Array doesn't get filled correctly

I have these JavaScript entities: Item and Items.

var exports = {};
exports.Item = function(item) {
    if (item) {
        for (var attr in this.attributes) {
            var value = item[attr];
            if (value !== undefined) {
                this.attributes[attr] = value;
            }
        }
    }
    return this;
};

exports.Item.prototype.attributes = {
    _id: "",
    title: ""
};

exports.Items = function(items) {
    if (items && items.length > 0) {
        for (var i = 0; i < items.length; i++) {
            this.add(items[i]);
        }
    }
};

exports.Items.prototype.arr = [];
exports.Items.prototype.add = function(item) {
    if (item) {
        item = new exports.Item(item);
        this.arr.push(item.attributes);
    }
};
exports.Items.prototype.toJSON = function() {
    var json = [];
    for (var i = 0; i < this.arr.length; i++) {
        json.push(this.arr[i]);
    }
    return json;
};

var i1 = new exports.Item({
    _id: "1",
    title: "1"
});

var i2 = new exports.Item({
    _id: "2",
    title: "2"
});

var i3 = new exports.Item({
    _id: "3",
    title: "3"
});

var items = new exports.Items([i1,i2,i3]);
console.log(items.toJSON());

There is a problem which I cannot find. When I execute the following code I get the last item 3 times instead of all the items.
I am sure the mistake is something small I cannot see. Maybe you can help me?