Třeba tak:

interface IProduct {
    name: string;
    type: string;
}
this.products = ko.observableArray<IProduct>([
    { name: "Sušenky", type: "free" },
    { name: "Alkohol", type: "adult" },
    { name: "Mléko", type: "free" }
]);
return ko.utils.arrayFilter(self.products(), (prod: IProduct) => prod.type === self.currentFilter());