safaribook-knockout-essentials/ko-cart/js/custom/koBindings.js

ko.bindingHandlers.toggle = {
  init: function (element, valueAccessor) {
    var value = valueAccessor();
    ko.applyBindingsToNode(element, {
      click: function () {
        value(!value());
      },
    });
  },
};

ko.bindingHandlers.currency = {
  symbol: ko.observable("$"),
  update: function (element, valueAccessor, allBindingsAccessor) {
    return ko.bindingHandlers.text.update(element, function () {
      var value = +(ko.unwrap(valueAccessor()) || 0);
      var symbol = ko.unwrap(
        allBindingsAccessor().symbol !== undefined
          ? allBindingsAccessor().symbol
          : ko.bindingHandlers.currency.symbol
      );
      return symbol + value.toFixed(2).replace(/(\d)(?=(\d{3})+\.)/g, "$1,");
    });
  },
};