Typescript Interface, reference one instance property for dynamic second property

Question

I want to use a property within a Typescript Interface's instance to compute a second property. I've been looking into Generics but I'm not successfully implementing this yet. :

Here's an example

// Here I define all valid names - works fine!

type ModalNames = 'payment' | 'source'

// This interface implements all expected payloads
//  for each name 

interface ModalPayloads {
  payment: {
    plan: string
  }
  source: {
    metadataId: string
  }
}


// How do I use the instance name
//  to grab the correct payload property?
export interface ShowModalPayload {
  name: ModalNames
  modalProps?: ModalPayloads[instance.name]
}

In other words, for a given instance of ShowModalPayload, I want to assign modalProps to a type derived from instance.name. Hopefully this makes sense!

Answer 1

I'd use generics for this:

export interface ShowModalPayload<K extends ModalNames> {
  name: K
  modalProps?: ModalPayloads[K]
}

Note the use of ModalPayloads[K] in place of ModalPayloads[instance.name]. That's called a lookup type and it allows you to describe the type of m[k] where m has type ModalPayloads and k has type K.

Anyway the ShowModalPayload interface is now generic in K, which should be one of the ModalNames literals:

declare const smpPayment: ShowModalPayload<'payment'>;
smpPayment.name // "payment"
smpPayment.modalProps // {plan: string} | undefined

declare const smpSource: ShowModalPayload<'source'>;
smpSource.name // "source"
smpSource.modalProps // {metadataId: string} | undefined

Those correspond to what you're looking for, I think. Note that the following is still possible:

declare const smpWhoKnows: ShowModalPayload<ModalNames>
smpWhoKnows.name // ModalNames
smpWhoKnows.modalProps // {plan: string} | {metadataId: string} | undefined

which is possibly not what you want, but is not prohibited. The above definition is probably sufficient for most use cases, though.

Hope that helps. Good luck!