Formatar moeda com Knockout JS

formatar.js

//baseado em http://jsfiddle.net/digitalbush/R6MPU/
jQuery(function ($) {
    function empenhoViewModel() {
        var self = this;
        self.valorTotal = ko.observable().dinheiro();
    };

    var format = function (value) {
        if (value === '' || isNaN(value)) value = 0;
        toks = Number(value).toFixed(2).replace('-', '').split('.');
        var display = 'R$ ' + $.map(toks[0].split('').reverse(), function (elm, i) {
            return [(i % 3 === 0 && i > 0 ? '.' : ''), elm];
        }).reverse().join('') + ',' + toks[1];

        return value < 0 ? '-' + display : display;
    };

    ko.subscribable.fn.dinheiro = function () {
        var target = this;

        var writeTarget = function (valor) {
            valor = String(valor).replace('.', '').replace(',', '.').replace(/[^0-9.-]/g, '');
            target(parseFloat(valor));
        };

        var result = ko.computed({
            read: function () {
                return target();
            },
            write: writeTarget
        });

        result.formatted = ko.computed({
            read: function () {
                return format(target());
            },
            write: writeTarget
        });

        result.isNegative = ko.computed(function () {
            return target() < 0;
        });

        return result;
    };

    var viewModel = new empenhoViewModel();
    ko.applyBindings(viewModel);
    viewModel.init();
});

index.html

<!-- imput em razor (ASP.NET MVC) -->
@Html.TextBoxFor(x => x.ValorTotal, new { @data_bind = "value: valorTotal.formatted" })

<!-- imput simples em html -->
<input data-bind="value: valorTotal.formatted" type="text" value="">

<span data-bind="text: valorTotal.formatted"></span>