Retrieve an enumeration from a value within an enumeration

Question

Retrieve an enumeration from a value within an enumeration

In my coding project, I have an enum called Animals and I've been working on a function that should return the value as an enum if it is valid.

enum Animals {
  WOLF = 'wolf',
  BADGER = 'badger',
  CAT = 'cat',
 }

const coerceEnumValue = <T> (value: string, E: T): T | undefined => {
  const keys = Object.keys(E);
  const values = keys.map(k => E[k as any]);
  const obj = lodash.zipObject(keys, values);
  const key = lodash.findKey(obj, item => item === value);
  if (!key) return undefined;
  return E[key];
};

const selectedAnimal: Animals | undefined = coerceEnumValue('cat', Animals);
console.log(selectedAnimal);

Despite my efforts, the function is returning typeof Animals instead of casting to Animals.

typescript

Answer 1

Answer №1

An enum not only defines a type for each item in the enum (Animal), but it also establishes a type for the object that holds all the values at runtime. This object is named Animal, however, it is distinct from the type Animal as it encompasses all entries within the enum. To revert from the type of the enum container object (typeof Animal) back to the enum type itself, we must use

typeof Animal[keyof typeof Animal]

In your situation, T is deduced as typeof Animal, representing the enum container object that you are passing to the function. To return to the enum type, T[keyof T] must be implemented.

enum Animals {
  WOLF = 'wolf',
  BADGER = 'badger',
  CAT = 'cat',
}

const coerseEnum = <T> (s: string, E: T): T[keyof T] | undefined => {
  const keys = Object.keys(E);
  const values = keys.map(k => E[k as any]);
  const obj = lodash.zipObject(keys, values);
  const key = lodash.findKey(obj, item => item === s);
  if (!key) return undefined;
  return E[key];
};

const x: Animals | undefined = coerseEnum('cat', Animals);
console.log(x);

Note It's easy to overlook this distinction, but when utilizing Animal in a type annotation, it refers to an enum item type. When used as a value, it represents the container object which is not the same type as Animal. Just clarifying in case the previous explanation was ambiguous.

Answer 2

An enum not only defines a type for each item in the enum (Animal), but it also establishes a type for the object that holds all the values at runtime. This object is named Animal, however, it is distinct from the type Animal as it encompasses all entries within the enum. To revert from the type of the enum container object (typeof Animal) back to the enum type itself, we must use

typeof Animal[keyof typeof Animal]

In your situation, T is deduced as typeof Animal, representing the enum container object that you are passing to the function. To return to the enum type, T[keyof T] must be implemented.

enum Animals {
  WOLF = 'wolf',
  BADGER = 'badger',
  CAT = 'cat',
}

const coerseEnum = <T> (s: string, E: T): T[keyof T] | undefined => {
  const keys = Object.keys(E);
  const values = keys.map(k => E[k as any]);
  const obj = lodash.zipObject(keys, values);
  const key = lodash.findKey(obj, item => item === s);
  if (!key) return undefined;
  return E[key];
};

const x: Animals | undefined = coerseEnum('cat', Animals);
console.log(x);

Note It's easy to overlook this distinction, but when utilizing Animal in a type annotation, it refers to an enum item type. When used as a value, it represents the container object which is not the same type as Animal. Just clarifying in case the previous explanation was ambiguous.

Retrieve an enumeration from a value within an enumeration

Answer №1

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