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Category: Knockout

Knockout 3.2.0 released

This one has been slow-cooking since February and it’s good. For the first time in a while, this release focuses on adding some significant new features to Knockout. Almost any developer using KO should seriously consider making use of these, as they can greatly streamline your code.

So what’s new?


Components bring a new and hugely more scalable way of structuring a large application. They:

  • Combine a viewmodel and template in an encapsulated package
  • Can represent either small reusable widgets or entire sections/pages in a larger app
  • Can be preloaded, or loaded on demand
  • Can be nested or inherited
  • Can work with custom conventions for configuration or loading.

I’ve built a few apps with KO components recently (including the KO triage site, and an iOS/Android game called Blockpick (not open source)) and can confidently say this is how I’ll be structuring my KO apps from now on.

See: components overview documentation and explore from there. Or if you’re in the mood to get a hot drink and settle in for a demo, see this video from NDC Oslo 2014.

Custom elements

Custom elements provide many of the benefits of the emerging Web Components spec, but cleanly integrated with KO’s new components feature described above. Plus, they’re ready to use in production today – compatible all the way back to IE6!

See: custom elements documentation

Pure computeds

ko.computed has always been the powerhouse behind KO, underpinning just about all of its features. It makes your code reactive without you having to declare any dependencies explicitly.

ko.pureComputed is a new specialisation of ko.computed with better performance and memory management in many cases. It automatically pauses/resumes evalution, and automatically releases/reclaims references to dependencies, according to whether any other part of your app is watching it at the time. This can be extremely valuable for large single-page applications, where you may be tearing down and rebuilding different parts of your UI over long periods of time, and need to ensure your memory/CPU use is as streamlined as possible.

See: pure computed documentation

The textInput binding

This new binding transparently resolves numerious browser event quirks related to text entry. You might not believe how many different types of events are raised (or worse: not raised) by different browsers when a user types, cuts, pastes, drags, or autocompletes text in an <input> or <textarea>. Or if you’ve been a web developer for a while, you probably would believe it, and have the battle scars to show.

Well, textInput is something of an expert on the subject, so hopefully you don’t have to be.

See: textInput documentation


Of course, there are smaller bugfixes in there too. You can find the exact set of changes on GitHub.

Hopefully these enhancements will prove valuable for your projects. I’ve been using most of them for a while and have certainly found them useful. Big thanks to everyone who’s participated in implementing and discussing these changes!

Architecting large Single Page Applications with Knockout.js

Last week in Oslo I was lucky enough to attend the fantastic NDC 2014 conference, at which I gave the following talk about using brand-new not-yet-released Knockout.js features, along with Yeoman, Gulp, and Karma to build large single-page web apps in a very modern style (while still retaining backward compatibility all the way to IE 6!).

This should be of interest to anyone who uses Knockout or cares about scalable front-end architecture.


  • 00:00 Benefits and challenges of single-page apps; relevance of Knockout.js
  • 11:20 Demo: Knockout 3.2′s components and custom elements
  • 15:40 Demo: Using Yeoman to generate (“scaffold”) a scalable application architecture
  • 22:20 Demo: Scaffolding and building a new Knockout component
  • 30:50 Understanding Grunt and Gulp
  • 34:00 Demo: Using Gulp and require.js to produce control both preloading and incremental loading
  • 41:50 Testing – your options
  • 43:50 Demo: Adding Jasmine and Karma to the technology stack
  • 59:10 Summary: Overcoming the challenges of single-page app development

Watch the video

Architecting large Single Page Applications with Knockout.js

Want more? I also gave a talk about WebGL and GPU coding for the browser.

Knockout Projections – a plugin for efficient observable array transformations

knockout-projections is a new Knockout.js plugin that adds efficient “map” and “filter” features to observable arrays. This means you can:

  • Write to get a new, read-only array-valued observable containing the mapped version of each input item.
  • Write myObservableArray.filter(someFilterFunction) to get a new, read-only array-valued observable containing a subset of the input items.

When the underlying myObservableArray changes, or whenever any observable accessed during the mapping/filtering changes, the output array will be recomputed efficiently, meaning that the map/filter callbacks will only be invoked for the affected items.

The point of all this: it can scale up to maintain live transforms of large arrays, with only a fixed cost (not O(N)) to propagate typical changes through the graph of dependencies.

Trivial mapping example

To illustrate the mechanics in an obvious way, consider this underlying array:

var numbers = ko.observableArray([1, 2, 3]);

Now if you’ve referenced knockout-projections, you can write:

var squares = { return x*x; });

Initially, squares will contain [1, 4, 9]. It’s observable, so you can use squares.subscribe to get notifications when it mutates, or you can bind it to a DOM element (e.g., foreach: squares) in KO’s usual way.

Now if you transform the underlying array:


… then squares updates (to [1, 4, 9, 64]) and only calls your mapping function for the new item, 8.

Any transformation is permitted, e.g.:


This has the effect of reversing squares (to [64, 9, 4, 1]), again without remapping anything.

If you remove item(s):

numbers.splice(1, 1);

… then squares is updated (here, to [64, 1]) without remapping anything.

In summary, is an efficient, observable equivalent to Also, it can be arbitrarily chained (and combined in chains with filter) to produce graphs of transformations.

Trivial filtering example

The filter feature works exactly as you’d expect, given the above. For example,

var evenSquares = squares.filter(function(x) { return x % 2 === 0; });

Initially, evenSquares will contain just [64]. When you mutate the underlying array,

numbers.push(4); // evenSquares now contains [64, 16]
numbers.push(5); // evenSquares doesn't change
numbers.push(6); // evenSquares now contains [64, 16, 36]

Again, it responds to arbitrary transformations:

numbers.sort();  // evenSquares now contains [16, 36, 64]

A more realistic use case

Typically you won’t just be playing around will small collections of numbers. Most Knockout apps work with collections of model objects – sometimes very large collections.

Here’s a simple model object:

function Product(data) { =; = ko.observable(;
    this.price = ko.observable(data.price);
    this.isSelected = ko.observable(false);

Many KO applications involve fetching a large collection of model objects and exposing that from a viewmodel:

function PageViewModel() {
    // Some data, perhaps loaded via an Ajax call
    this.products = ko.observableArray([
        new Product({ id: 1, name: 'Klein Burger', price: 3.99 }),
        new Product({ id: 2, name: 'Mobius Fries', price: 1.75 }),
        new Product({ id: 3, name: 'Uncountable Chicken Chunks', price: 3.59 }),
        new Product({ id: 4, name: 'Mandelbrot Salad', price: 2.40 }),
        ... etc ...
ko.applyBindings(new PageViewModel());

Now this might be bound to the UI:

<ul data-bind="foreach: products">
        <input type="checkbox" data-bind="checked: isSelected" />
        <strong data-bind="text: name"></strong>
        (Price: £<span data-bind="text: price().toFixed(2)"></span>)

This is all fine, and Knockout is already good at efficiently (i.e., incrementally) updating the UI when the products array changes. But what if you want to track which subset of products is “selected”?

The traditional approach would be something like:

this.selectedProducts = ko.computed(function() {
    return this.products().filter(function(product) {
        return product.isSelected();
}, this);

This works (using Array.prototype.filter, not the Knockout-projections filter function). However, it’s inefficient. Every time the products array changes, and every time any of their isSelected properties changes, it re-evaluates the isSelected property of every product. It has to do so, because it has no built-in understanding of what you’re doing, so it can’t be clever and incremetally update the earlier selectedProducts array.

However, if you use Knockout-projections filter function, e.g.:

this.selectedProducts = this.products.filter(function(product) {
    return product.isSelected();

… it’s both syntactically cleaner, and way faster for large arrays: it now updates the selectedProducts array incrementally whenever either products changes or any of the isSelected values changes.

Similarly you might want to output the names of the selected items. So, you could chain on a new selectedNames property, and perhaps furthermore chain on selectionSummary:

this.selectedNames = {
this.selectionSummary = ko.computed(function() {
    return this.selectedNames().join(', ')  || 'Nothing';
}, this);

Now when products changes, or when an isSelected or name property changes, the effect will propagate out incrementally through the whole dependency graph with the minimum of callback-invoking, meaning a snappy user experience even with large data sets and low-end mobile devices.

Licence and origin

knockout-projections is open source under the Apache 2.0 license. It was built as a tiny component in a very large project I’m working on at Microsoft, and thanks to my boss (and his boss, and probably multiple people up the chain) we’ve decided to open-source it. Hopefully my team will be producing plentiful nuggets of OSS goodness for you as we go about our work :)

Just a clarification for the avoidance of any confusion, Knockout.js itself remains MIT licensed and is run by the KO community and the KO core team members (which includes me personally).