeScript/issues/35847

[predragnikolic]

Thanks @fatcerberus , now it is clear why typescript behaves like this.

Having an Exact type would solve my issue, but until something like that exist I would have to stick with casting.

Have a nice day. 👋