Typescript how do I extend an interface which uses a generic type?

Question

I have this interface structure set up:

export interface Response<T> {
  hasMoreResults: boolean
  pageToken: string | null
  status: string
  result: T
}

export interface ArrayResponse<T> {
  hasMoreResults: boolean
  pageToken: string | null
  status: string
  result: T[]
}

As there are many shared fields I want to write something like this instead:

interface ArrayResponse extends Response<T>{
    result: T[]
}

However this throws an error I cannot fix. Is there a straightforward way of changing the type of the property which uses the generic in both cases? I don't want to do T | T[] T | T[] as the result in one interface, as this will cause problems in my code. Thanks.

Answer 1

Looking at your code the only difference between ArrayResponse and Response is that result is an array.

You could use:

export interface Response<T> {
  hasMoreResults: boolean
  pageToken: string | null
  status: string
  result: T
}

export type ArrayResponse<T> = Response<T[]>

As Response can already take any type as an argument.

Answer 2

Sure, you can omit it like this

export interface ArrayResponse<T> extends Omit<Response<T>, "result"> {
  result: T[]
}

Answer 3

As far as I can tell, ArrayResponse is just an instantiation of Response where T is an array. If we want to define ArrayResponse to just pass in the item type we could do this:

export interface Response<T> {
  hasMoreResults: boolean
  pageToken: string | null
  status: string
  result: T
}

export interface ArrayResponse<T> extends Response<T[]>  {
}

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