Incorrect type deduction on generic array mapping

I have observed an unexpected interaction in the following scenario. When attempting to iterate using the map function with the generic options, I noticed that the item inside is not of the expected type.

const foo = <T extends string[]>(options: T, labels: Record<T[number], string>): string[] => {
    return options.map(o => labels[o]);
}

The above situation presents a simplified issue that should, in theory, be completely type safe. Given that labels is a record indexed by the options' index type, it should be capable of being accessed by it. However, the type inferred for the control variable o in the map function is string.

Could this discrepancy be due to incorrect typing within the map function or is it actually expected behavior?

The simplest solution I have found so far is to specify the type as (o: T[number]) => ..., but my main interest lies in understanding why this unexpected behavior occurs and exploring the underlying principles behind it.

Edit: Sample usage

foo(["A", "B"] as ["A", "B"], {
    A: "Label A",
    B: "Label B",
})
typescriptgenerics

Answer №1

Implement a type parameter T for the array of strings called items rather than for the overall array:

const foo = <T extends string>(
    options: T[], labels: Record<T, string>): string[] => {
    return options.map(o => labels[o]);
}

foo(["A", "B"] as ["A", "B"], {
    A: "Label A",
    B: "Label B0",
})

Sample code on the playground

Is this due to an incorrect type assigned to the map function or is it expected behavior?

The error at labels[o] is anticipated. In your scenario, labels is a type of Record<T[number]>, so you can only access it with keys of type T[number].

When you apply map on options, TypeScript will infer the item type string based on the base constraint of T (string[]) - which represents the most common base type in this case. Since string does not match T[number], an error is generated.

By using a fixed key, we can ensure consistency between the type of record keys and array items.

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