I would like to get to know if there are classes in JavaScript ECMAScript 5? I read some literature and there are different opinions about it. I know that it is possible to emulate classes, but I am not sure if they are called classes. Do you maybe also have some proofs?
Are these classes simply pseudo classes or what is the correct term for it?
JavaScript/ECMAScript is a prototype-based programming language.
Wikipedia’s definition of this is:
Prototype-based programming is a style of object-oriented programming in which behaviour reuse (known as inheritance) is performed via a process of cloning existing objects that serve as prototypes. This model can also be known as prototypal, prototype-oriented, classless, or instance-based programming.
Well, know it is your decision whether to call them classes.
But some people in the JS world call them classes and with ECMAScript 6 there will be a keyword to create such prototypes, called class. (Which implies that these prototypes should be called class.)
Since you are writing a paper, you could take a definition of class (there are a few out there), mention which you used and whether that is applicable in JavaScript. (If that somehow belongs to your work.)
Some definitions of class are also summarized in a great book by John C. Mitchell. (Mitchell, John: Concepts in Programming Languages. Cambridge University Press, 2003.)
On classes in Simula he writes: (p. 326)
Class: A Simula class is a procedure that returns a pointer to its activation record. The body of a class may initialize the object it creates.
On classes in Smalltalk he writes: (p. 327)
Class: A Smalltalk class defines class variables, class methods, and the instance methods that are shared by all objects of the class. At run time, the class data structure contains pointers to an instance variable temple, a method dictionary, and the superclass.
He also states that the concept of classes in Java and C++ is very similar to the concept in Smalltalk, which was a OO pioneer.
We can see that class can be defined differently, but both definitions above don’t seem to quite match JavaScript prototypes.
"class" is a term for many things. The classes you are looking for are a concept, and are certainly used in JavaScript. We implement the concept by reification, which is easily possible with JavaScripts powerful, object-oriented prototype inheritance. This model is quite different from the object model used in other programming languages, where classes are more a compiler artifact than concrete objects.
That the concept is implemented by means of generic language features allows for multiple slightly different implementations (some of which even don't rely on prototypes). So when the term "class" is used, we don't know how exactly it would look like. Only if we refer to a specific class with an implementation, it's clear what structure is meant (assuming your familiar with the kind of implementation).
There is however a quite standard pattern (implementation) with a constructor function that is invoked with new to create instances, and a prototype object that is used for sharing methods. The pattern also defines how to setup inheritance and subclassing.
You're right that there is no syntactical class structure in ES5 (though the keyword is reserved, and implemented in ES6 by the standard structure).
From MDN:
Classes are syntax sugar over constructor functions
You can use this new construction in ES5 by using some compiler:
Babel
Traceur
JSX
TypeScript
Related
Please fill in this gap of knowledge that I have:
So as of ECMA5, Object.create() is the standard method of creating objects because the value that is passed in, is the what the created object's prototype will point to
VS
using the 'new' constructor which will make the created object's prototype point to the prototype of the constructed object. Also using new runs the constructor code in the function.
Now with the ECMA6, and the introduction of Classes, which I know is just syntactical sugar, is the standard still to use Object.create()? Im assuming Classes are just syntactical sugar for function constructors, or equivalent to using the 'new' method to create objects.
And if this is the case, why do frameworks such as React use Classes, if Object.create is more efficient?
ES6 classes are not syntactic sugar, for ES5 they are transpiled into traditional prototype based constructs, but in native ES6 support it works far more efficient.
We use classes in React to get used to this syntax, as in next few years this will be fully supported native mechanism.
For object creation itself right now you can use Object.create and new. but also object literals.
The method of having closured object factories is right now one of 'clean-code-recommended' - benefits of better control of coupling, local('private') variables and methods.
As you said, classes just are syntactic sugar for constructors. They don't change the picture at all. Sometimes you will want to use new, sometimes you will want to use Object.create, they just do different things:
Also using new runs the constructor code in the function
And React needs this, so they use constructors.
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prototype based vs. class based inheritance
This question came up at work the other day - what's the difference between a class-based language like Python, and a prototype-based language like Javascript? Aside from differing approches, both ways seem very similar and we struggled to find something that a class-based language could do that a prototype-based language couldn't, or vice-versa.
Can anybody elaborate or go into any detail on how they differ fundamentally?
I haven't found much online about the differences, just sites that show you how to accomplish one with the other (such as this: Simulating classes with prototypes in JavaScript)
Any enlightenment appreciated!
Check out this article. It is a detailed article discussing the differences between class-based and prototype-based languages.
Copy of the table summarizing the differences:
Class-based (Java)
Class and instance are distinct entities.
Define a class with a class definition; instantiate a class with constructor methods.
Create a single object with the new operator.
Construct an object hierarchy by using class definitions to define subclasses of existing classes.
Inherit properties by following the class chain.
Class definition specifies all properties of all instances of a class. Cannot add properties dynamically at run time.
Prototype-based (JavaScript)
All objects are instances.
Define and create a set of objects with constructor functions.
Same.
Construct an object hierarchy by assigning an object as the prototype associated with a constructor function.
Inherit properties by following the prototype chain.
Constructor function or prototype specifies an initial set of properties. Can add or remove properties dynamically to individual objects or to the entire set of objects.
It seems like you're familiar with the actual languages, so you know what the difference is, right? I guess you're asking about the differences at a deeper, maybe more "philosophical", level.
Class-based languages tend to work from the top down, general to particular. The classic example would be where you define a 'Vehicle' class, and then subclasses like 'Car', 'Train'.
A prototype-based language would instead tend to start with the particular, in fact start with an instance of the particular and modify that.
I like this: http://steve-yegge.blogspot.ie/2008/10/universal-design-pattern.html
In the end it's not a question of if you can do inheritance in JS or whether there is something that you can do in one language but not the other. It's a deep difference in their ways of approaching problem solving. For a particular problem a good idiomatic solution that made best use of the language's features would probably be quite different in a prototype-based language from one in a class-based language.
The JavaScript guide of MDN has some good points, take a look: https://developer.mozilla.org/en/JavaScript/Guide/Details_of_the_Object_Model
Well, they're both (usually) Turning Complete. :)
Classes are about enforcing encapsulation, at the expense of some (or a lot of) run-time extensibility. Prototypes encourage encapsulation and allow almost unlimited run-time extension.
Seriously, prototype-based OO makes it very easy -- some would say dangerously easy -- to dynamically extend/derive from an object. Prototype languages therefore do not coexist easily with static typing. So if you like how compilers catch type errors, you will almost certainly be excluding prototype-based languages.
There's actually a very good discussion here: http://yehudakatz.com/2011/08/12/understanding-prototypes-in-javascript/ although there are others which are just as good I liked that one.
I just finished Doug Crockford's The Good Parts, and he offers three different ways to go about inheritance: emulation of the classical model, prototype-based inheritance, and functional inheritance.
In the latter, he creates a function, a factory of sorts, which spits out objects augmented with desired methods that build upon other objects; something along the lines of:
var dog = function(params) {
// animal is the 'super class' created
// the same way as the dog, and defines some
// common methods
var that = animal(params);
that.sound = 'bark';
that.name = function () {};
return that;
}
Since all objects created this way will have a reference to the same functions, the memory footprint will be much lower than using when using the new operator, for instance. The question is, would the prototype approach offer any advantages in this case? In other words, are object prototypes somehow 'closer to the metal' that provide performance advantages, or are they just a convenience mechanism?
EDIT: I'll simplify the question. Prototypes vs. their emulation through object composition. So long as you don't require all object instances to get updated with new methods which is a convenience offered only by prototypes, are there any advantages to using prototypes in the first place?
I emailed Doug Crockford, and he had this to say:
[Using the functional approach above vs. prototypes] isn't that much memory. If you have a huge number of objects times a huge number of methods, then you might want to go prototypal. But memories are abundant these days, and only an extreme application is going to notice it.
Prototypes can use less memory, but can have slightly slower retrieval, particularly if the chains are very long. But generally, this is not noticeable.
The instances don't all reference the same functions etc. Each call to "dog" will create a new "name" function instance.
There are many opinions about this, and Crockford's isn't necessarily the right one.
The main disadvantage to modifying the prototypes is that it may make it harder to work with other javascript libraries.
But the disadvantage of Crockford's functional way of creating classes is that you can't add a method or field to all instances of a type as easily.
See Closure: The Definitive Guide for some critical comments on Crockford's view about classes and inheritance in javascript:
http://my.safaribooksonline.com/9781449381882/I_sect1_d1e29990#X2ludGVybmFsX0ZsYXNoUmVhZGVyP3htbGlkPTk3ODE0NDkzODE4ODIvNTE0
Libraries like Dojo, Google Closure Library (which seems to copy Dojo's style), and perhaps YUI have their own class system that seems to be a good middle ground. I like Dojo's system probably the best because it has support for Mixins, unlike Closure. Some other class systems not tied to gui toolkits include Joose, JS.Class, and JavascriptMVC (check out last one esp. if using jquery).
I once created a GUI that was backed by a class that didn't have great event-based support for the information it needed. The GUI implemented a generic interface where an instance of that class is passed in, so I cannot inherit from that class, so I did something similar to prototype-based inheritance, I created a proxy object that wraps the instance of that class when it's passed to my GUI. The proxy intercepts methods of interest that mutate state, and report them as events. The entire GUI then revolved around using these events.
In this case, it would have just been annoying to create a separate class that wraps the passed in instance and reproduces the same interface. This would actually have lead to more overhead (not that it matters) due to creating all the redundant functions.
There was simply no other viable alternative. The only alternatives were:
Create an object that composes the object passed to the GUI (like I mentioned above). This object would then reproduce every function in the original class to implement the same interface, and add events, so it's effectively just the same as the proxy object way.
Have the GUI track state mutation in its passed in object. This would have been a pain and bug prone.
I know both languages are from the same ECMA-262 standard. It seems that the two are becoming very similar with JavaScript adding event listeners for core Object instances through methods like freeze and seal in EMCAScript-262 5th edition and such. I was wondering what the differences are?
First of all ActionScript 3 and JavaScript are both defined in ECMA-262 so they have a lot in common. Both languages feature prototype inheritance for instance. It is however not correct that ActionScript fully implements ES4.
ActionScript implements a couple of features that are not defined in ECMA-262 and some -- but definitely not all -- of ES4.
So what does AS3 add to ECMA-262? Those are also the differences to JavaScript:
Dynamically and statically typed code
Packages, Classes and Interfaces
Standard OO inheritance model (not prototype based, statically typed)
uint and int datatype
E4X (ECMA-357)
Type-safe conditional compilation (ES4)
Vector.<T> datatype (ES4)
Maybe I have forgotten some features. I am not sure if XML, XMLList etc. are already defined in 262 or came with 357.
The key difference however is the standard library. JavaScript comes with a couple of predefined classes like DOMElement and browser dependent additions. ActionScript has a fairly large standard library with features like video streaming and is consistent over all platforms.
I've been programming in both ActionScript and Javascript, and from a less-technical standpoint, I see two main differences.
1) JavaScript is more powerful. You are allowed to do much more with the language because it does not have a "compiler" or types. There are some great frameworks out there like ExtJS and jQuery that try and simplify things for you, but even with them, you are really allowed to do an amazing amount of damage if you want to.
2) ActionScript is much more confining and hence, much easier to maintain. Adobe did a lot of work to keep you out of the difficult parts of ECMAScript. ECMAScript Objects, prototypal inheritance, and closures are three concepts that you really don't need to understand to program in ActionScript. You just need to understand how to use Adobe's "Class" object.
For simple uses, I prefer JavaScript. However, once the project gets large, it depends who you are coding for. If I had a team of 5 developers programming at a scrappy start-up, I'd choose JavaScript in a heartbeat. However, in the girth of a large corporation, or academia, you might be safer relying on Adobe's platform.
Hope that helps.
One is type Safetly. Actionscript requires that you set a type for all objects, and JavaScript doesn't (for that matter, in JavaScript, one variable may be one type and then immediately set to another type).
Actionscript is object oriented. Although you can sort of have this in JavaScript, Actionscript allows for object inheritance, etc.
Essentially the main difference I find is that ActionScript is more a verbose statically-typed class-based language where as javascript is a prototypal language.
Unfortunately there is no type-inference in ActionScript so using Flex Builder gives a warning every time you leave something untyped which I find unnecessary and overly verbose, not only does it make it more verbose than javascript but I find equivalent code to be more verbose than C#.
However the extra verbosity does have yield perf improvements and extra type safety at compile-time. Unfortunately this also adds to build time quite significantly, in Java Script apps of any size I'm used to instant feedback whereas my last ActionScript project had build time exceeding 2 minutes.
From a developer point of view, what matter most:
1) Javascript is not really OOP, it has NO super keyword, which means if you override( by any means ) something, you can't call it through super, and this is the deal breaker for complex programs for which OOP is the key, and Actionscript3 is all OOP, you can have millions line of Actionscript3 code working together, and well maintained.
2) Actionscript3 runs in Flash Player which has only one implementation from Adobe, this means it's consistent all the time, all browsers( as long as installed Flash Player), but Javascript runs in browsers directly, but each browser has its own implementation, which means your Javascript code has to be tested against all targeted browsers to ensure working.
The key differences are that ActionScript 3 supports both class-based inheritance and prototypal inheritance, enforces namespace bindings between class names and file names, and does not support some global JavaScript methods such as eval. Fortunately, you can do several things to bridge the gap.
You can globally set the namespace using ES for ECMAScript or AS3 for ActionScript 3:
use namespace ES;
use namespace AS3;
If you are using the AS3 namespace, any method override must use the AS3 namespace
and the override attribute.
If you are not using the AS3 namespace, you can use the prototype methods and propertyIsEnumerable.
You can selectively use the AS3 namespace version of a property or method in a dynamic function:
var nums:Array = new Array(1, 2, 3);
nums.AS3::pop();
trace(nums); // output: 1,2
To turn off class based inheritance, you can also use the following compiler options:
compc -as3=false -strict=false -es=true
import *
class foo
{
dynamic function foo()
{
}
}
If you do not use the AS3 namespace, an instance of a core class inherits
the properties and methods defined on the prototype object.
If you decide to use the AS3 namespace, an instance of a core class inherits the
properties and methods defined in the class definition.
Here is a common features between ECMAScript-4 and ECMAScript-2017 or later:
Feature ES4/ES6+ ES4 Only
Rest parameter ☑
Destructuring ☑
ByteArrays ☑
Class ☑
Interface ☑
Static fields ☑
Parameter default ☑
Rest Parameters ☑
Bound methods ☑
dynamic this value ☑
multiple catch clauses ☑
short-circuit-and (&&=) ☑
short-circuit-or (||=) ☑
Type Annotations ☑
References
Simulating AS3 language features in JavaScript using AMD and ES5
JavaScript 2 Draft
ECMAScript 4th Edition[proposed]:Predefined Types and Objects
The New Browser War
ECMAScript 1-on-1
The Trouble with JavaScript: JavaScript has no Class
What's New in ECMAScript-4
David Flanagan on JavaScript 2
JavaScript 2 and the Future of the Web
Writing Classes
Faster byte array operations with ASC2
Managing event listeners
frankly it's not the same, cuz action script is loaded with EMQJ24, the new language for high development website. while JS still with it EMCA22, the difference between those are the style and format of the code. and also action script are ages enough, that's why most of programmer nowdays using CSX01 updated language from cSS,it can recognize all type off language without any line.
One of the major advantages with Javascript is said to be that it is a prototype based language.
But what does it mean that Javascript is prototype based, and why is that an advantage?
Prototypal inheritance is a form of object-oriented code reuse. Javascript is one of the only [mainstream] object-oriented languages to use prototypal inheritance. Almost all other object-oriented languages are classical.
In classical inheritance, the programmer writes a class, which defines an object. Multiple objects can be instantiated from the same class, so you have code in one place which describes several objects in your program. Classes can then be organized into a hierarchy, furthering code reuse. More general code is stored in a higher-level class, from which lower level classes inherit. This means that an object is sharing code with other objects of the same class, as well as with its parent classes.
In the prototypal inheritance form, objects inherit directly from other objects. All of the business about classes goes away. If you want an object, you just write an object. But code reuse is still a valuable thing, so objects are allowed to be linked together in a hierarchy. In javascript, every object has a secret link to the object which created it, forming a chain. When an object is asked for a property that it does not have, its parent object will be asked... continually up the chain until the property is found or until the root object is reached.
Each function in JavaScript (which are objects themselves) actually has a member called "prototype", which is responsible for providing values when an object is asked for them. Having this member allows the constructor mechanism (by which objects are constructed from functions) to work. Adding a property to the prototype of a function object will make it available to the constructed object, as well as to all of the objects which inherit from it.
Advantages
There may not be a hard and fast rule as to why prototypal inheritance is an advantageous form of code-reuse. Code reuse itself is advantageous, and prototypal inheritance is a sensible way of going about it. You might argue that prototypal inheritance is a fairly simple model of code reuse, and that code can be heavily reused in direct ways. But classical languages are certainly able to accomplish this as well.
Sidenote: #Andrew Hedges makes a good point, that there are actually many prototypal languages. It's worth noting that these others exist, but also worth noting that none of them are anything close to mainstream. NewtonScript seemed to have some traction for a while, but died with its platform. It's also possible to extend some modern languages in ways which add prototypal capabilities.
A prototype-based language, does not make the distinction of classes vs objects: it simply has objects. A prototype-based language has the notion of a prototypical object, an object used as a template from which to get the initial properties for a new object. Any object can specify its own properties, either when you create it or at run time. In addition, any object can be associated as the prototype for another object, allowing the second object to share the first object's properties.
Prototype-based programming is a style of object-oriented programming where classes are not present, and behavior reuse (or inheritance in class-based languages) is performed by cloning existing objects that serve as prototypes.
The advantage/disadvantage is that, we can create new kinds of objects at run time without need for defining classes (static code). Like most features it is upto the developer to turn it to a advantage/disadvantage.
Above is possible because objects are essentially functions in java script (closures too).
Instead of declaring a class structure, you can create objects of the same type, and add to their definition any time you like using the object's prototype.
It's more flexible than the normal way of doing things.
After reading all the answers this is the conclusion
1) Inheritance in which objects are inherited directly from other objects
2) That does not use classes
3) Also called instance based programming or classless prototype oriented programming
4) Behaviour reuse is performed by cloning existing objects that serve as prototypes
5) Object used as template from the new object get initial properties
If you just use objects at runtime instead of a class at compile to build new objects, this opens up the possibility of extending an object without knowing any details about it. Of course, it may become a disadvantage pretty quickly depending on usage. I make no assumptions about the language here, so it is applicable to languages other than javascript which are not as dynamic.
myobject.prototype=unkownobject;
myobject.newproperty=1;
You may get the object from just about anywhere; your own code, from the network, from the database, from external linkage and so on.
Note that, a language don't have to implement prototype inheritance like javascript. In javascript, a prototype object is merely shared, so is its properties, among the inheritors. The alternative is copying over all the properties of the prototype to the new object. Each approach has its strengths in different situations. I like the second more but it isn't what javascript does.
Memory conservation is one of the benefits of prototypal inheritance in JS. In a language like Java, objects generate their own copies of the superclass' instance variables and methods, while in JS, the "super"-object offers get-access to its variables and methods to each "sub"-object that inherits from it without the need to recreate them.