binding viewmodel to existence of a property in knockout

I am using Knockout.js to populate a set of HTML5 <details> elements. Here is the structure:

<div class="items" data-bind="foreach: Playlists">
    <details class="playlist-details" data-bind="attr: {id: 'playlist-details-' + $index()}">
        <summary>
            <span data-bind="text: name"></span> - <span data-bind="text: count"></span> item(s)
            <div class="pull-right">
                <button data-bind="click: $parent.play, css: {disabled: count() == 0}, attr: {title: playbtn_title}" class="btn"><i class="icon-play"></i> Play</button>
                <button data-bind="click: $parent.deleteList" class="btn btn-danger"><i class="icon-trash"></i> Delete</button>
            </div>
        </summary>
        <div class="list" data-bind="with: items" style="padding-top: 2px;">
            ...
        </div>
    </details>
</div>

The data in the ViewModel looks something like this:

var VM = {
    Playlists: [
        {
            name: "My Playlist1",
            count: 3,
            items: [<LIST OF SONG ID'S>],
            playbtn_title: "Play this playlist"
        },
        {
            name: "My Playlist2",
            count: 5,
            items: [<LIST OF SONG ID'S>],
            playbtn_title: "Play this playlist"
        },
        {
            name: "My Playlist3",
            count: 0,
            items: [],
            playbtn_title: "You need to add items to this list before you can play it!"
        }
    ]
};

I want to add the ability to remember the open or closed state of the details view. I have implemented this behavior previously using jQuery and localStorage¹, but for this project I want to use Knockout natively instead of using jQuery.

I have added an isOpen property to the playlists in the ViewModel which is retrieved from localStorage when the page loads. However, it seems that I can’t use the attr binding in Knockout because the HTML5 spec says to look only for the presence or absence of the open attribute, not for a value.

How would I get Knockout to add and remove the open property of the <details> element as the isOpen property of the ViewModel changes?

---

¹: Like this:

// On the initial page load.
contents += '<details ' + ((localStorage['tl_open_playlist-details-' + counter] == 1) ? 'open' : '') ' class="playlist-details" id="playlist-details-' + counter + '" data-name="' + escape(listname) + '">'

...

// Update storage when things are clicked.
$(document).on('DOMSubtreeModified', 'details.playlist-details', function() {
    if ($(this).prop('open')) {
        localStorage['tl_open_' + this.id] = 1;
    } else {
        delete localStorage['tl_open_' + this.id];
    }
});

Answer

You can modify the attr binding to take into account another binding option (named attrRemoveWhenFalse here) and remove the attribute for you:

<input class='testInput' type="text" 
       data-bind="attr: { disabled: isDisabled }, attrRemoveWhenFalse: true" />

var originalAttr = { init: ko.bindingHandlers.attr.init, 
                     update: ko.bindingHandlers.attr.update }
ko.bindingHandlers.attr.update = function (element, valueAccessor, 
                                           allBindingAccessor, viewModel, 
                                           bindingContext) {
    if (typeof originalAttr.update === 'function') 
        originalAttr.update(element, valueAccessor, allBindingAccessor, 
                            viewModel, bindingContext);
    if (allBindingAccessor().attrRemoveWhenFalse) {
        for (var prop in valueAccessor()) {
            if (!ko.utils.unwrapObservable(valueAccessor()[prop])) {
                element.removeAttribute(prop);
            }
        }
    }
}

Demo