Just Enough To Be Dangerous

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AngularJS-ish filters in KnockoutJS

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As I learn more about AnuglarJS, I find filters to be one of the most exciting features of the framework. Filters allow common, mostly-simple display-level modifications to expressions in a view without having to include that logic in the model (i.e. $scope).

Applying a filter to an expression in an Angular view is super easy: {{ expression | filter }} and you’re done. Angular comes with a handful of filters for scalar values (number, currency, date) and arrays (filter, limit, order) but it’s also pretty easy to write your own.

In addition to “write once, use everywhere” convenience, filters are also chainable. This is particularly useful with Angular’s array filters: {{ customerList | filter:searchText | orderBy:'earnings' }} works just like it reads (only show customers with a property value containing the value of the model property searchText and then order those customers by the earnings property). And of course it live-updates when you change searchText.

OK, so Angular is awesome. But what if I’ve already done a bunch of stuff using another library (KnockoutJS in my case)? Can I do something like a filter in Knockout? Hint: yes.

Knockout has two ways that this could work: “fn” functions and extenders.

How fn works

Each of Knockout’s subscribable types exposes a public fn object to which you can add new functions to extend that type with cool new stuff. From then on, that stuff will be available on any new instance of that type. You can add new functions to any of the following:

  • ko.subscribable.fn
  • ko.observable.fn
  • ko.observableArray.fn
  • ko.computed.fn

And because of inheritance, functions that you add to a type will be usable by objects of any of its derived types. We’ll see why this is great for filters in a second.

Filters with fn

So how do we turn this into something like filters? Take a look at this fiddle.

You can see that I’ve added 4 functions to ko.subscribable.fn:

  • filter to filter an array by a string value
  • orderBy to order an array by an element property value
  • currency to format a number as currency
  • date to format a date

Why ko.subscribable.fn?

Why not put filter and orderBy on ko.observableArray.fn and put currency and date on ko.observable.fn? Simple: adding these functions to ko.subscribable maximizes chainability.

Let me explain. First, you want to return a ko.computed from filter functions. This ensures automatic UI updates because Knockout tracks subscribables accessed in the function body of a ko.computed and automatically re-evaluates the function when any of those values changes. But a ko.computed is not an observable or an observableArray. It is a ko.subscribable, though. So adding these functions to ko.subscribable.fn allows the inclusion of clear, chainable filters in data bindings like this one: data-bind="foreach: dudes.filter(search, 'name').orderBy(orderBy)".

It even makes sense to put the scalar (currency and date) filters on ko.subscribable.fn in case you ever add a filter (or filters) that whittles an array to a single value (i.e. first, last, findOne, imFeelingLucky, etc.).

By putting filter functions on ko.subscribable.fn, they are available to all subscribable types, giving you lots and lots of flexibility.

How extenders work

Extenders and fn are really similar. To create and use an extender, throw in some code like this:

ko.extenders.awesomeSauce = function (target, options) {
  //target is the subscribable object you are extending
  // add stuff to it and you can use it in your viewModel and bindings

  //options is the object provided to the initialization of the extender
  // use it to control what to add or how those additions behave
  return target; //if you don't return target, you might lose track of it

foo = ko.observable().extend({ awesomeSauce: { thing1: true, thing2: false }});

Ultimately, there’s little difference between fn and extenders as far as the end result is concerned. They both allow you to extend a subscribable. These are really the only differences:

  • You have to (or “can be”) be more explicit when you want to add extenders to a subscribable by calling .extend(). No automatic inclusion as with fn.
  • You can (or “have to”) provide options when you add an extender. This allows you to be more precise about what exactly your subscribable is extended with or how that stuff behaves.

Filters with extenders

In the fiddle above, instead of putting all the filter functions I might want to use into the same bag, they are separated by extender name.

ko.extenders.arrayExtensions is used to extend an observableArray with the filter functions filter and orderBy. These functions work exactly the same as in the fn example. However, in order to make the arrayExtensions methods chainable, I have to return a subscribable extended with arrayExtensions. This could be easy to forget to do when writing your own filter function. Also, this approach to chainability is not a frugal use of memory since two new functions are added to the target each time you call extend({ arrayExtensions: true }).

ko.extenders.formatting is used to extend an observable with filter functions that are appropriate to its data type (either "date" or "currency", provided as the extender option). Due to their simplicity, I did not make these filter functions chainable. I suppose they could be, but I couldn’t think of a good reason to do that here.

To reiterate, in order to take advantage of these filter functions, I have to call .extend() for my amount, date, and dudes observables.

Discussion and Gotchas

You can see that there is no difference between the fn and the extender approach in how the data-bind attributes in the HTML consume our filter functions. That’s pretty cool.

<input type="text" data-bind="value: search, valueUpdate: 'afterkeydown'" />
<select data-bind="value: orderBy">...</select>

<table class="table">
    <tbody data-bind="foreach: dudes.filter(search, 'name').orderBy(orderBy)">
            <td data-bind="text: name"></td>
            <td data-bind="text: amount.currency()"></td>
            <td data-bind="text: date.date('dddd MMMM Do YYYY')"></td>

While it’s convenient for all functions you add to fn to be available to all subsequent instances of that type, ALL functions you add will be available to instances of that type. In the examples above, I added all my functions to ko.subscribable.fn to maximize chainability, but that small set of functions is already mutually-exclusive. filter and orderBy only make sense for observable arrays, currency only makes sense for numbers, and date only makes sense for dates. They are all there whether they make sense to be or not. Fortunately this is not a big concern for performance or scalability because each fn function object is shared by every subscribable.

Also, with the fn approach, putting everything in the same bucket is nice for availability, but if you start defining lots of fn functions in multiple files and possibly loading different subsets of those files on each page, you need to be pretty good at keeping those things straight, using unique function names, and keeping function names meaningful as to which date type they should be used for.

Chainability is powerful, but it can lead to long filter chains. In life, as in Knockout, you don’t get money for nothing and chicks for free. Because we have used ko.computed at each step of the way, a change at a point in the filter chain will cause re-evaluation of the remainder of the chain. If you have 10 filters chained together and you’re wondering why your page is sluggish, look there first.

I mentioned this above, but I wanted to reiterate it. Using the extender approach can be costly for memory consumption. Each time you .extend() a subscribable, new functions are added to the target. If you end up extending hundreds or thousands of objects on your page, you might start feeling the pain.

You may have noticed that arrayExtensions does not use its options parameter so it does not matter what the value is, but in order to call .extend() with valid JSON, a value must be provided. For no particular reason, I chose to provide true.


I hope this was a useful demonstration of the power of fn and extender functions in KnockoutJS. In summary, if you are looking for simplicity and ease of use with respect to filtering, I’d say go with fn functions. If you need more precision but perhaps don’t care as much about convenience, try extenders.

Enjoy, and let me know what you think!