extending knockout observables, to be easily resetted.

ko-default.js

// adding default value when initializing an observable
// for later resetting, preventing the observable from
// becoming undefined ( still you can set it to `null` )
// ====================================================
// example[1]: primitive types ( passed by value )
// -----------------------------------------------
// var username = ko.observable().default( 'guest' );
// username();              // 'guest'
// username( 'admin' );     // 'admin'
// username( undefined );   // 'guest'
// username( 'john doe' );  // 'john doe'
// username.reset();        // 'guest'
//
// example[2]: non-premitive types ( passed by reference )
// -------------------------------------------------------
// var userList = ko.observableArray([]).default(function() { return []; });
// var settings = ko.observable().default(function() { return { foo: 'bar' } });
// userList.reset();
// settings.reset();
ko.observable.prototype.reset = function() {
  this( typeof this._default === 'function' ? this._default() : this._default );
}

// * **param:** {object|factory}
ko.observable.prototype.default = function( value ) {
  if( typeof value === 'undefined' ) {
    return;
  }

  // store default value
  this._default = value;

  // observable defined with no arguments
  // use default
  if( typeof this() === 'undefined' ) {
    this.reset();
  }

  // whenever value changed to undefined, reset to default
  this.subscribe(function( newValue ) {
    typeof newValue === 'undefined' && this.reset();
  });
}