Typescript: index signature is missing in type when extending interface

Question

I'm trying to do an extension of one of my interfaces, but keep getting the error "index signature is missing in type {dcs: Relationship} with the following snippet:

interface Relationship {
    data: {
        id: string;
        type: string;
    }
}

interface T {
    relationships: {[key: string]: Relationship};
}

interface Test extends T {
    relationships: {
        dcs: Relationship;
    };
}

The goal is for the relationships property in T to be an object with any number of keys that are all of the Relationship type. Test is supposed to be a specific implementation of type T.

Not sure what the correct way to approach this is. Thanks in advance!

Answer 1

Declare Test like so:

interface Test extends T {
  relationships: {
    [key: string]: Relationship;
    dcs: Relationship;
  };
}

However, as you've noticed you end up having to declare the indexer again when you actually implement the interface. You could save some typing by doing this:

interface T {
  relationships: {
    [key: string]: Relationship;
  };
}

interface TestRelationships {
   [key: string]: Relationship;
   dcs: Relationship;
}

interface Test extends T {
  relationships: TestRelationships;
}

class T1 implements Test {
  relationships: TestRelationships;
}

Answer 2

A solution too if the type doesn't need to be an interface! Is to use an object literal type

(( However adding the index signature to the interface can be a better option )) It's to show that we can do that too!

ex:

export type SignalProcessorEvents =  {
    valueCandle: (
        value: WhalesCandle,
        storageType: StorageType | undefined
    ) => void
}

Illustration:

When using the interface (typescript complaining)

When switching to object literal

Extension code for the one that want to see it:

export class Events<EventsObject extends { [eventName: string]: (...args: any[]) => void}> {
    protected _events: EventsObject = {} as any;