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Re: Differences between [[Class]] and constructor

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  Re: Differences between [[Class]] and constructor Andreas Bergmaier <andber93@web.de> - 2015-06-02 21:32 +0200
    Re: Differences between [[Class]] and constructor Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2015-06-02 23:05 +0200

#27963 — Re: Differences between [[Class]] and constructor

Stefan Ram schrieb:
>    I have observed that the class ([[Class]]) name and the
>    constructor name of an object often is the same!

This might be the case for plain objects with the native constructors.
It is not for almost every other object, like

     var p = new (function Person(){});
     className(p) // "object"
     p.constructor // Person

>    The first difference I found is: You can change the
>    constructor name, but not the class name.

Not exactly. You cannot change constructor names either, as the `.name` 
property of functions is typically non-writable (if it exists at all, it 
is non-standard, as opposed to [[Class]]). See 
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/name 
for more information.

What you can arbitrarily change is the `.constructor` property of any 
normal object, there are no limitiations here.

  Bergi

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#27968

Andreas Bergmaier wrote:

> Stefan Ram schrieb:
>>    I have observed that the class ([[Class]]) name and the
>>    constructor name of an object often is the same!
> 
> This might be the case for plain objects with the native constructors.
> It is not for almost every other object, like
> 
>      var p = new (function Person(){});
>      className(p) // "object"

No, with this code it would be a capital “O”.

>      p.constructor // Person
> 
>>    The first difference I found is: You can change the
>>    constructor name, but not the class name.
> 
> Not exactly. You cannot change constructor names either, as the `.name`
> property of functions is typically non-writable (if it exists at all, it
> is non-standard, as opposed to [[Class]]). See
> https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/name
> for more information.
> 
> What you can arbitrarily change is the `.constructor` property of any
> normal object, there are no limitiations here.

Whereas “normal object” would be one whose prototype is not constructed 
using (an emulation of) Object.create(…), as I pointed out in an update of 
my <http://stackoverflow.com/a/12593269/855543> today:

With a conforming implementation of Object.create(), following

  WeatherWidget.prototype =
    Object.create(Widget.prototype, {constructor: {value: WeatherWidget}});

the “constructor” property of WeatherWidget instances is neither writable, 
nor enumerable or configurable.  Of those, an emulation of Object.create() 
can only accomplish read-onlyness, see Function.prototype.extend() in 
JSX:object.js.

-- 
PointedEars
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