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It's not. In python, classes are entities like any other object. In JS, classes literally don't exist. Yes, even in ES6. I know there's a class keyword, but it

It's not. In python, classes are entities like any other object. In JS, classes literally don't exist. Yes, even in ES6. I know there's a class keyword, but it doesn't actually create a class. What the class keyword creates is a constructor function which then initializes new objects with a set of properties, and also sets a prototype, when called with the new keyword. JS objects don't have a superclass, they have a prototype. A prototype isn't a class: it's another object. When a method is called, or a variable is looked up on a JS object, JS scans up the prototype chain, all the way to Object.protype. But there's nothing special about these objects. If you wrote, say: var Person = function(){}; Person.prototype = {name:"bill", addr:"foo"}; var charlie = new Person(); charlie.name = "charlie"; charlie.addr = "baz"; var NewPerson = function(){}; NewPerson.prototype = charlie; var bill = new NewPerson(); This code is totally valid. bill's prototype chain looks like this: bill -> charlie (or NewPerson.prototype) -> Person.prototype -> ... -> Object.prototype Welcome to prototypical inheritance.

deathanatos 10y ago
> A prototype isn't a class: it's another object. When a method is called, or a variable is looked up on a JS object, JS scans up the prototype chain, all the way to Object.protype. A type in Python is also just another object. When a method is called, or a variable is looked up on an object, Python scans the MRO (effectively the same thing as the prototype chain, except it supports multiple inheritance too) all the way up to `object`. Python's type system is very similar, if not nearly a superset (though probably not exactly), of JavaScript's. I struggle to see the real material differences in them. Consider your example, translated: class Person: name = 'bill' addr = 'foo' class Charlie(Person): name = 'charlie' addr = 'baz' class NewPerson(Charlie): pass bill = NewPerson()
- qwertyuiop924 10y ago
  That looks the same, but conceptually (and actually, code-wise), that's not what JS is doing. Here's what's actually happening, in psudo-python: class Person({name:"bill", addr:"foo"}): #there's no syntax for object literals in Python #such a thing doesn't even make sense: so I'm improvising. pass charlie = Person() charlie.name = "charlie" charlie.addr = "baz" class NewPerson(charlie): #note charlie is an object, not a class pass bill = NewPerson() In JS, charlie isn't a class, charlie is an object: you can change all its properties and everything, just like any other object. It's not a class, because there aren't any.
  - OOPMan 10y ago
    I notice you conveniently ignored the meta-programming capabilities of Python which would allow your "nonsensical" improvisation to work just fine. Brush up on your Python skills before talking rubbish.
    - qwertyuiop924 10y ago
      Classless object literals actually don't make sense in the Python model. It seems like you're the one who needs to brush up. In any case you're not actually providing any links or facts, so unless I dig through long articles on metaclasses in python, I can't verify anything you say. Please prove your point, or at least provide I link so I can verify.
      - OOPMan 10y ago
        Challenge accepted :-) It should be interesting to see if I can actually abuse Python's meta-programming enough to make it work :-)
    - deleted 10y ago
      [deleted]
  - deathanatos 10y ago
    > In JS, charlie isn't a class, charlie is an object: you can change all its properties and everything, just like any other object. It's not a class, because there aren't any. Right; this is actually why I like Python's way a bit better: it makes a more formal distinction between the two. In Python, Charlie is also an object, and you can change all its properties and everything, just like most¹ other objects. Most of what you try to use to distinguish JS from Python applies just as well to Python. No, it isn't exactly the same, and I think the semantically equivalent code in Python will often contain more information than the JS. E.g., in my conversion of your JS, the class is an instance of `type`, whereas — as you correctly note — it's "just" an object in JS. It's not really though; in most JS that devs will write, the object is a type, but that distinction isn't explicit in the code. It's implicit in the dev's intentions, expressed only in the general sense that the code is being used. e.g., if I saw, in JS, var Person = function(){}; Person.prototype = {name:"bill", addr:"foo"}; var charlie = new Person(); while it is true, technically, that `Person` is a function and `Person.prototype` is an object, the semantic meaning being expressed in this manner by the author of this setup is that `Person` is a type, and that charlie is an instance of that type. It's just explicit in Python. (To some degree, JS is aware of this fact too; `charlie instanceof Person` is `true`, after all.) In a single-inheritance class in Python, during attribute lookup, Python will attempt to look the attribute up on the object's __dict__; if it doesn't exist, then it looks up the parent type (by looking at __class__) and then recurses, trying to look the attribute up there, and proceeding up the "chain".² In JS, during attribute lookup, JS will attempt to look the attribute up on the object's internal list of attribute (I don't believe this is exposed like __dict__ in Python); if it doesn't exist, then it looks up the parent type (by looking at __proto__) and then recurses, trying to look the attribute up there, and proceeding up the "chain". What material difference exists here? ¹objects in Python can forbid attributes from being set, or limit the available attributes. I'm not sure to what extend JS lets you do that. e.g., `object` in Python is very restrictive about allowing additional objects. ²I'm explicitly ignoring multiple inheritance here, so I'm vastly simplifying. Python will actually use the MRO for this procedure, but in the case of simple inheritance, that's equivalent to following __class__. I'm also ignoring __slots__ for simplicity.
- chriswarbo 10y ago
  > A type in Python is also just another object. Yes, but it's (roughly speaking) an instance of a "Class" class. In the "Person" example, the prototype of a "Person" object is another "Person" object (until the chain reaches an "Object", at least).
codedokode 10y ago
This code doesn't look good to me. What is the benefit of inheriting objects from objects instead of inheriting classes? When we have distinct functions, classes and objects we can easily understand how they are supposed to be used: you can call a function, create an instance of a class, call a method of an object. With Javascript you cannot easily see whether you have a plain function or a constructor and how you should use it. And interpreter won't tell you anything if you forget to put a `new` keyword. (And by the way why do you write 'var Person = function' instead of just 'function Person'? The latter is shorter and more readable) Your code can easily cause a bug. Imagine Person's constructor looks like this: function Person() { this.friends = []; } ... var NewPerson = function() {}; NewPerson.prototype = charlie; Do you see the problem here? To fix the problem you have to rewrite the code using Object.create() and call parent constructor with Person.call(...). I have written applications in Javascript and I think that Java-style classes are the best for readable and maintainable code. All these tricks with prototypes and dynamic objects (where you can add or patch properties in objects in runtime) might look good in short samples of code but they are only the source of bugs and cause waste of time to understand what the author meant in real world applications. Your code example proves that because adding a single field to a Person's constructor requires you to find and modify all of the objects inherited from it and this task is not easy too.
- chriswarbo 10y ago
  It sounds like you're trying to write Javascript as if it were Java. I've worked with developers who did this (e.g. http://chriswarbo.net/blog/2014-03-12-trolled.html http://chriswarbo.net/blog/2014-03-12-trolled.html ) and I would advise that you instead try to write more idiomatic Javascript, or else use some kind of Java-to-Javascript compiler. Your critique of Javascript is basically "it's not Java". I could make a similar critique of Java not being StandardML, but it would be about as meaningless: When we have distinct structures, signatures and functors we can easily understand how they are supposed to be used: you can open a structure, create an implementation of a signature, apply a functor to a structure. > (And by the way why do you write 'var Person = function' instead of just 'function Person'? The latter is shorter and more readable) Personally, I prefer "var Person = function Person(...) {...}" since it makes the code more symmetric (the second "Person" makes stack traces more informative). The "function Person(...) {...}" style makes an artificial dinstinction between functions and everything else. > All these tricks with prototypes and dynamic objects (where you can add or patch properties in objects in runtime) might look good in short samples of code but they are only the source of bugs and cause waste of time to understand what the author meant in real world applications. Javascript is a dynamic language; there is no ahead-of-time compilation. Things can only be done at runtime, including adding properties to objects. If you mean that mutating values should be avoided, then I would emphatically agree. That's certainly not a Javascript-specific issue though, and Java is just as riddled with mutability as anything else. I haven't had enough experience with prototypical inheritance to comment on it vs class-based inheritance; I've been burned enough by class-based inheritance that I tend to avoid inheritance in all forms (and OO too, these days).

deathanatos 10y ago

> A prototype isn't a class: it's another object. When a method is called, or a variable is looked up on a JS object, JS scans up the prototype chain, all the way to Object.protype. A type in Python is also just another object. When a method is called, or a variable is looked up on an object, Python scans the MRO (effectively the same thing as the prototype chain, except it supports multiple inheritance too) all the way up to `object`. Python's type system is very similar, if not nearly a superset (though probably not exactly), of JavaScript's. I struggle to see the real material differences in them. Consider your example, translated: class Person: name = 'bill' addr = 'foo' class Charlie(Person): name = 'charlie' addr = 'baz' class NewPerson(Charlie): pass bill = NewPerson()

qwertyuiop924 10y ago
That looks the same, but conceptually (and actually, code-wise), that's not what JS is doing. Here's what's actually happening, in psudo-python: class Person({name:"bill", addr:"foo"}): #there's no syntax for object literals in Python #such a thing doesn't even make sense: so I'm improvising. pass charlie = Person() charlie.name = "charlie" charlie.addr = "baz" class NewPerson(charlie): #note charlie is an object, not a class pass bill = NewPerson() In JS, charlie isn't a class, charlie is an object: you can change all its properties and everything, just like any other object. It's not a class, because there aren't any.
- OOPMan 10y ago
  I notice you conveniently ignored the meta-programming capabilities of Python which would allow your "nonsensical" improvisation to work just fine. Brush up on your Python skills before talking rubbish.
  - qwertyuiop924 10y ago
    Classless object literals actually don't make sense in the Python model. It seems like you're the one who needs to brush up. In any case you're not actually providing any links or facts, so unless I dig through long articles on metaclasses in python, I can't verify anything you say. Please prove your point, or at least provide I link so I can verify.
    - OOPMan 10y ago
      Challenge accepted :-) It should be interesting to see if I can actually abuse Python's meta-programming enough to make it work :-)
  - deleted 10y ago
    [deleted]
- deathanatos 10y ago
  > In JS, charlie isn't a class, charlie is an object: you can change all its properties and everything, just like any other object. It's not a class, because there aren't any. Right; this is actually why I like Python's way a bit better: it makes a more formal distinction between the two. In Python, Charlie is also an object, and you can change all its properties and everything, just like most¹ other objects. Most of what you try to use to distinguish JS from Python applies just as well to Python. No, it isn't exactly the same, and I think the semantically equivalent code in Python will often contain more information than the JS. E.g., in my conversion of your JS, the class is an instance of `type`, whereas — as you correctly note — it's "just" an object in JS. It's not really though; in most JS that devs will write, the object is a type, but that distinction isn't explicit in the code. It's implicit in the dev's intentions, expressed only in the general sense that the code is being used. e.g., if I saw, in JS, var Person = function(){}; Person.prototype = {name:"bill", addr:"foo"}; var charlie = new Person(); while it is true, technically, that `Person` is a function and `Person.prototype` is an object, the semantic meaning being expressed in this manner by the author of this setup is that `Person` is a type, and that charlie is an instance of that type. It's just explicit in Python. (To some degree, JS is aware of this fact too; `charlie instanceof Person` is `true`, after all.) In a single-inheritance class in Python, during attribute lookup, Python will attempt to look the attribute up on the object's __dict__; if it doesn't exist, then it looks up the parent type (by looking at __class__) and then recurses, trying to look the attribute up there, and proceeding up the "chain".² In JS, during attribute lookup, JS will attempt to look the attribute up on the object's internal list of attribute (I don't believe this is exposed like __dict__ in Python); if it doesn't exist, then it looks up the parent type (by looking at __proto__) and then recurses, trying to look the attribute up there, and proceeding up the "chain". What material difference exists here? ¹objects in Python can forbid attributes from being set, or limit the available attributes. I'm not sure to what extend JS lets you do that. e.g., `object` in Python is very restrictive about allowing additional objects. ²I'm explicitly ignoring multiple inheritance here, so I'm vastly simplifying. Python will actually use the MRO for this procedure, but in the case of simple inheritance, that's equivalent to following __class__. I'm also ignoring __slots__ for simplicity.
chriswarbo 10y ago
> A type in Python is also just another object. Yes, but it's (roughly speaking) an instance of a "Class" class. In the "Person" example, the prototype of a "Person" object is another "Person" object (until the chain reaches an "Object", at least).

codedokode 10y ago

This code doesn't look good to me. What is the benefit of inheriting objects from objects instead of inheriting classes? When we have distinct functions, classes and objects we can easily understand how they are supposed to be used: you can call a function, create an instance of a class, call a method of an object. With Javascript you cannot easily see whether you have a plain function or a constructor and how you should use it. And interpreter won't tell you anything if you forget to put a `new` keyword. (And by the way why do you write 'var Person = function' instead of just 'function Person'? The latter is shorter and more readable) Your code can easily cause a bug. Imagine Person's constructor looks like this: function Person() { this.friends = []; } ... var NewPerson = function() {}; NewPerson.prototype = charlie; Do you see the problem here? To fix the problem you have to rewrite the code using Object.create() and call parent constructor with Person.call(...). I have written applications in Javascript and I think that Java-style classes are the best for readable and maintainable code. All these tricks with prototypes and dynamic objects (where you can add or patch properties in objects in runtime) might look good in short samples of code but they are only the source of bugs and cause waste of time to understand what the author meant in real world applications. Your code example proves that because adding a single field to a Person's constructor requires you to find and modify all of the objects inherited from it and this task is not easy too.

chriswarbo 10y ago
It sounds like you're trying to write Javascript as if it were Java. I've worked with developers who did this (e.g. http://chriswarbo.net/blog/2014-03-12-trolled.html http://chriswarbo.net/blog/2014-03-12-trolled.html ) and I would advise that you instead try to write more idiomatic Javascript, or else use some kind of Java-to-Javascript compiler. Your critique of Javascript is basically "it's not Java". I could make a similar critique of Java not being StandardML, but it would be about as meaningless: When we have distinct structures, signatures and functors we can easily understand how they are supposed to be used: you can open a structure, create an implementation of a signature, apply a functor to a structure. > (And by the way why do you write 'var Person = function' instead of just 'function Person'? The latter is shorter and more readable) Personally, I prefer "var Person = function Person(...) {...}" since it makes the code more symmetric (the second "Person" makes stack traces more informative). The "function Person(...) {...}" style makes an artificial dinstinction between functions and everything else. > All these tricks with prototypes and dynamic objects (where you can add or patch properties in objects in runtime) might look good in short samples of code but they are only the source of bugs and cause waste of time to understand what the author meant in real world applications. Javascript is a dynamic language; there is no ahead-of-time compilation. Things can only be done at runtime, including adding properties to objects. If you mean that mutating values should be avoided, then I would emphatically agree. That's certainly not a Javascript-specific issue though, and Java is just as riddled with mutability as anything else. I haven't had enough experience with prototypical inheritance to comment on it vs class-based inheritance; I've been burned enough by class-based inheritance that I tend to avoid inheritance in all forms (and OO too, these days).