constructor, super, and data members issue

Isiah Meadows isiahmeadows at
Sun Aug 26 17:35:27 UTC 2018

Every object, including functions, have an internal prototype. Functions
normally have one set to `Function.prototype`, and objects normally inherit
from `Object.prototype` at least indirectly. But because of how prototypes
work, the only requirement for something to be used as a prototype is that
it must be an object. So you can do `Object.create(someFunction)` and
although you can't call it (it's not a callable object), that object
inherits all the properties and methods from that function. `class` in
JavaScript is just sugar over a common pattern (really complex sugar
requiring `` to emulate, but still sugar), not an entirely new
concept, and it all builds off of prototypes. Specifically, the instance
prototype inherits from the parent prototype, and the class constructor
itself inherits from the parent constructor. That's why if you declare a
static `call` method on a parent class, you can still access and use it in
the subclass.
On Sat, Aug 25, 2018 at 19:58 Ben Wiley <therealbenwiley at> wrote:

> How can they be prototypically inherited if they don't live on the
> prototype? I feel like I'm missing something.
> Le sam. 25 août 2018 19 h 53, Isiah Meadows <isiahmeadows at> a
> écrit :
>> Class fields are prototypically inherited just like via `Object create`.
>> This is more useful than you might think, and it's the main reason anyone
>> actually cares about static fields beyond namespacing.
>> On Sat, Aug 25, 2018 at 14:36 Ben Wiley <therealbenwiley at>
>> wrote:
>>> All this just reminds me of *my opinion* that class fields is a borrowed
>>> concept from statically typed languages that is misplaced in a dynamically
>>> typed languages like JavaScript.
>>> In C++ I use class fields to declare what properties will be allocated
>>> and instantiated when a new class member is constructed.
>>> In the ES proposal for class fields we mimic this type of behavior by
>>> instantiating properties on the object when it's constructed, but there's
>>> no runtime guarantee that this set of properties will remain the same.
>>> There's no reason not to put this in the constructor, and although
>>> putting class fields on the prototype is debatably not the best idea, it
>>> would be the only scenario where we get some kind of new helpful behavior
>>> out of it.
>>> Ben
>>> Le sam. 25 août 2018 14 h 25, Augusto Moura <augusto.borgesm at>
>>> a écrit :
>>>> 24-08-2018 19:29, Aaron Gray <aaronngray.lists at>:
>>>> >
>>>> > Yeah it does look like its badly "broken by design".
>>>> >
>>>> Why this behaviour is broken? Every OOP language that I worked with
>>>> behaves de same way, and there's not many developers complaining about
>>>> it. If you want to use a property that might be overrided in a
>>>> subclasss you need to use a method and make the source of the data
>>>> more versatile (in Java and others similiar languages we have to
>>>> implement it using getter methods). Luckily Javascript doesn't need
>>>> getter and setters methods to make a property overridable because of
>>>> getter and setters descriptors, so we can workaround the first example
>>>> easily:
>>>> ``` js
>>>> class Bar {
>>>>   bar = 'in bar';
>>>>   constructor() {
>>>>     console.log(
>>>>   }
>>>> }
>>>> class Foo extends Bar {
>>>>   _initiedSuper = false;
>>>>   _bar = 'in foo';
>>>>   constructor() {
>>>>     super();
>>>>     this._initiedSuper = true;
>>>>   }
>>>>   get bar() {
>>>>     return this._bar;
>>>>   }
>>>>   set bar(val) {
>>>>     if (this._initiedSuper) {
>>>>       this._bar = val;
>>>>     }
>>>>   }
>>>> }
>>>> new Foo(); // will log 'in foo'
>>>> ```
>>>> *I have to say the relaying that the super constructor will use the
>>>> bar property and workarounding it **is a bad practice** and should be
>>>> avoided at any costs. The contract with the super class constructor
>>>> should rely only on the super call, these situations just reveal bad
>>>> design choices in the super class. Logan Smyth example is the correct
>>>> answer to this problem*
>>>> 25-08-2018 01:28, Jordan Harband <ljharb at>:
>>>> >
>>>> > Personally I think a design where the superclass relies on any part
>>>> of the
>>>> > subclass is "broken by design"; but certainly there's ways you can
>>>> achieve
>>>> > that.
>>>> >
>>>> Of course is not broken. The super class has a contract with a
>>>> parametrized option, it can be used in subclasses or just in a
>>>> constructor call `new Base({ idAttribute: 'foo' })`, if it has a
>>>> default value for that is not a sub class concern. When refactoring
>>>> code adding defaults and "lifting" parameters are very common ~not
>>>> only on OOP~ and relying that the super class is using some property
>>>> in the constructor is the real "broken by design".
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