TypeScript enum composition experiments

enumComposition.ts

// based on:
// https://github.com/microsoft/TypeScript/issues/17592#issuecomment-320805415
// https://www.typescriptlang.org/docs/handbook/enums.html#objects-vs-enums

enum GlobalEnum {}

// example enum 1
enum Example1 {
  A = 'A',
  B = 'B'
}

// example enum 2
enum Example2 {
  C = 3,
  D = 4
}

// https://github.com/microsoft/TypeScript/issues/17592#issuecomment-320805415
function composeEnum<
  Enum1 extends typeof GlobalEnum,
  Enum2 extends typeof GlobalEnum
>(enum1: Enum1, enum2: Enum2): typeof GlobalEnum & Enum1 & Enum2 {
  return { ...enum1, ...enum2 } as typeof GlobalEnum & Enum1 & Enum2;
}

// https://www.typescriptlang.org/docs/handbook/enums.html#objects-vs-enums
const ComposedEnum = composeEnum(Example1, Example2);
type ComposedEnum = typeof ComposedEnum[keyof typeof ComposedEnum];

    
// testing
    
function doSomethingWithComposedEnum(input: ComposedEnum) {
  console.log(input); // note: for some reason can't use enum members on input
}

doSomethingWithComposedEnum(ComposedEnum.A);  // "A"
console.log(ComposedEnum.C);  // 3