knockout.js MVVM: .extend keyword in several SPA template js files found

The implementation of knockout.js MVVM pattern uses the .extend - functionality e.g. in login.viewmodel.js on behalf of the knockout validation plugin. (knockout.validation.js). This plugin script also resides within the Visual Studio SPA template in Folder scripts.

Please refer to: https://github.com/Knockout-Contrib/Knockout-Validation

self.userName = ko.observable("").extend({ required: true });

unction LoginViewModel(app, dataModel) {
    // Private state
    var self = this,
        validationTriggered = ko.observable(false);

    // Private operations
    function initialize() {
        dataModel.getExternalLogins(dataModel.returnUrl, true /* generateState */)
            .done(function (data) {
                self.loadingExternalLogin(false);
                if (typeof (data) === "object") {
                    for (var i = 0; i < data.length; i++) {
                        self.externalLoginProviders.push(new ExternalLoginProviderViewModel(app, data[i]));
                    }
                } else {
                    self.errors.push("Unbekannter Fehler.");
                }
            }).fail(function () {
                self.loadingExternalLogin(false);
                self.errors.push("Unbekannter Fehler.");
            });
    }

    // Data
    self.userName = ko.observable("").extend({ required: true });
    self.password = ko.observable("").extend({ required: true });
    self.rememberMe = ko.observable(false);
    self.externalLoginProviders = ko.observableArray();
    self.validationErrors = ko.validation.group([self.userName, self.password]);

    // Other UI state
    self.errors = ko.observableArray();
    self.loadingExternalLogin = ko.observable(true);
    self.loggingIn = ko.observable(false);
...

Afte some CTRL-H search through the template's code it seems that there is no custom ko.extenders - instruction set found. According to the knockout.js tutorial it is possible to create custom extender functionality:

http://knockoutjs.com/documentation/extenders.html