Our application uses a big conditional type to handle the possible structures of a given array of objects based on one of the properties of said objects.

We use the array of objects to define a sequence of actions to run, these actions can have different properties which our engine uses to run the corresponding code. We then use a big map to run code based on the type property in the example below.

Here is a simplified sample:

type payloadA = {
  propA: string;
}

type payloadB = {
  propB: boolean;
}

enum Payloads {
  A = 'A',
  B = 'B',
}

type conditionalPayload<T extends Payloads> =
  T extends Payloads.A ? payloadA
  : T extends Payloads.B ? payloadB
  : never;

type myObj<T extends Payloads> = {
  type: T;
  payload: conditionalPayload<T>
}

// Using an array of arrays because that is how it is in our code
const myArray: myObj<Payloads>[][] = [
  [
    {
      type: Payloads.A,
      payload: {
        propA: 'dummy',
      },
    },
    {
      type: Payloads.B,
      payload: {
        propA: 'dummy', // I would like this to throw an error, currently it does not
      },
    },
  ],
];

We’d like the type checker to be able to validate the payload property while writing the code.