enum-support

Convenient enumeration pattern for JavaScript.

NB: Credit is due to https://github.com/rauschma/enumify, which appears to be no longer maintained as of this writing (November, 2019).

NB2: This release contains breaking changes since v3.0.0. See details at the end of this document.

Overview

This library uses objects to represent enumerations, not just simple integer values. It's a little bit like the way Java implements enums.

In this implementation, you ask the Enumeration class to create your enum, including

• its name,
• its values,
• optionally, its instance methods, and
• optionally, unique instance methods for each of its values.

Here's a simple example.

const Enumeration = require('@ballista/enum-support')
const Color = Enumeration.new({ name: 'Color', values: ['RED', 'GREEN', 'BLUE'] })

Stock features

The enumeration class you get has certain characteristics, including the following.

• They are sealed.
• They are frozen.
• They are not extendable, throwing ClassNotExtendableError from the constructor if subclassed then instantiated.
• They are not instantiable, throwing EnumClassNotInstantiableError from the constructor if newed.

Each Enumeration class created via Enumeration.new() has the following public properties and methods.

• values: a static, frozen property containing an array of each of the instances, also frozen, of the enumeration; the order of the array determines each enum's ordinal (zero-based).
• $ERROR$: a static property containing an UnknownEnumValueError subclass that has a name equal to Unknown${name}EnumValue code equal to E_UNKNOWN_${toUpperSnake(name)}_ENUM_VALUE.
• of(nameOrOrdinal): a static method that returns the enumeration instance given its name or zero-based ordinal; throws an instance of $ERROR$ if not found.
• isInstance(it): a static method that returns truey if it is an instance of the enumeration's class (and yes, instanceof works, too, since these are just classes).
• isClass(it): a static method that returns truey if the given class is the enumeration class.
• Symbol.iterator: a static property that returns an iterator over values.

Each value (or, technically, Enumeration subclass instance) is frozen & has the following public properties & methods:

• name: a string property containing the name of the value.
• ordinal: a number representing the value's position amongst all values, as determined by the array of values you give to Enumeration.new().
• toString(): returns a concatenation of the class's name, followed by a ., followed by the value's name (ie, Color.RED.toString() === 'Color.RED').

Examples

For full usage, see the tests.

Simple enumeration of values only

const Color = Enumeration.new({
  name: 'Color',                   // 1
  values: ['RED', 'GREEN', 'BLUE'] // 2
})

1. Give the enumeration its name
2. Provide each enumerated value as a literal string.

Enumeration with an instance method

  const Day = Enumeration.new({
    name: 'Day',
    values: ['MONDAY', 'TUESDAY', 'WEDNESDAY', 'THURSDAY', 'FRIDAY', 'SATURDAY', 'SUNDAY']
  },
  // 1
  {
    isWeekend() {
      return this === Day.SATURDAY || this === Day.SUNDAY
    },
    isWeekday () {
      return !this.isWeekend()
    }
  })

1. Simply provide a second parameter as a literal object with methods that you wan to add to the enumeration class

Enumeration with values that have their own methods

  const TicTacToePiece = Enumeration.new({
    name: 'TicTacToePiece',
    values: {                                       // 1
      O: {                                          // 2
        get inverse () { return TicTacToePiece.X },
        oh () { return 'o' }                        // 3
      },
      X: {                                          // 2
        get inverse () { return TicTacToePiece.O },
        ex () { return 'x' }                        // 4
      }
    }
  })

1. Instead of providing an array of literal strings, provide an object whose keys are used as enumeration value names
2. The value of each key is a literal object with methods that go on that specific value.
3. Method unique to TicTacToePiece.O
4. Method unique to TicTacToePiece.X

Breaking changes since v3.0.0

• All enumeration classes & instances are frozen & sealed, throwing ClassNotExtendableError.
• Enumeration class constructors now throw EnumClassNotInstantiableError if instantiated.
• $ERROR$ class now has its name set correctly.
• Removed static property INITIALIZED from enumeration classes.
• Removed methods isEnumerationInstance & isEnumerationClass from Enumeration.