using constructor

gistfile1.js

function Set() {
    this._elements = [];
}

Set.prototype.filter = function(fn) {
    var result = new this.constructor();
    for (var i = 0; i < this._elements.length; i++) {
        if (fn(this._elements[i]) {
            result.add(this._elements[i]);
        }
    }
};

// ObservableSet inherits from Set
// 
// Note that calling filter on an observable set will
// return an observable set.
//
function ObservableSet() {
    Set.apply(this, arguments);
}

// mixin the addEventListener, removeEventListener, dispatchEvent methods.
mixinObservable(ObservableSet.prototype);

ObservableSet.prototype.add = function(element) {
    var modified = Set.prototype.add.call(this, element);
    if (modifed) {
        this.dispatchEvent({type:'add', relatedTarget: element});
    }
    return modified;
};

ObservableSet.prototype['delete'] = function(element) {
    var modified = Set.prototype['delete'].call(this, element);
    if (modifed) {
        this.dispatchEvent({type:'delete', relatedTarget: element});
    }
    return modified;
};