Function outcome influenced by variable type

I am working with an enum that represents different types of nodes in a tree structure.

enum MyEnum {
  'A' = 'A',
  'B' = 'B',
  'C' = 'C',
  // ...
}

Each node in the tree has specific types of children nodes that are allowed.

type ChildrenAllowed<T extends MyEnum> = T extends MyEnum.A
  ? MyEnum.B | MyEnum.C
  : T extends MyEnum.B
  ? MyEnum.C
  : undefined;

I am trying to create a function that will return an array of allowed node types, but I am encountering type errors. Can you help me figure out what I am missing?

const getChildrenAllowedTypes = <T extends MyEnum>(
  p: T,
): ChildrenAllowed<T>[] => {
  switch (p) {
    case MyEnum.A:
      return [
        MyEnum.B, // Error: Type 'E.B' is not assignable to type 'O'.
        MyEnum.C, // Error: Type 'E.C' is not assignable to type 'O'.
      ];
    case MyEnum.B:
      return [
        MyEnum.C, // Error: Type 'E.C' is not assignable to type 'O'.
      ];
    default:
      return [];
  }
};

Here is a simplified object structure that is not directly related to the problem at hand.

// simplified object structure

type ChildrenOwner<TCurrent extends MyEnum> = {
  children?: Children<TCurrent>;
};

type Children<
  TCurrent extends MyEnum,
  TChildrenAllowed = ChildrenAllowed<TCurrent>,
> = {
  [TKey in TChildrenAllowed & string]?: TKey extends MyEnum
    ? ChildrenOwner<TKey>
    : undefined;
};

Answer №1

It seems there is a recurring issue with TS not being able to calculate the value of ChildrenAllowed<T> due to the nature of T being a generic and therefore unknown.

I am puzzled as to why the error message states not assignable to type 'O' instead of

not assignable to 'typeChildrenAllowed<T>'
, but ultimately you will need to use explicit assertions to resolve this.

type ChildrenAllowed<T extends MyEnum> =
  T extends MyEnum.A
  ? MyEnum.B | MyEnum.C
  : T extends MyEnum.B
  ? MyEnum.C
  : never; // I presume it should indeed be never

const getChildrenAllowedTypes =
  <T extends MyEnum>(p: T): ChildrenAllowed<T>[] => {
    switch (p) {
      case MyEnum.A:
        return [
          MyEnum.B,
          MyEnum.C,
        ] as ChildrenAllowed<T>[]; // at this point
      case MyEnum.B:
        return [
          MyEnum.C,
        ] as ChildrenAllowed<T>[]; // at this point
      default:
        return [] as ChildrenAllowed<T>[]; // at this point
    }
  };

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