Javascript (ES6) Enum

Enum.js

export class Enum {
  isObject (o) {
    return (!!o) && (o.constructor === Object);
  }
  isArray (a) {
    return (!!a) && (a.constructor === Array);
  }
  flipObject (o) {
    return Object.entries(o).reduce((obj, [key, value]) => ({ ...obj, [value]: key }), {})
  }

  constructor (keys) {
    if (!this.isArray(keys) && this.isObject(keys)) {
      keys = this.flipObject(keys)
    }
    for (const [key, value] of Object.entries(keys)) {
      this[value] = key;
    }
    Object.freeze(this);
  }

  *[Symbol.iterator]() {
    for (let key of Object.keys(this)) yield key;
  }

  getKeys () {
    return Object.keys(this)
  }
  
  getValues () {
    return Object.values(this)
  }
  
  getKey (key) {
    return this.flipObject(this)[key]
  }
}

How to use with Array notation

const days = new Enum([
  'monday',
  'tuesday',
  'wednesday',
  'thursday',
  'friday',
  'saturday',
  'sunday'
]);

console.log(days);
/* {monday: 0, tuesday: 1, wednesday: 2, thursday: 3, friday: 4, saturday: 5, sunday: 6} */

console.log([...days]);
/* ["monday", "tuesday", "wednesday", "thursday", "friday", "saturday", "sunday"] */

How to use with Object notation

const sizes = new Enum({
  xsmall: 'xs',
  small: 'sm',
  medium: 'md',
  large: 'lg'
})

console.log(sizes);
/* { large: "lg", medium: "md", small: "sm", xsmall: "xs"} */

console.log(sizes.small);
/* "sm" */

console.log(sizes.getKey(sizes.small));
/* "small" */

console.log([...sizes]);
/* ["xsmall", "small", "medium", "large"] */

console.log(sizes.getKeys());
/* ["xsmall", "small", "medium", "large"] */

console.log(sizes.getValues());
/* ["xs", "sm", "md", "lg"] */