Generic type mapping of TypeScript interface properties

Question

Generic type mapping of TypeScript interface properties

I am struggling to get this to function correctly

interface ObjectPool<Ids, T> {
  pool: {
    [K in Ids]: T<K>;
  };
};

interface Player<Id> {
  id: Id;
}

let playerPool: ObjectPool<0 | 1 | 2, Player>;

in a way that

playerPool[0].id === 0;
playerPool[1].id === 1;
playerPool[2].id === 2;
// playerPool[3] error

however TypeScript is indicating that I require a generic parameter at Player in

let playerPool: ObjectPool<0 | 1 | 2, Player>;

so I attempted

let playerPool: ObjectPool<0 | 1 | 2, Player<_>>;

but it did not resolve the issue either

typescript typescript-generics

Answer 1

Answer №1

When using T<K>, the type T must be a specific type operation (e.g., type T<K> = ... or interface T<K> { ... or class T<K> ...). It is not possible to have T<K> where T is a generic type parameter. To achieve this, higher-kinded types would be required, as mentioned in microsoft/TypeScript#1213, which TypeScript does not support directly.

If you rethink your approach and find a way to represent what you need without using higher-kinded types, it's better. For example:

type ObjectPool<P extends PropertyKey, T> =
  { [K in P]: { id: K } & T };

You can then define types like Player and use them accordingly:

interface Player {
  id: number,
  name: string,
}

This allows an

ObjectPool<0 | 1 | 2, Player>

to behave as expected:

function processPlayerPool(playerPool: ObjectPool<0 | 1 | 2, Player>) {
  playerPool[0].id === 0;
  playerPool[1].id === 1;
  playerPool[2].id === 2;
  playerPool[2].name;
  playerPool[3] // error
}

Keep in mind that trying to index over numeric literals may cause issues with analysis by the compiler. Consider this when working with wallPool.

Answer 2

When using T<K>, the type T must be a specific type operation (e.g., type T<K> = ... or interface T<K> { ... or class T<K> ...). It is not possible to have T<K> where T is a generic type parameter. To achieve this, higher-kinded types would be required, as mentioned in microsoft/TypeScript#1213, which TypeScript does not support directly.

If you rethink your approach and find a way to represent what you need without using higher-kinded types, it's better. For example:

type ObjectPool<P extends PropertyKey, T> =
  { [K in P]: { id: K } & T };

You can then define types like Player and use them accordingly:

interface Player {
  id: number,
  name: string,
}

This allows an

ObjectPool<0 | 1 | 2, Player>

to behave as expected:

function processPlayerPool(playerPool: ObjectPool<0 | 1 | 2, Player>) {
  playerPool[0].id === 0;
  playerPool[1].id === 1;
  playerPool[2].id === 2;
  playerPool[2].name;
  playerPool[3] // error
}

Keep in mind that trying to index over numeric literals may cause issues with analysis by the compiler. Consider this when working with wallPool.

Generic type mapping of TypeScript interface properties

Answer №1

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