And it feels like an odd move in that class based OOP isn't the zeitgeist it once was. Shoehorning it in feels like a throw back to 90's and 00's.
One way to look at it is that you could consider not teaching the underlying prototypal inheritance model at all. If students will rarely run into code that relies on it, it may be vestigial semantics.
Think of it like how people used to teach patterns in C for doing object-oriented programming. For example, you could have a struct whose first field was a pointer to a table of functions for the operations you could perform on the object. In other words, how to implement your own vtables from scratch.
When C++ came along, it was still useful to understand vtables sometimes, but it wasn't something every programmer needed to fully internalize. C++ let them program at a higher level of abstraction.
> And it feels like an odd move in that class based OOP isn't the zeitgeist it once was.
Citation needed. :) Two of the newest, fastest growing languages are class-based: Kotlin and Swift. I'm not aware of any new prototype-based language that has noticeable growth.
Prototypes are a neat idea, but I think the market has pretty clearly shown that classes are a better practical approach to OOP.
In particular, I still run into usage of `call` and `apply` on a regular basis.
class Foo {
constructor () { this.foo = 0; }
log () { return this.foo; }
}
(new Foo()).log.call({ foo: 2 });
Or, much worse:
class Bar { constructor () { return new Foo(); } }
However, I would say there's some difference in severity between a surprising behavior that results from the intersection between two features, and a surprising behavior that results because the way a feature is described is literally inaccurate.
You can break Javascript classes without using any other feature of the language. You don't have to resort to something like Proxies, or overriding native prototypes. This is because it's not that other parts of Javascript provide escape hatches to get around behavior, it's that the class syntax itself does not cover the entirety of the prototype system it is trying to obscure.
It's not ideal to have any breakage in an abstraction, but some breaks are worse than others. To me, edge cases like this:
let foo = function () { this.x = 5; }
class Bar extends foo { //No error? But why?
constructor () { super(); }
}
foo = function () { this.x = 7; }
console.log((new Bar()).x); // 5, not 7 or undefined.
To understand what that means, you need to understand what `this` is in Javascript -- that classes don't actually bind methods to an instance. So we have this idea of methods from other languages that turn out to be kind of the wrong way of thinking about things.
From `this`, the obvious next question is, "so what's the difference between adding a method to a class and just attaching it to an object?" And it turns out that practically, the difference is very small -- and in fact it's very easy for us to remove or add methods to a class that look and act very similarly to the methods in a class descriptor. So what we're calling a "class" is no longer a descriptor or a set of laws about what an instance is and what it can do, it's just a blueprint of how to build an instance at runtime.
We haven't gotten rid of classes yet, but we're already working with something that feels foreign to a number of programmers coming out of traditional OOP languages. `this` means that what a traditional OOP programmer from Java thinks of as a "method" doesn't exist. It means that the context of a function is determined at runtime, not at compile time.
You don't need to use the word "prototype" to talk about that, but it's what I would call very prototype-ish behavior. Arguably, by the time someone understands `this`, they've already learned about half of what they need to know to grok prototypes anyway -- all that's left is to tell them that if a property isn't found on an object, there's a special property JS will traverse to look for it elsewhere. But sure, we can delay talking about that for right now.
> A constructor can override its return value and return something else besides the new instance of its class.
Constructors complicate things more. We already know from above that a class isn't really a descriptor of what an object can do, instead we're thinking about it like a blueprint that tells us how to create an object at runtime (this is also incorrect, but we're sticking with that definition because we don't want to use the word "prototype"). Now we find out we can override the return value of a constructor. The obvious question a good student will ask is, "why? What use-case is there for treating a constructor function like a normal function?"
Again, we don't have to jump to prototype from here, but we do kind of need to explain that the `new` keyword is actually doing most of the work when we build a class, and the reason we can do weird things with a constructor is because a constructor is just another function. But we'll avoid prototype by saying that `new` just makes an instance of a class, and that behavior can be overridden by returning another object from the constructor that `new` invokes.
And when the student figures out that they can call `new` on an anonymous function without returning anything and it still creates an object, we can delay talking about prototype even farther, because we'll just lie and say that pure functions are a shorthand for making classes that are only a constructor.
But increasingly the question becomes, why wouldn't we talk about prototype now? Ostensibly, the point of classes is that it hides all the weirdness of the prototype system. In my experience, people struggle with prototypes because they're not Java. But JS classes also aren't Java, or at least they stop being Java as soon as you do anything even mildly interesting with them. So if people who learn classes in depth still have to grapple with the fact that class definitions are constructed at runtime, and that instance methods can be modified after they're created, and that methods aren't inherently bound to instances at all, what have we gained?
I just think people really over-emphasize the difference between "class-based" and "prototype-based" languages as if they're completely different in how users code with them. In my experience, the primary interesting consequences of JS's prototype-basedness vs class-based languages is that you can call methods with arbitrary `this` values, monkey-patch classes at runtime, or even change the class of an object at runtime. In other languages, these kinds of features would be called reflection, they would be in a late chapter labeled "advanced features", and people wouldn't worry about teaching them to students before they had a good handle on the rest of the language.
I think prototype inheritance is often more useful than class inheritance; I do not agree that classes are necessarily better. Prototype inheritance can be used for more kind of things (especially with Object.create and Object.getPrototypeOf; I sometimes use these).
In the mobile space, Swift, Java, and now Kotlin are all class based.
“The zeitgeist” is different than “what companies are paying real money for”.
Besides, even with OOP based inheritance, the language is still “lying” just as much. Most developers who don’t understand how the lowest level of computers work don’t understand that when you call a function it’s always basically passing in a this pointer to one static instance of the function.
Object orientation as in "There must be classes, there must be inheritance, each method and instance value must be able to be private, public, or protected, and have official constructors and destructors" and probably a couple of other things I'm missing, is also alive and well and not going anywhere any time soon, but is no longer the only definition of Object Orientation.
If you were not around at the time, you'll have to take my word for this, although the echos still reverberate in Javascript tutorials to this day (because of the way they talk about prototype-based inheritance as if you've got to be placated long enough about the fact that it isn't "true" OO long enough to learn what it actually is), but when it was released, Javascript (prior to any sort of "class" keyword) was not considered an "object oriented" language, because it didn't have "real" inheritance, or private values, etc. etc.
The point is that in a world where the definition of "OO" has expanded enough to encompass the original JS, it's a bit odd to bodge on a particular definition of OO so late in the process.
(People also argued back then about whether Python was object oriented (!), because it didn't have "true" support for private instances or methods. By contrast, I find it amusing when sometimes the question comes up "Is Go really Object Oriented if it has no inheritance?" and the answer generally amounts to "Honestly, who cares?")
But personally I prefer prototypal inheritance and the everything is an object mentality.
What does class mean?
It’s all just syntactic sugar in any language. Before the class keyword, it was just more explicit in JS.
A class is a function in JS, a class is a separate type in most other languages. I don’t see a meaningful difference.
In class oriented languages like C# and Java, sure. JS with ES modules has the alternative of using functions exported directly from a module.
The latter approach has the benefit of better tree shaking than the class based equivalent.
class a {}
typeof a == 'function'
Objects in JavaScript are not class based. You can read here about the diferences:
As that link says, objects are not fundamentally class-based because objects don't have to have a class. But you could add classless objects to almost any language without making their classes fake. And when it talks about inheritance, well, you can inherit state in C++ too, it's called 'static'.
Or in shorter form: It's not class-based but it does have classes.
You can implement anything in any general purpose language. But that doesn't mean the language has that concept.
It's pretty normal for runtimes to not know about certain language concepts. The C runtime has never heard of switches or while loops, no big deal. If it's in the compiler it's part of the language.
And it doesn't matter what typeof says. I could take a C++ program, replace every instance of the word 'class' with 'struct', and it would work fine. It would still be using classes, even though typeof says 'struct'.
Typeof in javascript calls the constructor a function and it calls objects objects. That's correct and fine. You can't put a class itself into a variable in C++ anyway.
And if you really want to get into the gritty details, it's possible to make a very similar argument that C++ of all languages doesn't have classes. Sure they exist in the source code, but all that pretty syntax gets thrown away and you end up with a naked object or one that just has a pointer to a struct it inherits from. And there's not even a 1:1 relationship between source-code 'classes' and those structs!
Yes, but prototype is not fixed. Prototype chain is not fixed (you can extend it at any point). All of that is changeable at runtime. You can also change the object's prototype at runtime.
It's all dynamic, and prototype ("class" as you say) doesn't provide any structure/constraints to the object. Thus it's pretty pointless to call it a class. It's just property sharing via a prototype chain. It's nothing like classes in languages like C++, Java, PHP, etc. And it doesn't help anyone understand the language, if you try to see it through that paradigm.