1、由于在原型中查找值的过程是一次搜索,因此我们对原型对象所做的任何修改都能够立即从实例上反映出来---即便是先创建了实例后修改原型也是如此。代码如下:
function Person(){
}
var friend=new Person();
Person.prototype.sayHello=function(){
alert("Hello");
}
friend.sayHello(); //输出:Hello
上面这段代码先创建了Person的一个实例,并将其保存在friend中。接着下一个代码块在Person.prototype(Person的原型属性对对象)中添加了一个方法sayHello();
根据输出,我们发现即使friend实例是在添加新方法之前创建的，但它任然可以访问这个新方法。
其原因可以归结为实例与原型之间的松散连接关系，当我们调用friend.sayHello()时,首先会在实例中搜索名为sayHello的属性方法,如果实例属性中没有,则会继续去搜索原型。而原型与实例之间是通过[[prototype]]指针相连接的(实例通过[[prototype]]指针指向原型属性对象来调用原型中的方法和属性),正因为[[prototype]]是一个指针,并不是Person的副本,所以即使修改(添加)原型的方法在创建完实例之后,该实例任然可以访问完成修改之后的原型对象!
2、尽管利用上面所说的实例与连接的松散连接关系,我们可以随时为原型添加属性和方法，并且修改能够在所有的实例中反映出来。
但是如果重写整个原型对象，那么情况就不一样了,在前面的随笔中曾说过,当调用构造函数创建一个对象实例时JS会为实例添加一个指向最初原型(也就是Person.prototype)的[[prototype]]指针,而像下面的代码中那样,把原型修改成另外一个对象，并不能改变实例中的[[prototype]]指针的值(说明白点,就是实例还是指向最初原型)；
注意:实例中的[[prototype]]仅指向最初原型;
function Person(){
}
var friend=new Person();
Person.prototype={
constructor:Person,
name:"张三",
age:22,
sayHello:function(){
alert("Hello James");
}
};
friend.sayHello(); //error 控制台报错!
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seYphe/vnSxJWnuTrvo4ejMvykg/ikqF0UvGv1cBDr5LjPKbxdvn4u6nwTDzodVKezYj0pahcF71o9HMQ6Oe5zCm+X/FLU1OcUSFGnk6J2UfCu1cNBrJPjPqfwdskuRU1+KYwUdTopahcF71o9HMQ6Oe5zCm+XjFLU6Hvk+6So00lRuyh41+rhINbJcZ9TeLs0sJNfdtqfbevHCt61ejiIdXLc52gXiKXg0argXauHg1gnx32OdoFYCh6tTruQCPzZ2eNHUNoFYjFaAeTCeA2xSFEAuTBeQyxSFEAujNcQixQFkAvjNcQiRQHkwngNsUhRALkwXkMsUhRALozXEIsUBZAL4zXEIkUB5MJ4DbFIUQC5MF5DLFIUQC6M1xCLFAWQC+M1xCJFAeTCeA2xSFEAuTBeQyxSFEAujNcQixQFkAvjNcQiRQHkwngNsUhRALkwXkMsUhRALozXEIsUBZAL4zXEIkUB5MJ4DbFIUQC5MF5DLFIUQC6M1xCLFAWQC+M1xCJFAeTCeA2xSFEAuTBeQyxSFEAujNcQixQFkAvjNcQiRQHkwngNsUhRALkwXkMsUhRALozXEIsUBZAL4zXEIkUB5MJ4DbFIUQC5KHy8bmAnv+y0P9sWAH9R+HhtQmIXUhQAqcLHaxMSu5CiAEgVPl6bkNiFFAVAqvDx2oR079pL0zRNc2m79M3bdeaNP27vettzlQFIUQCkCh+vq5+QlmNS116kqFWkKABShY/XtU9IXXvZOyWt2aQUlcu2APiLwsfr2iekA+7YfSJF5bQtAP6i8PG65gnpdu1/2tGl7Z6Z6vZ4UOrSdmnK+v8Q1fBBqmcw6r05TEn9LV1bKSqjbQHwF4WP17VPSMOU9Ew7vYgzeH/wQ/+Rqa6Xu/6v5rWSiZ+kqGy2BcBfFD5e1z4hTaSoQcDpvd+1l+F772eqxg+h927apbcM3dHLaVsA/EXh43XtE1KaooaR5/1K/15ez/V2T4PR++eJyCRF5bQtAP6i8PG69gnpyxQ1E36kKCkKgAmFj9e1T0jrU9TSP+ebT1Ezq5SictkWAH9R+Hhd+4T0RYp63tN755+uvTx/WEhRj9C09AB7GaQoAFKFj9e1T0jfpKj76OGo5JMO7vfpnwcfdHBzRy+nbQHwF4WP1yYkdiFFAZAqfLw2IbELKQqAVOHjtQmJXUhRAKQKH69NSOxCigIgVfh4bUJiF1IUAKnCx2sTEruQogBIFT5eT32pCWzxy077s20B8BeFj9cmpAUaZz0pCoBU4eO1CWmBxllPigIgVfh4bUJaoHHWk6IASBU+XpuQFmic9aQoAFKFj9cmpAUaZz0pCoBU4eO1CWmBxllPigIgVfh4bUJaoHHWk6IASBU+XpuQFmic9aQoAFKFj9cmpAUaZz0pCoBU4eO1CWmBxllPigIgVfh43TTN/XZ9foXH9Xa/38c/VqBrL489vrRd/0ez9XpSFACpwsfrZ4p6xIeH0Y97ul2/y2bfLv8XXXvp7/czSv1m2wWQogBIFT5eS1H/SVF/IkUBkCp8vF5OUbdr01zarn+LqxdqHu/eHm++1tBfuLf48OXeBlcv/yymV/zrleU6x6ubzohS1J9IUQCkCh+vP6eoNCH9DyjPd/t55XYdvNC1l0FoSa4tfbX8OEa9312uc/iL4230ty1FbSZFAZAqfLxekaIGsadrL69XknfTq0WD5ZMU9e3yw6DTe3Oxzq69pO+lMSp0imoYOvuAALBK4eN1s+aOXv8Xei+N3x0moHT5cSr6dvlB0hmEnqU6R7cG/0setwqdouL7ZVs5LgC5KHy8zitFvaNOellqMUWteERdivoTKQqAVOHj9ecUlT7INJuxjr6jd39lnVHkWaxzYitTpKg/0VYApAqfG1akqFGqmXlo+36/J091J+lm8urVF8v/L+96vSQPZM3XOd5K114mrk1JUX+irarlSTVgQeGjw5o7ereZDxCYvsjz+ujzppl4+/+7w7tx3yz/fGniytNcnffRw1E+6eAA2gqAVOFzw9fPRZ1v4kGnPeqUov5EWwGQKnxuyC9FTX1OgRR1Om0FQKrwuaFZ/DbigClq8kth/linbyP+O20FQKrwuWF58guVorrRV830HFSnZLCetgIgVfjc0MBOzu7LAIRjbgAA2EKKAgDYQooCANhCigIA2EKKAgDYQooCANhCigIA2EKKAgDYQooCANhCigIA2EKKAgDYQooCANhCigIA2EKKAgDYQooCANhCigIA2EKKAgDYQooCANhCigIA2EKKAgDYQooCANhCigIA2EKKAgDYQooCANhCigIA2EKKAgDYQooCANhCigIA2OIf4sAaXKFjjXQAAAAASUVORK5CYII=" 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" alt="" />
从上面的图我们可以看出,重写原型对象之后,并没有是对象实例的[[prototype]]指针的值发生改变，它还是指向最初的原型对象。