在构造函数内外为 prototype 属性赋值的区别

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function Person(){
    Person.prototype.talk=function(){alert("saying")};
}

function Men(){
}

Men.prototype = new Person();

var men = new Men();
men.talk();

可以执行成功，但

function Person(){
    Person.prototype.talk=function(){alert("saying")};
}

function Men(){
    Men.prototype = new Person();
}   

var men = new Men();
men.talk();

报错。请问在构造函数内部和外部为 prototype 赋值有何区别

赋值

function

men.talk

4 replies • 2015-04-19 11:36:20 +08:00

iyangyuan

Apr 18, 2015 via iPhone

因为第二个在new的时候，你还没有指定原型。

new相当于创建一个新对象，然后把这个新对象的原型指向函数(类)的prototype对象，第二个在构造方法里才指定原型，此时新对象的创建已经完成，它早已指向别处，所以报错。

在第二个例子中，如果再new一次，应该就不会报错了。

bombless

Apr 18, 2015

function Men(){
function ret() {}
ret.prototype = new Person();
ret = new ret;
Men = ret;
return ret
}
作弊可以吗，哈哈哈

xavierchow

Apr 18, 2015

请看https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/new
```
When the code new foo(...) is executed, the following things happen:

1. A new object is created, inheriting from foo.prototype.
2.The constructor function foo is called with the specified arguments and this bound to the newly created object. new foo is equivalent to new foo(), i.e. if no argument list is specified, foo is called without arguments.
3.The object returned by the constructor function becomes the result of the whole new expression. If the constructor function doesn't explicitly return an object, the object created in step 1 is used instead. (Normally constructors don't return a value, but they can choose to do so if they want to override the normal object creation process.)
```
第一步的时候，Men的prototype还不是Person对象，第二步才是执行constructor，而在你的代码中这时才改Men的prototype，晚了。

xavierchow

Apr 19, 2015

补充一点，对象初始化之后prototype是不可以改变，但是可以修改prototype的属性。
所以new之后再assign prototype是不行的，但是(new Person()).talk()是OK的。