Using a computed observable to filter or project an observable array is an incredibly common operation; I don't think I've ever seen a Knockout project that didn't do this at least once. Knockout Projections is a plugin that adds a map and filter function to observable arrays, which creates a computed observable that only recomputes it's callback on dependent elements that have changed instead of re-evaluating every single dependent element.

To better understand the problem this plugin solves, we are going to look at the example Sanderson uses on his blog to illustrate the differences between a normal computed observable array and an array made with Knockout Projections.

Consider the following model:

function Product(data) {
    this.name = ko.observable(data.name);
    this.isSelected = ko.observable(false);
}
function PageViewModel() {
    // Some data, perhaps loaded...