Index interface with template literal type and mapped type

Question

I'm experimenting with TypeScript's type system to build an API class that's able to infer the expected data based on a passed string (which is a specific route on the backend).

However some of those routes include a number which makes my current implementation a bit ugly. I was wondering if you could improve that generic type.

My current implementation looks like this:

type ResourceType = 'api_keys' | 'message_forwardings' | `devices/${number}/user_devices`
type ResourceMap = {
  api_keys: IAkey,
  user_devices: IUserDevice
  message_forwardings: IMessageForwarding
}

type DataType<T extends ResourceType> =
    T extends keyof ResourceMap
        ? ResourceMap[T]
        : T extends `devices/${number}/user_devices`
            ? ResourceMap['user_devices']
            : never

class RestApiService<T extends ResourceType> {
    resource: T
    data: DataType<T>

    constructor(resource: T){
        this.resource = resource
    }

    // all the request methods go here
}

with this i can do:

const data = new RestApiService('api_keys').data // typescript knows it's Array<IAkey>

new RestApiService('some_route') // Argument of type 'some_route' is not assignable to parameter of type 'ResourceType'

const data = new RestApiService('devices/1234/user_devices').data // typescript knows it's Array<IUserDevice>

This is already working well but the more routes I want to add to the ResourceType and ResourceMap the more unclear my DataType narrowing can get, especially if there are even more routes that need a template literal type.

Is there a way to tidy things up and that I don't need to add more conditional types?

Answer 1

TypeScript supports template string pattern index signatures, so you can write your ResourceMap type directly as

type ResourceMap = {
    api_keys: IAkey,
    [k: `devices/${number}/user_devices`]: IUserDevice
    message_forwardings: IMessageForwarding
}

and then do a pure indexed access instead of worrying about conditional types:

type DataType<T extends ResourceType> = ResourceMap[T]

Playground link to code