ES Example (ES6 Symbol + ES7 Decorators + ES7 Object properties)

example.js

import Observable from "Observable";

class AuthController {
  @Observable
  user = null;
  
  @Observable
  isAuth = false;
  
  constructor() {
    // Update isAuth if user variable exists
    this.user.after(value => this.isAuth = value instanceof User);
    
    // Remove user if isAuth == false
    this.isAuth.after(value => {
      if (!value) { this.user = null; }
    });
  }
  
  loginAs(user) {
    this.user = user;
  }
  
  logout() {
    this.isAuth = false;
  }
}

Observable.js

/**
 * ObservablePrimitive instance
 */
class ObservablePrimitive {
    /**
     * @constructor
     * @param value
     */
    constructor(value = null) {
        this.events = {
            before: [],
            after:  [],
            all:    []
        };


        Object.defineProperty(this, '$value', {
            enumerable: false,
            value: value
        });
    }

    /**
     * @param value
     * @return void
     */
    set value(value) {
        var exists   = typeof value !== "undefined" && value !== null;
        var oldValue = this.$value;

        if (exists) {
            this.events.before.forEach((c) => c(oldValue));
            this.$value = value;
            this.events.all.forEach((c) => c(oldValue, value));
            this.events.after.forEach((c) => c(value));
        }
    }

    /**
     * Return value
     * @return {*}
     */
    get value() {
        return this.$value;
    }

    /**
     * @param callback
     * @returns {ObservablePrimitive}
     */
    after(callback) {
        this.events.after.push(callback);
        return this;
    }

    /**
     * @param callback
     * @returns {ObservablePrimitive}
     */
    before(callback) {
        this.events.before.push(callback);
        return this;
    }

    /**
     * @param callback
     * @returns {ObservablePrimitive}
     */
    subscribe(callback) {
        this.events.all.push(callback);
        return this;
    }

    /**
     * @returns {*}
     */
    toString() {
        return this.value;
    }

    /**
     * @returns {*}
     */
    [Symbol.toPrimitive]() {
        return this.value;
    }
}

/**
 * Observable decorator
 *
 * @param target
 * @param key
 * @param descriptor
 * @returns {{enumerable, get, set}}
 * @constructor
 */
export default function Observable(target, key, descriptor) {
    return (function (descriptor) {
        var value = descriptor.initializer();

        var property = new ObservablePrimitive(value);

        return {
            enumerable: true,
            get:        () => property,
            set:        (value) => property.value = value
        };
    })(descriptor);
}

@trikadin, это конечно круто, но код по-моему нафига не читаем. Используй нормальное ооп, раз боженька (мозилла и ко) в ES6 его дал =)

А что не читаемо-то? О_о