enum-type

A TypeScript inspired Enum type for plain JavaScript.

INSTALLATION

npm i -S @fkurz/enum-type

USAGE

const CardSuit = Enum.fromObject({
  CLUB: "club",
  DIAMOND: "diamond",
  HEART: "heart",
  SPADE: "spade"
});

See chapter THE Enum CLASS below for more details.

SUMMARY

ENUMERATED TYPES

Enumerated types (or, enumeration/ enum) are most commonly implemented as a set of constants of a common type whose elements are bound to unique identifiers.

EXAMPLE: In Java an enum for the suits of (french) playing cards could be defined by

enum CardSuit {
    CLUB,
    DIAMOND,
    HEART,
    SPADE
}

and is getting compiled to instances CLUB, DIAMOND, HEART and SPADE available as public static properties of CardSuit. Therefore

CardSuit.class == CardSuit.CLUB.getClass() // true

This package provides a comparatively more flexible enum implementation for plain JavaScript which is inspired by TypeScript enums.

The underlying concept is that an enum type simply is a

• finite collections of values $C$
• which can be enumerated by the natural numbers
• and defines unique names/symbols for each value

TRANSLATION TO JAVASCRIPT

An enumerable type as specified above can be implemented in JavaScript as an Object with mixed String and Number keys where the number keys enumerate arbitrary values and the string keys give names to the index numbers—and implicitely to the values—for better readability.

EXAMPLE: The enumerable type for card suits could be defined as

const CardSuit = {
  // value enumeration
  0: "club",
  1: "diamond",
  2: "heart",
  3: "spade",
  // name bindings
  CLUB: 0,
  DIAMOND: 1,
  HEART: 2,
  SPADE: 3
};

PROPERTIES OF THIS IMPLEMENTATION

This enum implementation has some convenient properties:

• Values may be of arbitrary and potentially heterogenous type

• Implicit ordering/comparability: e.g.

  CardSuit.SPADE > CardSuit.CLUB; // true

• Value access by name (like an Object): e.g.

  CardSuit[CardSuit.CLUB]; // 'club'

• Fixed iteration order (like an Array): e.g.

  for (const suit of CardSuit) {
    console.log(suit);
  }
  
  // 'club'
  // 'diamond'
  // 'heart'
  // 'spade'

• Readability/self-documentation:

  const colorOf = (suit) => {
      switch(suit) {
          case CardSuit.CLUB:
          case CardSuit.SPADE: return 'black';
          case CardSuit.DIAMOND:
          case CardSuit.HEART: return 'red';
          default:
              throw new Error('Unknown suit.');
      }
  }

NOTE: Retrieving values by name as shown in the second property requires two access operations on the enum: first to access the index for the given name and the second to retrieve the value for that index. E.g. we have to write

CardSuit[CardSuit.CLUB]

to obtain the value associated with the name CLUB.

NOTE: The compatibility with the for-of loop shown (third implementation property) is due to Enum's implementation of the Iterable interface (otherwise properties bound to string keys would also be looped over).

THE Enum CLASS

Static methods

Enum provides two static constructors .fromArray and .fromObject.

• Enum.fromArray(entries: Array<string[]>): Enum

  Construct an Enum from an Array of Arrays containing the key at index zero and the value at index 1.

• Enum.fromObject(object: { [key: string]: any }): Enum

  Construct an Enum from an Object. This method simply recurs to Enum.fromArray by passing Object.entries(object).

In addition to constructing it, both methods will make the enum instance unmodifiable using Object.freeze.

EXAMPLE:

CardSuit can be constructed using the Enum class provided by this package as follows:

const CardSuit = Enum.fromArray([
  ["CLUB", "club"],
  ["DIAMOND", "diamond"],
  ["HEART", "heart"],
  ["SPADE", "spade"]
]);

or—if the specific ordering is not relevant as long as there is some order—by

const CardSuit = Enum.fromObject({
  CLUB: "club",
  DIAMOND: "diamond",
  HEART: "heart",
  SPADE: "spade"
});

NOTE: The second enum does not necessarily return the values in insertion order of the passed object because the implementation uses Object.entries and this does not guarantee a fixed order. (1)

Instance methods

Enum provides the following instance methods:

• .keys(): string[]

  Return the key set as an Array.

• .entries(): Array<string[]>

  Return the entry set as an Array of Arrays. The inner Arrays have fixed length two where the first element stores the index and the second element stores the value.

• .values(): any[]

  Return the value collection as an Array.

• .keyOf(value: any): string

  Return the key associated with the given value. This is the reverse of the value enumeration.

EXAMPLES:

CardSuit.keys(); // ['CLUB', 'DIAMOND', 'HEART', 'SPADE']
CardSuit.entries(); // [['CLUB', 'card'], ['DIAMOND', 'diamond'], ['HEART', 'heart'], ['SPADE', 'spade']]
CardSuit.values(); // ['card', 'diamond', 'heart', 'spade']
CardSuit.keyOf("card"); // 'CARD'