The "Observer" pattern

observer.js

const Subject = function () {
    const observersList = [];

    const addObserver = observer => {
        observersList.push(observer);
    }

    const removeObserver = toRemove => {
        const i = observersList.findIndex(observer => observer == toRemove);

        observersList.splice(i, 1);
    }

    const notify = context => {
        observersList.forEach(observer => observer.update(context));
    }

    this.addObserver = addObserver;
    this.removeObserver = removeObserver;
    this.notify = notify;
};

const Observer = function () {
    // ...
}

Observer.prototype.update = function (context) {}