Can TypeScript infer a function return type based on an argument in this scenario?

Question

I'm writing a function that fetches data from an API. I know what the data will be based on the API path, so I tried an approach with 'ApiSignature' types.

I simplified the code I have at the moment:

interface Order {
    id: string
}

type ApiSignature<P, D> = {
  path: P,
  data: D
};

type OrdersApiSignature = ApiSignature<'/orders', Order[]>
type OrderApiSignature = ApiSignature<'/order', Order>

type AnyApiSignature = OrdersApiSignature | OrderApiSignature

function useApi<T extends AnyApiSignature>(path: T['path']) {
    return null as any as T['data']
}

const data = useApi('/orders') // Order | Order[]

Playground link

Is it possible for TypeScript to infer that in this case the data will be Order[] and not Order, based on the function argument?

Answer 1

Building on @jcalz's comment,

But if your path is a string then you can just do this more conventionally with a key lookup like this.

type ApiSignatureMap = {
  "/orders": Order[];
  "/order": Order;
};

type AnyApiSignature = {
  [K in keyof ApiSignatureMap]: ApiSignature<K, ApiSignatureMap[K]>;
}[keyof ApiSignatureMap];

function useApi<P extends AnyApiSignature["path"]>(path: P): ApiSignatureMap[P] { ... }

We'll use a "map" to map paths to the type of data (ApiSignatureMap).

And to get all the possible API signatures, we use a mapped type:

{
  [K in keyof ApiSignatureMap]: ApiSignature<K, ApiSignatureMap[K]>;
}

This will give us an object whose values are all the API signatures we seek. To get a union of them we can use keyof ApiSignatureMap:

{
  [K in keyof ApiSignatureMap]: ApiSignature<K, ApiSignatureMap[K]>;
}[keyof ApiSignatureMap]

And finally in the function declaration, we have a generic P that can be any valid path in AnyApiSignature, then get the return type using the API signature map above.

Playground