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Closures VS Classes in modern Javascript

For closures which main goal it's to create another functions, I was wondering if in modern javascript, it's better to just use classes in modern javascript.
// Closure way private counter
const countPlusOne = () => {
let count = 0;
return () =>{
count++;
console.log(count);
}
}
let demoAdd = countPlusOne();
demoAdd(); // 1
demoAdd(); // 2
demoAdd(); // 3
To be honest, I never liked the use of closures in that way (but I think they're great for things like middlewares) as are hard to read.
So, should I refactor closures like the one up, to classes? Their behavior seem more analogous to typical Objects from other languages.
// Class way private counter
class countPlusClass{
count = 0;
add(){
this.count++;
console.log(this.count)
}
}
const demo = new countPlusClass();
demo.add(); // 1
demo.add(); // 2
demo.add(); // 3

No, classes aren't always better. They're just different. I'd say the main differences are
the interface of the thing returned by the constructor/factory function. A class instance has properties and methods, and usually multiple of them, whereas a closure is simply a function that you can call - with a single functionality. The syntax to invoke them is different, the extra method name is sometimes superfluous and sometimes beneficial.
In OOP, objects are expected to have an identity and a state. In FP, functions are expected to be pure. Sure, you don't need to follow a specific paradigm, and stateless objects are fine as are impure functions (though maybe call them "procedures" then), but keep these conventions in mind when arguing about readability and maintainability.
So choose wisely. Do you (possibly in the future) need multiple methods? Do you encapsulate state? Then use a class to create objects. Do you only need a function to call? Then create a closure.

A closure has a significant advantage over a class the way you're imagining: with a class, if you use a public class field like you are, any code with access to the instance can modify its value. This is usually undesirable - scope should generally be constrained as possible and you don't want the correctness of your class to depend on consumers of the class not modifying it (whether accidentally or deliberately).
class countPlusClass{
count = 0;
add(){
this.count++;
console.log(this.count)
}
}
const demo = new countPlusClass();
demo.add(); // 1
demo.add(); // 2
// some code elsewhere in the codebase that has access to the demo instance:
demo.count = 55555;
demo.add(); // not 3...
Closures, in contrast, are completely private (barring strange, exceptional circumstances).
If you were to use a class, and you wanted to emulate the privacy of closures, make sure to use private class fields instead, so they can't be modified outside the class.
class countPlusClass{
#count = 0;
add(){
this.#count++;
console.log(this.#count)
}
}
const demo = new countPlusClass();
demo.add(); // 1
demo.add(); // 2
// some code elsewhere in the codebase that has access to the demo instance
// cannot modify the private field
demo.count = 55555;
demo.add(); // not 3...
As for which is better, a closure-based object or a class? That's up to each individual developer.

Classes are not always better. It really depends upon the circumstances. Each has their place in your programming toolset.
A class has the following advantages:
There's defined syntax in the language
You can sub-class a class to extend it
You can more easily have many properties and many methods.
Methods are automatically non-enumerable and run in strict mode.
The syntax in the code that uses a class somewhat self-describes what is happening since new countPlusClass() makes it clear you're creating an object that will then have methods and probably state. That's not as obvious with the closure you show.
For object with a lot of methods there's some space-savings benefit to the prototype that a class uses.
Developer tools will recognize an instance of a class and know how to display it, auto-complete when typing code for it, how to represent it in the debugger, how to use it in error messages, etc...
A closure has these advantages:
The data in the closure is complete private. Nobody can get to it from outside the closure.
In some circumstances, the caller may find it takes less code to use the closure since you make one function call (sometimes passing arguments) and you get back another function that then implements the one thing this is supposed to do (your mention of middleware comes to mind).
So, I'd say that if you want or need or value any of the benefits of the class, then use the class.
If you don't want or need any of the benefits of the class and the simpler interface of the closure meets your needs or if you really need the privacy of the closure, then you can choose the closure.
I'd say that for a given circumstance, one of these tools may be a better "fit" for the problem at hand.
As has been mentioned in the comments, you can also have factory functions that may be closures (retain private state in a closure) and may return an object with methods and/or properties that can even be an object created by instantiating a class. So, these concepts can all be combined too to get some of the benefits of both.

Are getter/setters really useful? [duplicate]

What's the advantage of using getters and setters - that only get and set - instead of simply using public fields for those variables?
If getters and setters are ever doing more than just the simple get/set, I can figure this one out very quickly, but I'm not 100% clear on how:
public String foo;
is any worse than:
private String foo;
public void setFoo(String foo) { this.foo = foo; }
public String getFoo() { return foo; }
Whereas the former takes a lot less boilerplate code.

There are actually many good reasons to consider using accessors rather than directly exposing fields of a class - beyond just the argument of encapsulation and making future changes easier.
Here are the some of the reasons I am aware of:
Encapsulation of behavior associated with getting or setting the property - this allows additional functionality (like validation) to be added more easily later.
Hiding the internal representation of the property while exposing a property using an alternative representation.
Insulating your public interface from change - allowing the public interface to remain constant while the implementation changes without affecting existing consumers.
Controlling the lifetime and memory management (disposal) semantics of the property - particularly important in non-managed memory environments (like C++ or Objective-C).
Providing a debugging interception point for when a property changes at runtime - debugging when and where a property changed to a particular value can be quite difficult without this in some languages.
Improved interoperability with libraries that are designed to operate against property getter/setters - Mocking, Serialization, and WPF come to mind.
Allowing inheritors to change the semantics of how the property behaves and is exposed by overriding the getter/setter methods.
Allowing the getter/setter to be passed around as lambda expressions rather than values.
Getters and setters can allow different access levels - for example the get may be public, but the set could be protected.

Because 2 weeks (months, years) from now when you realize that your setter needs to do more than just set the value, you'll also realize that the property has been used directly in 238 other classes :-)

A public field is not worse than a getter/setter pair that does nothing except returning the field and assigning to it. First, it's clear that (in most languages) there is no functional difference. Any difference must be in other factors, like maintainability or readability.
An oft-mentioned advantage of getter/setter pairs, isn't. There's this claim that you can change the implementation and your clients don't have to be recompiled. Supposedly, setters let you add functionality like validation later on and your clients don't even need to know about it. However, adding validation to a setter is a change to its preconditions, a violation of the previous contract, which was, quite simply, "you can put anything in here, and you can get that same thing later from the getter".
So, now that you broke the contract, changing every file in the codebase is something you should want to do, not avoid. If you avoid it you're making the assumption that all the code assumed the contract for those methods was different.
If that should not have been the contract, then the interface was allowing clients to put the object in invalid states. That's the exact opposite of encapsulation If that field could not really be set to anything from the start, why wasn't the validation there from the start?
This same argument applies to other supposed advantages of these pass-through getter/setter pairs: if you later decide to change the value being set, you're breaking the contract. If you override the default functionality in a derived class, in a way beyond a few harmless modifications (like logging or other non-observable behaviour), you're breaking the contract of the base class. That is a violation of the Liskov Substitutability Principle, which is seen as one of the tenets of OO.
If a class has these dumb getters and setters for every field, then it is a class that has no invariants whatsoever, no contract. Is that really object-oriented design? If all the class has is those getters and setters, it's just a dumb data holder, and dumb data holders should look like dumb data holders:
class Foo {
public:
int DaysLeft;
int ContestantNumber;
};
Adding pass-through getter/setter pairs to such a class adds no value. Other classes should provide meaningful operations, not just operations that fields already provide. That's how you can define and maintain useful invariants.
Client: "What can I do with an object of this class?"
Designer: "You can read and write several variables."
Client: "Oh... cool, I guess?"
There are reasons to use getters and setters, but if those reasons don't exist, making getter/setter pairs in the name of false encapsulation gods is not a good thing. Valid reasons to make getters or setters include the things often mentioned as the potential changes you can make later, like validation or different internal representations. Or maybe the value should be readable by clients but not writable (for example, reading the size of a dictionary), so a simple getter is a nice choice. But those reasons should be there when you make the choice, and not just as a potential thing you may want later. This is an instance of YAGNI (You Ain't Gonna Need It).

Lots of people talk about the advantages of getters and setters but I want to play devil's advocate. Right now I'm debugging a very large program where the programmers decided to make everything getters and setters. That might seem nice, but its a reverse-engineering nightmare.
Say you're looking through hundreds of lines of code and you come across this:
person.name = "Joe";
It's a beautifully simply piece of code until you realize its a setter. Now, you follow that setter and find that it also sets person.firstName, person.lastName, person.isHuman, person.hasReallyCommonFirstName, and calls person.update(), which sends a query out to the database, etc. Oh, that's where your memory leak was occurring.
Understanding a local piece of code at first glance is an important property of good readability that getters and setters tend to break. That is why I try to avoid them when I can, and minimize what they do when I use them.

In a pure object-oriented world getters and setters is a terrible anti-pattern. Read this article: Getters/Setters. Evil. Period. In a nutshell, they encourage programmers to think about objects as of data structures, and this type of thinking is pure procedural (like in COBOL or C). In an object-oriented language there are no data structures, but only objects that expose behavior (not attributes/properties!)
You may find more about them in Section 3.5 of Elegant Objects (my book about object-oriented programming).

There are many reasons. My favorite one is when you need to change the behavior or regulate what you can set on a variable. For instance, lets say you had a setSpeed(int speed) method. But you want that you can only set a maximum speed of 100. You would do something like:
public void setSpeed(int speed) {
if ( speed > 100 ) {
this.speed = 100;
} else {
this.speed = speed;
}
}
Now what if EVERYWHERE in your code you were using the public field and then you realized you need the above requirement? Have fun hunting down every usage of the public field instead of just modifying your setter.
My 2 cents :)

One advantage of accessors and mutators is that you can perform validation.
For example, if foo was public, I could easily set it to null and then someone else could try to call a method on the object. But it's not there anymore! With a setFoo method, I could ensure that foo was never set to null.
Accessors and mutators also allow for encapsulation - if you aren't supposed to see the value once its set (perhaps it's set in the constructor and then used by methods, but never supposed to be changed), it will never been seen by anyone. But if you can allow other classes to see or change it, you can provide the proper accessor and/or mutator.

Thanks, that really clarified my thinking. Now here is (almost) 10 (almost) good reasons NOT to use getters and setters:
When you realize you need to do more than just set and get the value, you can just make the field private, which will instantly tell you where you've directly accessed it.
Any validation you perform in there can only be context free, which validation rarely is in practice.
You can change the value being set - this is an absolute nightmare when the caller passes you a value that they [shock horror] want you to store AS IS.
You can hide the internal representation - fantastic, so you're making sure that all these operations are symmetrical right?
You've insulated your public interface from changes under the sheets - if you were designing an interface and weren't sure whether direct access to something was OK, then you should have kept designing.
Some libraries expect this, but not many - reflection, serialization, mock objects all work just fine with public fields.
Inheriting this class, you can override default functionality - in other words you can REALLY confuse callers by not only hiding the implementation but making it inconsistent.
The last three I'm just leaving (N/A or D/C)...

Depends on your language. You've tagged this "object-oriented" rather than "Java", so I'd like to point out that ChssPly76's answer is language-dependent. In Python, for instance, there is no reason to use getters and setters. If you need to change the behavior, you can use a property, which wraps a getter and setter around basic attribute access. Something like this:
class Simple(object):
def _get_value(self):
return self._value -1
def _set_value(self, new_value):
self._value = new_value + 1
def _del_value(self):
self.old_values.append(self._value)
del self._value
value = property(_get_value, _set_value, _del_value)

Well i just want to add that even if sometimes they are necessary for the encapsulation and security of your variables/objects, if we want to code a real Object Oriented Program, then we need to STOP OVERUSING THE ACCESSORS, cause sometimes we depend a lot on them when is not really necessary and that makes almost the same as if we put the variables public.

EDIT: I answered this question because there are a bunch of people learning programming asking this, and most of the answers are very technically competent, but they're not as easy to understand if you're a newbie. We were all newbies, so I thought I'd try my hand at a more newbie friendly answer.
The two main ones are polymorphism, and validation. Even if it's just a stupid data structure.
Let's say we have this simple class:
public class Bottle {
public int amountOfWaterMl;
public int capacityMl;
}
A very simple class that holds how much liquid is in it, and what its capacity is (in milliliters).
What happens when I do:
Bottle bot = new Bottle();
bot.amountOfWaterMl = 1500;
bot.capacityMl = 1000;
Well, you wouldn't expect that to work, right?
You want there to be some kind of sanity check. And worse, what if I never specified the maximum capacity? Oh dear, we have a problem.
But there's another problem too. What if bottles were just one type of container? What if we had several containers, all with capacities and amounts of liquid filled? If we could just make an interface, we could let the rest of our program accept that interface, and bottles, jerrycans and all sorts of stuff would just work interchangably. Wouldn't that be better? Since interfaces demand methods, this is also a good thing.
We'd end up with something like:
public interface LiquidContainer {
public int getAmountMl();
public void setAmountMl(int amountMl);
public int getCapacityMl();
}
Great! And now we just change Bottle to this:
public class Bottle implements LiquidContainer {
private int capacityMl;
private int amountFilledMl;
public Bottle(int capacityMl, int amountFilledMl) {
this.capacityMl = capacityMl;
this.amountFilledMl = amountFilledMl;
checkNotOverFlow();
}
public int getAmountMl() {
return amountFilledMl;
}
public void setAmountMl(int amountMl) {
this.amountFilled = amountMl;
checkNotOverFlow();
}
public int getCapacityMl() {
return capacityMl;
}
private void checkNotOverFlow() {
if(amountOfWaterMl > capacityMl) {
throw new BottleOverflowException();
}
}
I'll leave the definition of the BottleOverflowException as an exercise to the reader.
Now notice how much more robust this is. We can deal with any type of container in our code now by accepting LiquidContainer instead of Bottle. And how these bottles deal with this sort of stuff can all differ. You can have bottles that write their state to disk when it changes, or bottles that save on SQL databases or GNU knows what else.
And all these can have different ways to handle various whoopsies. The Bottle just checks and if it's overflowing it throws a RuntimeException. But that might be the wrong thing to do.
(There is a useful discussion to be had about error handling, but I'm keeping it very simple here on purpose. People in comments will likely point out the flaws of this simplistic approach. ;) )
And yes, it seems like we go from a very simple idea to getting much better answers quickly.
Please note also that you can't change the capacity of a bottle. It's now set in stone. You could do this with an int by declaring it final. But if this was a list, you could empty it, add new things to it, and so on. You can't limit the access to touching the innards.
There's also the third thing that not everyone has addressed: getters and setters use method calls. That means that they look like normal methods everywhere else does. Instead of having weird specific syntax for DTOs and stuff, you have the same thing everywhere.

I know it's a bit late, but I think there are some people who are interested in performance.
I've done a little performance test. I wrote a class "NumberHolder" which, well, holds an Integer. You can either read that Integer by using the getter method
anInstance.getNumber() or by directly accessing the number by using anInstance.number. My programm reads the number 1,000,000,000 times, via both ways. That process is repeated five times and the time is printed. I've got the following result:
Time 1: 953ms, Time 2: 741ms
Time 1: 655ms, Time 2: 743ms
Time 1: 656ms, Time 2: 634ms
Time 1: 637ms, Time 2: 629ms
Time 1: 633ms, Time 2: 625ms
(Time 1 is the direct way, Time 2 is the getter)
You see, the getter is (almost) always a bit faster. Then I tried with different numbers of cycles. Instead of 1 million, I used 10 million and 0.1 million.
The results:
10 million cycles:
Time 1: 6382ms, Time 2: 6351ms
Time 1: 6363ms, Time 2: 6351ms
Time 1: 6350ms, Time 2: 6363ms
Time 1: 6353ms, Time 2: 6357ms
Time 1: 6348ms, Time 2: 6354ms
With 10 million cycles, the times are almost the same.
Here are 100 thousand (0.1 million) cycles:
Time 1: 77ms, Time 2: 73ms
Time 1: 94ms, Time 2: 65ms
Time 1: 67ms, Time 2: 63ms
Time 1: 65ms, Time 2: 65ms
Time 1: 66ms, Time 2: 63ms
Also with different amounts of cycles, the getter is a little bit faster than the regular way. I hope this helped you.

Don't use getters setters unless needed for your current delivery I.e. Don't think too much about what would happen in the future, if any thing to be changed its a change request in most of the production applications, systems.
Think simple, easy, add complexity when needed.
I would not take advantage of ignorance of business owners of deep technical know how just because I think it's correct or I like the approach.
I have massive system written without getters setters only with access modifiers and some methods to validate n perform biz logic. If you absolutely needed the. Use anything.

We use getters and setters:
for reusability
to perform validation in later stages of programming
Getter and setter methods are public interfaces to access private class members.
Encapsulation mantra
The encapsulation mantra is to make fields private and methods public.
Getter Methods: We can get access to private variables.
Setter Methods: We can modify private fields.
Even though the getter and setter methods do not add new functionality, we can change our mind come back later to make that method
better;
safer; and
faster.
Anywhere a value can be used, a method that returns that value can be added. Instead of:
int x = 1000 - 500
use
int x = 1000 - class_name.getValue();
In layman's terms
Suppose we need to store the details of this Person. This Person has the fields name, age and sex. Doing this involves creating methods for name, age and sex. Now if we need create another person, it becomes necessary to create the methods for name, age, sex all over again.
Instead of doing this, we can create a bean class(Person) with getter and setter methods. So tomorrow we can just create objects of this Bean class(Person class) whenever we need to add a new person (see the figure). Thus we are reusing the fields and methods of bean class, which is much better.

I spent quite a while thinking this over for the Java case, and I believe the real reasons are:
Code to the interface, not the implementation
Interfaces only specify methods, not fields
In other words, the only way you can specify a field in an interface is by providing a method for writing a new value and a method for reading the current value.
Those methods are the infamous getter and setter....

It can be useful for lazy-loading. Say the object in question is stored in a database, and you don't want to go get it unless you need it. If the object is retrieved by a getter, then the internal object can be null until somebody asks for it, then you can go get it on the first call to the getter.
I had a base page class in a project that was handed to me that was loading some data from a couple different web service calls, but the data in those web service calls wasn't always used in all child pages. Web services, for all of the benefits, pioneer new definitions of "slow", so you don't want to make a web service call if you don't have to.
I moved from public fields to getters, and now the getters check the cache, and if it's not there call the web service. So with a little wrapping, a lot of web service calls were prevented.
So the getter saves me from trying to figure out, on each child page, what I will need. If I need it, I call the getter, and it goes to find it for me if I don't already have it.
protected YourType _yourName = null;
public YourType YourName{
get
{
if (_yourName == null)
{
_yourName = new YourType();
return _yourName;
}
}
}

One aspect I missed in the answers so far, the access specification:
for members you have only one access specification for both setting and getting
for setters and getters you can fine tune it and define it separately

In languages which don't support "properties" (C++, Java) or require recompilation of clients when changing fields to properties (C#), using get/set methods is easier to modify. For example, adding validation logic to a setFoo method will not require changing the public interface of a class.
In languages which support "real" properties (Python, Ruby, maybe Smalltalk?) there is no point to get/set methods.

One of the basic principals of OO design: Encapsulation!
It gives you many benefits, one of which being that you can change the implementation of the getter/setter behind the scenes but any consumer of that value will continue to work as long as the data type remains the same.

You should use getters and setters when:
You're dealing with something that is conceptually an attribute, but:
Your language doesn't have properties (or some similar mechanism, like Tcl's variable traces), or
Your language's property support isn't sufficient for this use case, or
Your language's (or sometimes your framework's) idiomatic conventions encourage getters or setters for this use case.
So this is very rarely a general OO question; it's a language-specific question, with different answers for different languages (and different use cases).
From an OO theory point of view, getters and setters are useless. The interface of your class is what it does, not what its state is. (If not, you've written the wrong class.) In very simple cases, where what a class does is just, e.g., represent a point in rectangular coordinates,* the attributes are part of the interface; getters and setters just cloud that. But in anything but very simple cases, neither the attributes nor getters and setters are part of the interface.
Put another way: If you believe that consumers of your class shouldn't even know that you have a spam attribute, much less be able to change it willy-nilly, then giving them a set_spam method is the last thing you want to do.
* Even for that simple class, you may not necessarily want to allow setting the x and y values. If this is really a class, shouldn't it have methods like translate, rotate, etc.? If it's only a class because your language doesn't have records/structs/named tuples, then this isn't really a question of OO…
But nobody is ever doing general OO design. They're doing design, and implementation, in a specific language. And in some languages, getters and setters are far from useless.
If your language doesn't have properties, then the only way to represent something that's conceptually an attribute, but is actually computed, or validated, etc., is through getters and setters.
Even if your language does have properties, there may be cases where they're insufficient or inappropriate. For example, if you want to allow subclasses to control the semantics of an attribute, in languages without dynamic access, a subclass can't substitute a computed property for an attribute.
As for the "what if I want to change my implementation later?" question (which is repeated multiple times in different wording in both the OP's question and the accepted answer): If it really is a pure implementation change, and you started with an attribute, you can change it to a property without affecting the interface. Unless, of course, your language doesn't support that. So this is really just the same case again.
Also, it's important to follow the idioms of the language (or framework) you're using. If you write beautiful Ruby-style code in C#, any experienced C# developer other than you is going to have trouble reading it, and that's bad. Some languages have stronger cultures around their conventions than others.—and it may not be a coincidence that Java and Python, which are on opposite ends of the spectrum for how idiomatic getters are, happen to have two of the strongest cultures.
Beyond human readers, there will be libraries and tools that expect you to follow the conventions, and make your life harder if you don't. Hooking Interface Builder widgets to anything but ObjC properties, or using certain Java mocking libraries without getters, is just making your life more difficult. If the tools are important to you, don't fight them.

From a object orientation design standpoint both alternatives can be damaging to the maintenance of the code by weakening the encapsulation of the classes. For a discussion you can look into this excellent article: http://typicalprogrammer.com/?p=23

Code evolves. private is great for when you need data member protection. Eventually all classes should be sort of "miniprograms" that have a well-defined interface that you can't just screw with the internals of.
That said, software development isn't about setting down that final version of the class as if you're pressing some cast iron statue on the first try. While you're working with it, code is more like clay. It evolves as you develop it and learn more about the problem domain you are solving. During development classes may interact with each other than they should (dependency you plan to factor out), merge together, or split apart. So I think the debate boils down to people not wanting to religiously write
int getVar() const { return var ; }
So you have:
doSomething( obj->getVar() ) ;
Instead of
doSomething( obj->var ) ;
Not only is getVar() visually noisy, it gives this illusion that gettingVar() is somehow a more complex process than it really is. How you (as the class writer) regard the sanctity of var is particularly confusing to a user of your class if it has a passthru setter -- then it looks like you're putting up these gates to "protect" something you insist is valuable, (the sanctity of var) but yet even you concede var's protection isn't worth much by the ability for anyone to just come in and set var to whatever value they want, without you even peeking at what they are doing.
So I program as follows (assuming an "agile" type approach -- ie when I write code not knowing exactly what it will be doing/don't have time or experience to plan an elaborate waterfall style interface set):
1) Start with all public members for basic objects with data and behavior. This is why in all my C++ "example" code you'll notice me using struct instead of class everywhere.
2) When an object's internal behavior for a data member becomes complex enough, (for example, it likes to keep an internal std::list in some kind of order), accessor type functions are written. Because I'm programming by myself, I don't always set the member private right away, but somewhere down the evolution of the class the member will be "promoted" to either protected or private.
3) Classes that are fully fleshed out and have strict rules about their internals (ie they know exactly what they are doing, and you are not to "fuck" (technical term) with its internals) are given the class designation, default private members, and only a select few members are allowed to be public.
I find this approach allows me to avoid sitting there and religiously writing getter/setters when a lot of data members get migrated out, shifted around, etc. during the early stages of a class's evolution.

There is a good reason to consider using accessors is there is no property inheritance. See next example:
public class TestPropertyOverride {
public static class A {
public int i = 0;
public void add() {
i++;
}
public int getI() {
return i;
}
}
public static class B extends A {
public int i = 2;
#Override
public void add() {
i = i + 2;
}
#Override
public int getI() {
return i;
}
}
public static void main(String[] args) {
A a = new B();
System.out.println(a.i);
a.add();
System.out.println(a.i);
System.out.println(a.getI());
}
}
Output:
0
0
4

Getters and setters are used to implement two of the fundamental aspects of Object Oriented Programming which are:
Abstraction
Encapsulation
Suppose we have an Employee class:
package com.highmark.productConfig.types;
public class Employee {
private String firstName;
private String middleName;
private String lastName;
public String getFirstName() {
return firstName;
}
public void setFirstName(String firstName) {
this.firstName = firstName;
}
public String getMiddleName() {
return middleName;
}
public void setMiddleName(String middleName) {
this.middleName = middleName;
}
public String getLastName() {
return lastName;
}
public void setLastName(String lastName) {
this.lastName = lastName;
}
public String getFullName(){
return this.getFirstName() + this.getMiddleName() + this.getLastName();
}
}
Here the implementation details of Full Name is hidden from the user and is not accessible directly to the user, unlike a public attribute.

There is a difference between DataStructure and Object.
Datastructure should expose its innards and not behavior.
An Object should not expose its innards but it should expose its behavior, which is also known as the Law of Demeter
Mostly DTOs are considered more of a datastructure and not Object. They should only expose their data and not behavior. Having Setter/Getter in DataStructure will expose behavior instead of data inside it. This further increases the chance of violation of Law of Demeter.
Uncle Bob in his book Clean code explained the Law of Demeter.
There is a well-known heuristic called the Law of Demeter that says a
module should not know about the innards of the objects it
manipulates. As we saw in the last section, objects hide their data
and expose operations. This means that an object should not expose its
internal structure through accessors because to do so is to expose,
rather than to hide, its internal structure.
More precisely, the Law of Demeter says that a method f of a class C
should only call the methods of these:
C
An object created by f
An object passed as an argument to f
An object held in an instance variable of C
The method should not invoke methods on objects that are returned by any of the allowed functions.
In other words, talk to friends, not to strangers.
So according this, example of LoD violation is:
final String outputDir = ctxt.getOptions().getScratchDir().getAbsolutePath();
Here, the function should call the method of its immediate friend which is ctxt here, It should not call the method of its immediate friend's friend. but this rule doesn't apply to data structure. so here if ctxt, option, scratchDir are datastructure then why to wrap their internal data with some behavior and doing a violation of LoD.
Instead, we can do something like this.
final String outputDir = ctxt.options.scratchDir.absolutePath;
This fulfills our needs and doesn't even violate LoD.
Inspired by Clean Code by Robert C. Martin(Uncle Bob)

If you don't require any validations and not even need to maintain state i.e. one property depends on another so we need to maintain the state when one is change. You can keep it simple by making field public and not using getter and setters.
I think OOPs complicates things as the program grows it becomes nightmare for developer to scale.
A simple example; we generate c++ headers from xml. The header contains simple field which does not require any validations. But still as in OOPS accessor are fashion we generates them as following.
const Filed& getfield() const
Field& getField()
void setfield(const Field& field){...}
which is very verbose and is not required. a simple
struct
{
Field field;
};
is enough and readable.
Functional programming don't have the concept of data hiding they even don't require it as they do not mutate the data.

Additionally, this is to "future-proof" your class. In particular, changing from a field to a property is an ABI break, so if you do later decide that you need more logic than just "set/get the field", then you need to break ABI, which of course creates problems for anything else already compiled against your class.

One other use (in languages that support properties) is that setters and getters can imply that an operation is non-trivial. Typically, you want to avoid doing anything that's computationally expensive in a property.

One relatively modern advantage of getters/setters is that is makes it easier to browse code in tagged (indexed) code editors. E.g. If you want to see who sets a member, you can open the call hierarchy of the setter.
On the other hand, if the member is public, the tools don't make it possible to filter read/write access to the member. So you have to trudge though all uses of the member.

Getters and setters coming from data hiding. Data Hiding means We
are hiding data from outsiders or outside person/thing cannot access
our data.This is a useful feature in OOP.
As a example:
If you create a public variable, you can access that variable and change value in anywhere(any class). But if you create as private that variable cannot see/access in any class except declared class.
public and private are access modifiers.
So how can we access that variable outside:
This is the place getters and setters coming from. You can declare variable as private then you can implement getter and setter for that variable.
Example(Java):
private String name;
public String getName(){
return this.name;
}
public void setName(String name){
this.name= name;
}
Advantage:
When anyone want to access or change/set value to balance variable, he/she must have permision.
//assume we have person1 object
//to give permission to check balance
person1.getName()
//to give permission to set balance
person1.setName()
You can set value in constructor also but when later on when you want
to update/change value, you have to implement setter method.

When should I use data type classes in JavaScript

I come from a C# background. I've been working a lot with JavaScript lately. On a new app, I have a mysql/php back end. I'm going to be passing a lot of "types" back and forth.
So in my data base, I have several tables like
table1
id, fieldx,fieldy,fieldz
table2
id, fielda,fieldb,fielc
In c# I would definitely write classes for all those in the code. Which led me to implement things like so (in my JavaScript app):
function table1(id, x,y,z){
this.id=id;
this.x=x;
this.y=y;
this.z=z;
}
After about 6 tables worth of that, it suddenly occurred to me that maybe there was no point at all in making these classes.
So my question is, in a JavaScript app, do I use "classes" for data types? or should I just "document" which fields/types are expected and so in the code instead of
a.push(new table1(5,1,2,3));
I would just have
a.push({id:5,x:1,y:2,z:3});
This may seem like a preferences question but it's a fundamental language question that I have as I try to understand how to model my app's data in JavaScript. Is there any advantage of the classes (with only data fields) or is it just a mistake. Thanks.

It depends,
Note: Most of the programmers coming from a strong OO language will have trouble like you in regard to JavaScript's functional behavior (you are not alone).
If you want to create something closer to C# I would do the following:
function Table1(id, x, y, z) {
this.id=id;
this.x=x;
this.y=y;
this.z=z;
}
Table1.prototype.mySpecialTable1Method= function()
{
console.log(this.id);
};
Implementation:
var t = new Table1(1, 2, 3, 4);
t.mySpecialTable1Method();// outputs: 1
If you need to have methods that interact with the (soon to be) objects then I would definitely go with the code above. In addition it will make it clear when working with the objects that are related to a specific 'type' (naming the data).
But if your objects do not require any special "treatment" then I don't see any problem to use normal js object literals and pass them along (probably good for small projects).
Something along the lines:
var table1 = {};
table1.id = 1;
table1.x = 2;
table1.y = 3;
table1.z = 4;
console.log(table1.id); //outputs: 1
Extra reference:
http://www.youtube.com/watch?v=PMfcsYzj-9M
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Introduction_to_Object-Oriented_JavaScript
Update:
For the sake of readability and scalability and the point that you are coming from C# you may want to stick to the "class" implementation just because it will define the correlation between the raw data and the objects you are working with.
There is a good chance that you are going to work with some data that will probably be messy and unorganized.
MVC may be the solution for you. It tries to bring some order to the chaos that you are expecting. I recommend to check out some of them like: AngularJS or Ember.
Another solution may be reactive js - but mostly if you are going to interact with the DOM according to your data (ReactJS, and Facebook's React as some good ones).
As a note for security, I would like to add that mapping the data closely to the db isn't a best practice but its your call.

Javascript is a funny language, and there are plenty of ways to do things. An Object is an Object in Javascript with or without a name. {} is just a short-hand way to create one.
If you are going for readability, then your initial example would be the way to go.
If you just want to get the block of data into an array, then your second example is appropriate. Personally, I would use your later example if it is just data.
If you are using functions and what not as well as data storage, and plan on reusing it several times in your code, then yes, define your object and call it appropriately.

JavaScript has no classes, it is a functional language and a function is a first class citizen in js meaning that a function is an object.
From your example I can see that your intention for classes is simply to pass data and using json is perfect for this.

Why would I need to freeze an object in JavaScript?

It is not clear to me when anyone would need to use Object.freeze in JavaScript. MDN and MSDN don't give real life examples when it is useful.
I get it that an attempt to change such an object at runtime means a crash. The question is rather, when would I appreciate this crash?
To me the immutability is a design time constraint which is supposed to be guaranteed by the type checker.
So is there any point in having a runtime crash in a dynamically typed language, besides detecting a violation better later than never?

The Object.freeze function does the following:
Makes the object non-extensible, so that new properties cannot be added to it.
Sets the configurable attribute to false for all properties of the object. When - configurable is false, the property attributes cannot be changed and the property cannot be deleted.
Sets the writable attribute to false for all data properties of the object. When writable is false, the data property value cannot be changed.
That's the what part, but why would anyone do this?
Well, in the object-oriented paradigm, the notion exists that an existing API contains certain elements that are not intended to be extended, modified, or re-used outside of their current context. The final keyword in various languages is the most suitable analogy of this. Even in languages that are not compiled and therefore easily modified, it still exists, i.e. PHP, and in this case, JavaScript.

You can use this when you have an object representing a logically immutable data structure, especially if:
Changing the properties of the object or altering its "duck type" could lead to bad behavior elsewhere in your application
The object is similar to a mutable type or otherwise looks mutable, and you want programmers to be warned on attempting to change it rather than obtain undefined behavior.
As an API author, this may be exactly the behavior you want. For example, you may have an internally cached structure that represents a canonical server response that you provide to the user of your API by reference but still use internally for a variety of purposes. Your users can reference this structure, but altering it may result in your API having undefined behavior. In this case, you want an exception to be thrown if your users attempt to modify it.

In my nodejs server environment, I use freeze for the same reason I use 'use strict'. If I have an object that I do not want being extended or modified, I will freeze it. If something attempts to extend or modify my frozen object, I WANT my app to throw an error.
To me this relates to consistent, quality, more secure code.
Also,
Chrome is showing significant performance increases working with frozen objects.
Edit:
In my most recent project, I'm sending/receiving encrypted data between a government entity. There are a lot of configuration values. I'm using frozen object(s) for these values. Modification of these values could have serious, adverse side effects. Additionally, as I linked previously, Chrome is showing performance advantages with frozen objects, I assume nodejs does as well.
For simplicity, an example would be:
var US_COIN_VALUE = {
QUARTER: 25,
DIME: 10,
NICKEL: 5,
PENNY: 1
};
return Object.freeze( US_COIN_VALUE );
There is no reason to modify the values in this example. And enjoy the benefits of speed optimizations.

Object.freeze() mainly using in Functional Programming (Immutability)
Immutability is a central concept of functional programming because without it, the data flow in your program is lossy. State history is abandoned, and strange bugs can creep into your software.
In JavaScript, it’s important not to confuse const, with immutability. const creates a variable name binding which can’t be reassigned after creation. const does not create immutable objects. You can’t change the object that the binding refers to, but you can still change the properties of the object, which means that bindings created with const are mutable, not immutable.
Immutable objects can’t be changed at all. You can make a value truly immutable by deep freezing the object. JavaScript has a method that freezes an object one-level deep.
const a = Object.freeze({
foo: 'Hello',
bar: 'world',
baz: '!'
});

When you're writing a library/framework in JS and you don't want some developer to break your dynamic language creation by re-assigning "internal" or public properties.
This is the most obvious use case for immutability.

With the V8 release v7.6 the performance of frozen/sealed arrays is greatly improved. Therefore, one reason you would like to freeze an object is when your code is performance-critical.

What is a practical situation when you might want to freeze an object?
One example, on application startup you create an object containing app settings. You may pass that configuration object around to various modules of the application. But once that settings object is created you want to know that it won't be changed.

This is an old question, but I think I have a good case where freeze might help. I had this problem today.
The problem
class Node {
constructor() {
this._children = [];
this._parent = undefined;
}
get children() { return this._children; }
get parent() { return this._parent; }
set parent(newParent) {
// 1. if _parent is not undefined, remove this node from _parent's children
// 2. set _parent to newParent
// 3. if newParent is not undefined, add this node to newParent's children
}
addChild(node) { node.parent = this; }
removeChild(node) { node.parent === this && (node.parent = undefined); }
...
}
As you can see, when you change the parent, it automatically handles the connection between these nodes, keeping children and parent in sync. However, there is one problem here:
let newNode = new Node();
myNode.children.push(newNode);
Now, myNode has newNode in its children, but newNode does not have myNode as its parent. So you've just broken it.
(OFF-TOPIC) Why are you exposing the children anyway?
Yes, I could just create lots of methods: countChildren(), getChild(index), getChildrenIterator() (which returns a generator), findChildIndex(node), and so on... but is it really a better approach than just returning an array, which provides an interface all javascript programmers already know?
You can access its length to see how many children it has;
You can access the children by their index (i.e. children[i]);
You can iterate over it using for .. of;
And you can use some other nice methods provided by an Array.
Note: returning a copy of the array is out of question! It costs linear time, and any updates to the original array do not propagate to the copy!
The solution
get children() { return Object.freeze(Object.create(this._children)); }
// OR, if you deeply care about performance:
get children() {
return this._PUBLIC_children === undefined
? (this._PUBLIC_children = Object.freeze(Object.create(this._children)))
: this._PUBLIC_children;
}
Done!
Object.create: we create an object that inherits from this._children (i.e. has this._children as its __proto__). This alone solves almost the entire problem:
It's simple and fast (constant time)
You can use anything provided by the Array interface
If you modify the returned object, it does not change the original!
Object.freeze: however, the fact that you can modify the returned object BUT the changes do not affect the original array is extremely confusing for the user of the class! So, we just freeze it. If he tries to modify it, an exception is thrown (assuming strict mode) and he knows he can't (and why). It's sad no exception is thrown for myFrozenObject[x] = y if you are not in strict mode, but myFrozenObject is not modified anyway, so it's still not-so-weird.
Of course the programmer could bypass it by accessing __proto__, e.g:
someNode.children.__proto__.push(new Node());
But I like to think that in this case they actually know what they are doing and have a good reason to do so.
IMPORTANT: notice that this doesn't work so well for objects: using hasOwnProperty in the for .. in will always return false.
UPDATE: using Proxy to solve the same problem for objects
Just for completion: if you have an object instead of an Array you can still solve this problem by using Proxy. Actually, this is a generic solution that should work with any kind of element, but I recommend against (if you can avoid it) due to performance issues:
get myObject() { return Object.freeze(new Proxy(this._myObject, {})); }
This still returns an object that can't be changed, but keeps all the read-only functionality of it. If you really need, you can drop the Object.freeze and implement the required traps (set, deleteProperty, ...) in the Proxy, but that takes extra effort, and that's why the Object.freeze comes in handy with proxies.

I can think of several places that Object.freeze would come in very handy.
The first real world implementation that could use freeze is when developing an application that requires 'state' on the server to match what's in the browser. For instance, imagine you need to add in a level of permissions to your function calls. If you are working in an application there may be places where a Developer could easily change or overwrite the permission settings without even realizing it (especially if the object were being passed through by reference!). But permissions by and large can never change and error'ing when they are changed is preferred. So in this case, the permissions object could be frozen, thereby limiting developer from mistakenly 'setting' permissions erroneously. The same could be said for user-like data like a login name or email address. These things can be mistakenly or maliciously broken with bad code.
Another typical solution would be in a game loop code. There are many settings of game state that you would want to freeze to retain that the state of the game is kept in sync with the server.
Think of Object.freeze as a way to make an object as a Constant. Anytime you would want to have variable constant, you could have an object constant with freeze for similar reasons.
There are also times where you want to pass immutable objects through functions and data passing, and only allow updating the original object with setters. This can be done by cloning and freezing the object for 'getters' and only updating the original with 'setters'.
Are any of these not valid things? It can also be said that frozen objects could be more performant due to the lack of dynamic variables, but I haven't seen any proof of that yet.

The only practical use for Object.freeze is during development. For production code, there is absolutely no benefit for freezing/sealing objects.
Silly Typos
It could help you catch this very common problem during development:
if (myObject.someProp = 5) {
doSomething();
}
In strict mode, this would throw an error if myObject was frozen.
Enforce Coding Protocol / Restriction
It would also help in enforcing a certain protocol in a team, especially with new members who may not have the same coding style as everyone else.
A lot of Java guys like to add a lot of methods to objects to make JS feel more familiar. Freezing objects would prevent them from doing that.

I could see this being useful when you're working with an interactive tool. Rather than:
if ( ! obj.isFrozen() ) {
obj.x = mouse[0];
obj.y = mouse[1];
}
You could simply do:
obj.x = mouse[0];
obj.y = mouse[1];
Properties will only update if the object isn't frozen.

Don't know if this helps, but I use it to create simple enumerations. It allows me to hopefully not get duff data in a database, by knowing the source of the data has been attempted to be unchangeable without purposefully trying to break the code. From a statically typed perspective, it allows for reasoning over code construction.

All the other answers pretty much answer the question.
I just wanted to summarise everything here along with an example.
Use Object.freeze when you need utmost surety regarding its state in the future. You need to make sure that other developers or users of your code do not change internal/public properties. Alexander Mills's answer
Object.freeze has better performance since 19th June, 2019, ever since V8 v7.6 released. Philippe's answer. Also take a look at the V8 docs.
Here is what Object.freeze does, and it should clear out doubts for people who only have surface level understanding of Object.freeze.
const obj = {
name: "Fanoflix"
};
const mutateObject = (testObj) => {
testObj.name = 'Arthas' // NOT Allowed if parameter is frozen
}
obj.name = "Lich King" // Allowed
obj.age = 29; // Allowed
mutateObject(obj) // Allowed
Object.freeze(obj) // ========== Freezing obj ==========
mutateObject(obj) // passed by reference NOT Allowed
obj.name = "Illidan" // mutation NOT Allowed
obj.age = 25; // addition NOT Allowed
delete obj.name // deletion NOT Allowed

What is the point of Ext.apply versus simply setting values on the target?

Using Ext.js or sencha, what is the point of doing the following:
Ext.apply(app.views, {
contactsList: new app.views.ContactsList(),
contactDetail: new app.views.ContactDetail(),
contactForm: new app.views.ContactForm()
});
As opposed to this standard javascript:
app.views.contactsList = new app.views.ContactsList();
app.views.contactDetail = new app.views.ContactDetail();
app.views.contactForm = new app.views.ContactForm();
Is there any difference?

Ext.apply is generally more convenient (and possibly more efficient if there are fewer activation chain lookups required, as in your example, though that's a minor point) . There is also a variant Ext.applyIf that only applies members from the source object that do not exist in the target object, which is even more useful as it saves you from a boatload of manual if() checks. That's really useful, for example, when applying defaults to a config object that may already have user or app-defined properties assigned.
A note to future readers who look at the accepted answer: Ext also has Ext.extend which actually means "inherit from" a class, as opposed to Ext.apply[If] which merely merges an object instance into another, or Ext.override which overrides (without subclassing) a class definition. Lots of options, depending on what you need.

It's mostly there as a convenience method for code that is accepting an object as an argument, and needs to merge it. Merging objects is a common use case in JavaScript, and this type of helper is implemented by most frameworks. ($.extend in jQuery, Object.extend in Prototype, Object.append in MooTools, etc.)
In your case, there is little difference, other than offering a bit more readable code.

You need Ext.apply if you use Ajax requests to get the configurations from the server. Because Ajax responses are received later on, after the window is rendered. The second part of your code won't work.