function SampleObserver () {

    Rx.Observer.call(this);

    this.isStopped = false;

}

​

SampleObserver.prototype = Object.create(Rx.Observer.prototype);

SampleObserver.prototype.constructor = SampleObserver;

​

Object.defineProperties(SampleObserver.prototype, {

    onNext: {

        value: x => {

            if (!this.isStopped) {

                console.log(`Next: ${x}`);

            }

        }

    },

    onError: {

        value: err => {

            if (!this.isStopped) {

                this.isStopped = true;

                console.log(`Error: ${err}`);

            }

        }

    },

    onCompleted: {

        value: () => {

            if (!this.isStopped) {

                this.isStopped = true;

                console.log('Completed');

            }

        }

    } 

});

​

var sampleObserver = new SampleObserver();

​

var source = sampleObserver.asObserver();

​

console.log(source === sampleObserver);

// => false