Type narrowing does not work when accessing a Partial<Record> with a Union type as the key

[ChrisRZen]

Bug Report

🔎 Search Terms

type narrowing, union, if, undefined, partial

🕗 Version & Regression Information

I saw this initially with 5.0.4 I used the playground below to try with other versions as old as 3.3

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about narrowing union types

⏯ Playground Link

Playground link with relevant code

💻 Code

type ExplicitUnion = "foo" | "bar" | "baz"

const recordWithPartialUnionKey: Partial<Record<ExplicitUnion, string>> = {
    "foo": "fiz"
}


const useUnionTypeKeys = (key: ExplicitUnion):string => {
    if(recordWithPartialUnionKey[key] !== undefined) {
        return recordWithPartialUnionKey[key] // This fails the function const
    }
    return "test"
}

// Create a Union type with some literal strings
type StringUnion = "foo" | "bar" | "baz"

// Create a record with a partial Record using said Union
const recordWithPartialUnionKey: Partial<Record<StringUnion, string>> = {
    "foo": "fiz"
}


const useUnionTypeKeys = (key: StringUnion):string => {
    if(recordWithPartialUnionKey[key] !== undefined) {
        return recordWithPartialUnionKey[key] // Type 'string | undefined' is not assignable to type 'string'.
    }
    return "test"
}

// String narrowing works fine
type StringRecord = Record<string, string>

const stringRecord:StringRecord = {
    "foo": "fiz"
}

const useStringKeyRecord = (key: string):string => {
    if(stringRecord[key] !== undefined) {
        return stringRecord[key] // This passes the typecheck
    }
    return "test"
}

🙁 Actual behavior

When checking if a key exists on a Partial<Record> with a key value type set to a union of string literals, the type of the value is not narrowed down in the same way as it is for a Record with a string key in a similar situation.

🙂 Expected behavior

When checking if a key exists on a Partial<Record> with a key value type set to a union of string literals, the type of the value is narrowed to exclude undefined

[RyanCavanaugh]

There's no narrowing going on in the bottom example.

Duplicate #48227, etc

[ChrisRZen]

There's no narrowing going on in the bottom example.

Duplicate #48227, etc

Ah I see. I forgot about the noUncheckedIndexedAccess config which would cause the second example to fail.

@RyanCavanaugh I tried looking through the FAQ and it sounds like this has come up a lot. Did I miss a question that covers this? Is it valid to consider this a bug?

[RyanCavanaugh]

This should be in the FAQ.

I would say this is really more of a feature request, since to do it soundly requires tracking information that is currently not considered. Conservative implementations of this have also incurred too-large performance hits, so it's a trade-off.

[microsoft-github-policy-service]

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

[microsoft-github-policy-service]

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

[microsoft-github-policy-service]

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

[microsoft-github-policy-service]

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