澄清：Javascript原型更新混乱

Question

Possible answer , but answer shows what's observed but doesn't explain why it happens that way.

Let's create three function constructors.

function A() {
}

function B() {
}

function C() {
}
C.prototype.nm = "C";
B.prototype = new A()
var obj = new B()
B.prototype = new C()
console.log(obj.nm); // prints, undefined.

So after the last line I was expecting 'obj' to receive properties from prototype of C but it's not. So does it mean that once the object is created it is tied to whatever prototype it was assigned during creation ? Why is it that way, I mean I can receive live updates to object through the prototype but wouldn't it be better if can get updates from multiple objects just changing constructors prototype property ?

Answer 1

您没有为B分配任何特定的原型属性，因此继承链断开了。

Answer 2

Does it mean that once the object is created it is tied to whatever prototype it was assigned during creation ?

Yes, exactly. Objects inherit properties from the object in their [[Prototype]] internal property.

That property is set when you create the instance:

13.2.2 [[Construct]]

• Let proto be the value of calling the [[Get]] internal property of F with argument "prototype" .
• If Type ( proto ) is Object, set the [[Prototype]] internal property of obj to proto .

But if you change the prototype property of a constructor, it won't affect previous instances.

In fact, prototype is not special by itself, it's just that [[Construct]] internal method uses it while creating instances.

once the object is created it is tied to whatever prototype it was assigned during creation ?

Mostly yes. You can change it, but is not recommended (because of performance issues), using:

• __proto__ : this was a non-standard property in Object.prototype which could be used as a getter or setter of [[Prototype]]. ECMAScript 6 standardized it in Annex B (Additional ECMAScript Features for Web Browsers).

• Object.setPrototypeOf , a new method introduced by ECMAScript 6.