A function to create namespaced enums in typescript, where the return type is a literal reflection of the returned enum value itself

createNamespacedEnum.ts

/*
 * TS Playground: https://www.typescriptlang.org/play?noUncheckedIndexedAccess=true&noUnusedLocals=true&noUnusedParameters=true#code/PQKgUABCEEIKYHMCWA7CcAeBDAtgBwBs5IRgwwBjAexQGcAXdbfIiAXggoCc4t64Acrji08WCnAAmAURQBXHAAoA3pAi049AIIAuCAG0A5FkMAaCIYBGhgLqm1G+jD1GT5q2YsVb9gL4BKAG5yTFxCOAA6Ry0IrAhgYAgAImjYpJDmcKjNGAiKeMSUnLz0sFASCFlJJjCiEjIwADM5FAp6JBpOHj5BYVFxKVkFAB41IRwRMQkmfhRJWnV6LlQEewhK+RwAaTgATxm4OYWGZZRVtSHtvYXMWfmILS4uLF3hy53dgD41iABJFDwckYt0O9wASnBqFxJMNxpMBuZ9BFke9rjZPmBPoo1KhAfQ9P88fZ-HpVOt9Cg+lM4BBUJUMEtxPQPrRhoSgZ8bKS1OTDgoPrS0AAVOThaRECYoehC3Z4OBsgFAilUgborkQAAGABJlJSJv0JL4Ijq+Vddr4NcF1r5gr4IGSIPRZTTLgA1LAEOQ0jja5Rwg1wI061Hmy1qJ1yiAABSwXHaHv91Jkm3YEAhUJhiYRachVGhb02H3Mbo9Xs+n2CamodEYYjjSATKokyYUtD0j2erxj9cb+qTl0+qcUAHlLAArSH0CKHJZIESKXFA-wQLALDsvUbrckAaz2eizEnMADdS3A9CHaDY1J9-BEcFg8IpFMq+9nTSybMu2IOHetqwx0E2Ucx1TZQ7VXHMMwLfk9mLTZ3U9OAKx5CBGjzCBFH-YFCz2CAqEaQCYN2Whl1-LcsMInAEK9c94NPVNfT1eFDWNZR3z2C0VwWEtEKtLd1lNYD9HY3YbFTU1qLgPjrXIfieHoOQuDQZQDCYgN1UE8cIHAhZu3jAgD0GTY+ICSs-xoAC1P7FMOGAydYloWgkAQFAVF8cxkQiOt9MMlscBIvj5MUtArIGPzbUrCMXQZZ42hZYYhQOI57QMbc9BOFZ1SwFB9l8QcOESkFktQRo4C4CAAHE1AAfk1HUMrOCAADIIF3XZ8Mqi01D0FA4CPMrIudCARTFCUZxlOUEtFVgivuddXmyr58uG6a4ApBRLDKmxAiAA
 */

/*
 * A function to create namespaced enums, where the return type 
 * is a literal reflection of the returned enum value itself.
 *
 * Ex:
 *   createNamespacedEnum({
 *     setA: ['a', 'b'],
 *     setB: ['a', 'b'],
 *   });
 *
 *   // Generated object and type:
 *   {
 *     setA: { a: 'setA.a', b: 'setA.b' }
 *     setB: { a: 'setB.a', b: 'setB.b' }
 *   }
 */
const createNamespacedEnum = <
  Namespace extends string,
  EnumKey extends string,
  EnumKeys extends Array<EnumKey>,
  Input extends Record<Namespace, [...EnumKeys]>
>(
  input: Input,
): {
  [namespace in ExtractKeys<Input>]: {
    [enumKey in TupleElementType<Input[namespace]>]: `${namespace}.${enumKey}`;
  };
} => {
  type EnumValue = `${Namespace}.${EnumKey}`;
  type PartialNamespacedEnum = Record<Namespace, Record<EnumKey, EnumValue>>;

  const partialNamespacedEnums: Array<PartialNamespacedEnum> = (Object.entries(input) as Array<
    [key: Namespace, value: EnumKeys]
  >).map(([namespace, enumKeys]) => {
    const enumObj = {} as Record<EnumKey, EnumValue>;
    for (const enumKey of enumKeys) {
      const enumValue: EnumValue = `${namespace}.${enumKey}` as EnumValue;
      enumObj[enumKey] = enumValue;
    }

    return { [namespace]: enumObj } as PartialNamespacedEnum;
  });

  const namespacedEnum = Object.assign({}, ...partialNamespacedEnums);
  return namespacedEnum;
};

/*
 * Extracts the keys of a type, and casts them all to be strings
 * (a key can naturally be a string, number, or symbol).
 */
type ExtractKeys<T extends { [k: string]: any }> = T extends infer G
  ? `${string & keyof G}`
  : never;

/*
 * Gets all the element types of a tuple and returns them all as a union
 */
type TupleElementType<Tuple extends Array<any>> = Tuple[number];