Enum implementation for JS for simple Map-style enums

enum.js

// ES5 (with underscore.js)
function Enum(obj){
    return Object.freeze(_.extend(Object.create(null), obj));
}

// ES6
function Enum(obj){
    let target = Object.freeze(Object.assign(Object.create(null), obj));
    return new Proxy(target, {
        get(target, propKey, receiver){
            if (!(propKey in target)){
                throw new ReferenceError('Unknown property: ' + propKey);
            }
            return Reflect.get(target, propKey, receiver);
        }
    });
}

const MY_ENUM = Enum({
    VALUE_A: 'a',
    VALUE_B: 'b',
    VALUE_C: 'c'
});