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Prototypes Are Not Classes

raganwald.com

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Re: Prototypes Are Not Classes

#41

Post by roel_v »

Jare wrote: post #31

I've never heard anyone, experienced or newbie, say "JavaScript has classes". People at best will say "You can sort of make classes in JavaScript using objects". To me a Class has always been "a family of objects with the same structure and behaviour, and a way to create them". Stuff like hiding, constructors, type checking, polymorphism and inheritance are interesting, but secondary details. But then, I was taught O…

Your first and second paragraph seem contradictory to me. Javascript, through what is called 'prototypes', has "a family of objects with the same structure and behaviour, and a way to create them", and varies only in implementation and semantic details from other languages. So I agree with your second paragraph and would say that JS does have classes, but then I'm not sure what your first paragraph means.

Re: Prototypes Are Not Classes

#42

Post by roel_v »

angersock wrote: post #9
masklinn wrote: post #8

Earlier quoted context omitted.

I'm not sure that's a good idea, because then they'd be made very sad by constructors, by prototypes being something completely different[0], by not having mixins[1], by single-inheritance, by dynamic inheritance[2], by the lack of blocks (and non-local returns), etc… Essentially, by javascript not being a very good Self. [0] IIRC in Self the prototype is the object you copy, objects linked through parent slots are e…

MI still sends shivers down my spine, having spent time in the C++. Just say no.

OK, I'll bite. Please tell me what exactly your problem was.

Re: Prototypes Are Not Classes

#43

Post by Jare »

roel_v wrote: post #41
Jare wrote: post #31

I've never heard anyone, experienced or newbie, say "JavaScript has classes". People at best will say "You can sort of make classes in JavaScript using objects". To me a Class has always been "a family of objects with the same structure and behaviour, and a way to create them". Stuff like hiding, constructors, type checking, polymorphism and inheritance are interesting, but secondary details. But then, I was taught O…

Your first and second paragraph seem contradictory to me. Javascript, through what is called 'prototypes', has "a family of objects with the same structure and behaviour, and a way to create them", and varies only in implementation and semantic details from other languages. So I agree with your second paragraph and would say that JS does have classes, but then I'm not sure what your first paragraph means.

You can use objects and prototypes to build classes, but they are not classes by themselves. Similar to how you can use objects to create a linked list, but the language itself does not have linked lists.

Re: Prototypes Are Not Classes

#44

Post by raganwald »

egil wrote: post #37

Yet another shallow article describing the difference between prototypes and classes. Does anybody know of more in depth ones that also discusses multi-leven inheritance, polymorphism etc?

polymorphism is actually orthogonal to the ideas around metaobjects like classes. They are both powerful ideas. but can be expressed in different ways. Languages like Common Lisp and Haskell allow you to express polymorphism entirely through overloading of functions, for example.

Re: Prototypes Are Not Classes

#45

Post by xmonkee »

pygy_ wrote: post #32
raganwald wrote: post #28

Earlier quoted context omitted.

This is the best feedback so far.

How about this for an abbridged version? "You can add methods to any objects (no metaclass required), and any object can be inherited form, becoming the prototype of its descendants. In other words, any object can perform the job of a class in a class-based language."

How about this:

"In a class based model, skeletal templates (classes) serve as object factories, with something general (the class) serving as the starting point of something specific (an object).

In a prototype based model, the prototype object provides a fully fleshed out starting point for other objects, with something specific (the prototype) serving as the starting point of something even more specific (the inheriting object)"

Re: Prototypes Are Not Classes

#46

Post by CrossEye »

I think there is a serious problem in vocabulary. Wikipedia's definition of class ([1]) is quite reasonable, which, slightly paraphrased, reads, "A class is an extensible template for creating objects, providing initial values for state and implementations of behavior." While typically we don't use prototype objects for default state in Javascript, we always can if we choose, and they are widely used for providing behavior. And both "extensible" and "template" fit well. So by this definitions, prototypes really are classes.

But almost everyone who comes to Javascript from a class-based language brings a number of additional preconceptions. I don't know the Ruby community well enough to know if your descriptions are accurate, but I know that many of those coming from Java or C# backgrounds expect classes to be (1) distinct creatures from objects (and those who know much about the reflection class Class understand that the objects of that class are objects and not the classes themselves), (2) design/development/compile-time abstractions separate from the run-time realities, (3) unchanging at runtime (which is obviously different from Ruby), and (4) the enforcers of access policies of instance methods. None of these things are true for Javascript prototypes. So to these people, it's easiest to explain that Javascript does not have classes.

I have never implemented a serious programming language. But I understand from those who have that implementing classes and implementing prototypes can be very, very similar processes. So, while I won't stop insisting to those from other languages that Javascript doens't have classes, I do recognize that I'm talking more to their preconceptions and not to the deepest truth I know.

  [1]: http://en.wikipedia.org/wiki/Class_(computer_programming)
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