Clarification: Javascript prototype update confusion

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 ?

"wouldn't it be better if can get updates from multiple objects just changing constructors prototype property" --- nope, it wouldn't be. — zerkms, Mar 01 '15 at 20:09
@zerkms, so once the object is created, it is tied to a particular prototype and will only receive updates from that prototype and nothing else. — coder7891, Mar 01 '15 at 20:15

score 0 · Answer 1 · answered Mar 01 '15 at 20:11

0

You do not assign B any specific prototype properties so the inheritance chain is broken.

answered Mar 01 '15 at 20:11

Jack Shultz

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Oriol · Accepted Answer · 2015-03-01T20:27:49.273

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.

Yeah, tested it. '__proto__' can be used to change the prototype of a object to something else. But it is changing the 'constructor' property. That's strange. — coder7891, Mar 01 '15 at 20:37

Clarification: Javascript prototype update confusion

2 Answers2