enum.js

/**
 * Enum which supports attached methods.
 * Each method's `this` is the enum object.
 */
class Enum {
  /**
   * @param items {Object} Enum keys and values as a plain object
   * @param methods {Object} Enum methods as a plain object
   *                         (names are keys, values are methods)
   */
  constructor (items, methods) {
    Object
      .entries(items)
      .forEach(([ itemKey, itemValue ]) => {
        const itemStr = new String(itemValue);

        const boundMethods =
          Object.fromEntries(
            Object.entries(methods || {})
              .map(([ methodKey, methodValue ]) => (
                [methodKey, methodValue.bind(itemStr)]
              )));

        this[itemKey] = Object.assign(itemStr, boundMethods);
      });
  }
}

const Weekdays = new Enum({
  Monday: 'Monday',
  Tuesday: 'Tuesday',
  Wednesday: 'Wednesday',
  Thursday: 'Thursday',
  Friday: 'Friday',
  Saturday: 'Saturday',
  Sunday: 'Sunday'
}, {
  abbreviation() {
    return this.slice(0, 3);
  }
})

console.log(Weekdays.Tuesday.toString()) // "Tuesday"

console.log('Happy ' + Weekdays.Tuesday) // "Happy Tuesday"

console.log(Weekdays.Tuesday.abbreviation()) // "Tue"