神马是单例模式？简单的讲，就是某个构造函数，有且仅有一个实例存在，无论你调用该构造函数来创建实例多少次，如：
function Class(){} //仅仅用来举例
var instance_01 = new Class();
var instance_02 = new Class();
console.log( instance_01===instance_02 ); //输出：true
我们知道，在javascript，当且仅当 instance_01 与 instance_02 指向的是 同一个实例对象，上述表达式才有可能输出为true，换言之，即：
如果instance_01 与 instance_02 指向的不是同一个实例对象，即使 instance_01 与 instance_02 拥有完全相同的属性值，上述表达式也会输出 false
可以做个小测试：
function Wife( name ){
this.name = name;
}
Wife.prototype.shout = function(){
console.log( 'i\'m your only wife '+ this.name +' , do remember !');
};
var wife_01 = new Wife( 'Marry' );
var wife_02 = new Wife( 'Mary' );
console.log( wife_01===wife_02 ); //尽管名字相同，但输出为：false，妻子竟然有两个，天~~
那么，最上面举的那个例子是怎么实现的呢？
最简单直观的思路：第一次调用该构造函数创建实例对象的时候，可以将该实例对象的引用保存下来，接下来，当该构造函数被再次调用，无论多少次，都只是返回之前保存的那个实例对象
还是用 妻子 那个例子：
function Wife( name ){
if( Wife.instance ){
return Wife.instance;
}
Wife.instance = this;
this.name = name;
}
Wife.prototype.shout = function(){
console.log( 'i\'m your only wife '+ this.name +' , do remember !');
};
var wife_01 = new Wife( 'Marry' );
wife_01.shout(); //输出：i'm your only wife Marry , do remember !
var wife_02 = new Wife( 'Lucy' );
wife_02.shout(); //输出：i'm your only wife Marry , do remember !
console.log( wife_01===wife_02 ); //输出：true
很好，现在已经符合一夫一妻制的要求了，但总觉得有些不对劲——如果有人将 Wife.instance 恶意修改了，那不久悲剧了
针对于此，打个小补丁：
var Wife = (function(){
var instance = null; //外部无法直接访问，于是安全了
var _wife = function( name ){
if( instance ){
return instance;
}
instance = this;
this.name = name;
}
return _wife;
})();
Wife.prototype.shout = function(){
console.log( 'i\'m your only wife '+ this.name +' , do remember !');
};
var wife_01 = new Wife( 'Marry' );
wife_01.shout(); //输出：i'm your only wife Marry , do remember !
var wife_02 = new Wife( 'Lucy' );
wife_02.shout(); //输出：i'm your only wife Marry , do remember !
console.log( wife_01===wife_02 ); //输出：true
OVER
