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Ben Carp
Ben Carp

Reputation: 26518

How to properly use Mapped type for function signature

depending on the type of a parameter I'd like to type the return. Similar to function overload, but using mapped types.

const requestX = async(
    id?: string | undefined,
): Promise<(typeof id) extends string ? X: X[]> =>
    fetch(......)

However, the return is always typed as X[]. What is the proper way to do it?

Upvotes: 0

Views: 204

Answers (1)

brunnerh
brunnerh

Reputation: 184376

Unless you use overloads as suggested by Aleksey L., you need a generic type argument, otherwise typeof id is resolved statically to string | undefined which does not help in the mapping.

Generics example:

const requestX = async <ID extends string | undefined = undefined>(id?: ID)
    : Promise<ID extends string ? X : X[]> =>
    fetch(...);
    

const x = requestX() // Promise<X[]>
const y = requestX('123'); // Promise<X>

Upvotes: 1

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