Add support to Knockout for key.subkey bindings for attr, css, style, event, and more.
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Key.Subkey Binding plugin for Knockout

This plugin enables you to use a new syntax for Knockout’s built-in bindings attr, css, sytle, and event. This new syntax combines the name of the binding and the item you want to bind without having to use a sub-object.

Syntax examples

attr.href: url, attr.title: details someValue

css.profitWarning: currentProfit() < 0, css.majorHighlight: isSevere someValue

style.color: currentProfit() < 0 ? 'red' : 'black'

event.mouseover: enableDetails, event.mouseout: disableDetails

Advantages over the built-in syntax

This syntax is more concise because it eliminates the need for extra curly braces and quotes. For example, compare the following:

attr: { 'data-something': someValue } someValue

This syntax more clearly expresses that multiple bindings of the same type are separate. For example, compare:

attr: { href: url, title: details }

attr.href: url, attr.title: details

When combined with Knockout-Freedom, this syntax ensures that each of multiple attributes (etc.) is only updated when its own dependencies change. This is especially important when an update has side effects or involves some significant amount of work. For example, in the first case below, an update to myId will update both bindings and cause the iframe to reload (in some browsers); but in the second case, only an update to baseUrl will cause it to reload:

<iframe data-bind="attr: { src: baseUrl, id: myId }"></iframe>

<iframe data-bind="attr.src: baseUrl, myId"></iframe>

How this plugin works

To use this plugin, simply include knockout-key.subkey.js in your page after you’ve included Knockout. You can then start using the new syntax in your data bindings. If you have existing code that uses the original syntax, you do not have to change it, since this plugin doesn’t disable the old syntax.

Each time you use a new key.subkey binding in your view, this plugin dynamically creates a binding handler for it. It does this by wrapping two Knockout functions, ko.bindingProvider.instance.getBindings (used for most bindings) and ko.applyBindingsToNode (used for bindings within string-based templates such as jQuery-tmpl).

Using this syntax for custom bindings

If your binding follows the same pattern as the supported built-in bindings, it will work automatically with the new syntax. Here’s an example custom binding handler:

ko.bindingHandlers.dataAttr = {
    update: function(element, valueAccessor) {
        var value = ko.utils.unwrapObservable(valueAccessor()) || {};
        for (var subKey in value) {
            var dataValue = ko.utils.unwrapObservable(value[subKey]);
            if (!dataValue && dataValue !== 0)
                element.removeAttribute("data-" + subKey);
                element.setAttribute("data-" + subKey, dataValue.toString());

The binding could then be used like this:

<div data-bind=" 'one', dataAttr.two: 'two'"></div>

Alternatively, if your custom binding is only meant to work with the new syntax, you can use the new makeSubkeyHandler function to define how to handle the binding:

ko.bindingHandlers.dataAttr = {
    makeSubkeyHandler: function(baseKey, subKey, combinedKey) {
        return {
            update: function(element, valueAccessor) {
                var dataValue = ko.utils.unwrapObservable(valueAccessor());
                if (!dataValue && dataValue !== 0)
                    element.removeAttribute("data-" + subKey);
                    element.setAttribute("data-" + subKey, dataValue.toString());


This plugin exports ko.keySubkeyBinding.makeHandler, which can be used to manually generate a key.subkey binding handler. It both saves the handler in ko.bindingHandlers as key.subkey and returns the handler object. If the specified binding handler was already created before, makeHandler will create a new one, overwriting the old one. You can use this, for example, to create an alias for a binding: = ko.keySubkeyBinding.makeHandler('');

This plugin also exports ko.getBindingHandler with similar functionality to makeHandler. But getBindingHandler will simply return, rather than re-create, an existing binding handler.

As described in the section about custom bindings, when trying to dynamically create an x.y (for example) binding handler, this plugin will call ko.bindingHandlers.x.makeSubkeyHandler('x', 'y', 'x.y') if the function exists, which should return a binding handler object for that sub-key (with init and/or update functions).

License and Contact

License: MIT (

Michael Best