NodeJS Singleton pattern

singleton.js

exports = module.exports = (function () {
  // Instance stores a reference to the Singleton
  var instance;
 
  function init() {
 
    // Singleton
    // Private methods and variables
 		
    return {
      // Public methods and variables
      extend: function extend(extension, obj){
        for (var key in extension){
          obj[key] = extension[key];
        }
      }
    };
  };
 
  return {
    // Get the Singleton instance if one exists
    // or create one if it doesn't
    getInstance: function () {
      if (!instance) {
        instance = init();
      }
      return instance;
    }
  };
})();

Thanks a lot!