a knockout binding handler for date formatting with momentjs

knockoutExtensions.js

            /*
             * read-only date display with momentjs
             * use like this: data-bind="moment: dateVar, format: 'YYYY-MM-DD'"
             * The "format" is optional and will default to "MM/DD/YYYY"
             */
            ko.bindingHandlers.moment = {
                update: function (element, valueAccessor, allBindingsAccessor, viewModel) {
                    var val = valueAccessor();

                    var formatted = '**INVALID**'; // throw instead?
                    var date = moment(ko.utils.unwrapObservable(val));

                    var format = allBindingsAccessor().format || 'MM/DD/YYYY';

                    if (date && date.isValid()) {
                        formatted = date.format(format);
                    }

                    element.innerText = formatted;
                }
            };

I haven't tried using MomentJS with knockout, but I like the idea.

It doesn't look like you're unwrapping the value, which means that this will fail if you are binding to an observable. See http://jsfiddle.net/tlarson/6Aggd/1/ for a failure example.

I've updated line 9 to unwrap the observable in a forked Gist:
https://gist.github.com/codethug/6423433

Here is an updated fiddle with my changes: http://jsfiddle.net/tlarson/6Aggd/2/

With this change, it will work regardless of whether dateVar is an observable or not.

If you're using KO 2.3, you can use the shorthand version:
var date = moment(ko.unwrap(val));

Thanks @codethug, I added your change and also added checking for valid date formats. For now, I just return "** INVALID **", but when I think about it more, I'll figure out what I need to do for better error handling.