"Create a Typescript type that restricts keys to only those containing string values in their

After researching similar questions, I have found that none of them provide the exact solution I am looking for. For instance, the solution from this question falls short when used within a function: Playground

What I require is a function that takes an item T and a key of T such that the compiler can be confident the key always points to a string field, and therefore the return type of item[key] will be a string:

function f2<T> (item: T, key: StringOnlyKeys<T>): string {
    return item[key] as string;
}

This does not compile in the generic case, as shown in this playground link

Answer №1

It seems like this code snippet is functioning as intended:

function retrieveValue<T extends Record<any, any>>(data: T, key: StringOnlyKeys<T>): string {
    return data[key];
}

Although not entirely clear on the reason behind it, one possibility could be that TypeScript doesn't automatically limit generic type parameters. So, when using an unconstrained <T>, it could potentially resolve to a type like number. Instead of determining that "this function can't possibly be called with a number", the compiler might default to treating anything related to StringOnlyKeys<number> as any.

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