subscribe knockout with old value

subscribe.js

ko.subscribable.fn.subscribeChanged = function( callback ) {
  var oldValue;
  return [
    this.subscribe( function( _oldValue ) {
      oldValue = _oldValue;
    }, this, 'beforeChange' ),
    this.subscribe( function( newValue ) {
      callback( newValue, oldValue );
    } )
  ];
};