object.watch polyfill in ES5

object-watch.js

/*
 * object.watch polyfill
 *
 * 2012-04-03
 *
 * By Eli Grey, http://eligrey.com
 * Public Domain.
 * NO WARRANTY EXPRESSED OR IMPLIED. USE AT YOUR OWN RISK.
 * Modified by Michael Keyser
 * 2018-03-14
 */

Object.defineProperties(Object.prototype, {
    watch: {
        value: function (prop, handler) {
            if (handler === undefined) {
                return this.__watched__.hasOwnProperty(prop);
            } else if (this.__watched__.hasOwnProperty(prop)) {
                throw new Error('Cannot watch property more than once');
            }
            this.__watched__[prop] = Object.getOwnPropertyDescriptor(this, prop);
            var ref = this.__watched__[prop],
                oldval = ref.get(),
                newval = oldval,
                getter = function () {
                    return ref.get();
                },
                setter = function (val) {
                    oldval = newval;
                    newval = val;
                    ref.set(newval);
                    handler.call(this, newval, oldval);
                };
            if (delete this[prop]) { // can't watch constants
                Object.defineProperty(this, prop, {
                    get: getter,
                    set: setter,
                    enumerable: Boolean(ref.enumerable),
                    configurable: true
                });
            } else {
                throw new Error('Property cannot be watched');
            }
        }
    },
    unwatch: {
        value: function (prop) {
            if (!this.watch(prop)) {
                return false;
            }
            delete this[prop];
            Object.defineProperty(this, prop, this.__watched__[prop]);
            delete this.__watched__[prop];
            return true;
        }
    },
    __watched__: {
        value: {},
        writable: true
    }
});