Singleton模式

var User = (function() {

    var instance; 

    function _User(){}

    _User.prototype.say = function(){}

    function init() {

        return new _User()

    }

    return function() {

        if( instance == null) {

            instance = init();

        }

        return instance;

    }

})();

下面一种模式，可以根据执行的环境，来动态的创建不同的对象

var User = (function() {

    var instance; 

    function _Cat(){}

    _Cat.prototype.say = function(){}

    function _Dog(){}

    _Dog.prototype.say = function(){}    

    return function() {

        if( instance == null) {

            if( window.debug ) {

                instance = new _Cat();

            } else {

                instance = new _Dog();

            }

        }

        return instance;

    }

})();

// for use

User()