tag, for more than 10 lines – use a sandbox ( plnkr ,  jsbin ,  codepen …)
One of the most important principles of object oriented programming – delimiting internal interface from the external one.
That is “a must” practice in developing anything more complex than a “hello world” app.
To understand this, let’s break away from development and turn our eyes into the real world.
Usually, devices that we’re using are quite complex. But delimiting the internal interface from the external one allows to use them without problems.
For instance, a coffee machine. Simple from outside: a button, a display, a few holes…And, surely, the result – great coffee! :)
But inside… (a picture from the repair manual)
A lot of details. But we can use it without knowing anything.
Coffee machines are quite reliable, aren’t they? We can use one for years, and only if something goes wrong – bring it for repairs.
The secret of reliability and simplicity of a coffee machine – all details are well-tuned and  hidden  inside.
If we remove the protective cover from the coffee machine, then using it will be much more complex (where to press?), and dangerous (it can electrocute).
As we’ll see, in programming objects are like coffee machines.
But in order to hide inner details, we’ll use not a protective cover, but rather special syntax of the language and conventions.
In object-oriented programming, properties and methods are split into two groups:
If we continue the analogy with the coffee machine – what’s hidden inside: a boiler tube, heating element, and so on – is its internal interface.
An internal interface is used for the object to work, its details use each other. For instance, a boiler tube is attached to the heating element.
But from the outside a coffee machine is closed by the protective cover, so that no one can reach those. Details are hidden and inaccessible. We can use its features via the external interface.
So, all we need to use an object is to know its external interface. We may be completely unaware how it works inside, and that’s great.
In JavaScript, there are two types of object fields (properties and methods):
In many other languages there also exist “protected” fields: accessible only from inside the class and those extending it (like private, but plus access from inheriting classes). They are also useful for the internal interface. They are in a sense more widespread than private ones, because we usually want inheriting classes to gain access to them.
Protected fields are not implemented in JavaScript on the language level, but in practice they are very convenient, so they are emulated.
Now we’ll make a coffee machine in JavaScript with all these types of properties. A coffee machine has a lot of details, we won’t model them to stay simple (though we could).
Let’s make a simple coffee machine class first:
Right now the properties  waterAmount  and  power  are public. We can easily get/set them from the outside to any value.
Let’s change  waterAmount  property to protected to have more control over it. For instance, we don’t want anyone to set it below zero.
Protected properties are usually prefixed with an underscore  _ .
That is not enforced on the language level, but there’s a well-known convention between programmers that such properties and methods should not be accessed from the outside.
So our property will be called  _waterAmount :
Now the access is under control, so setting the water amount below zero becomes impossible.
For  power  property, let’s make it read-only. It sometimes happens that a property must be set at creation time only, and then never modified.
That’s exactly the case for a coffee machine: power never changes.
To do so, we only need to make getter, but not the setter:
But most of the time  get.../set...  functions are preferred, like this:
That looks a bit longer, but functions are more flexible. They can accept multiple arguments (even if we don’t need them right now).
On the other hand, get/set syntax is shorter, so ultimately there’s no strict rule, it’s up to you to decide.
If we inherit  class MegaMachine extends CoffeeMachine , then nothing prevents us from accessing  this._waterAmount  or  this._power  from the methods of the new class.
So protected fields are naturally inheritable. Unlike private ones that we’ll see below.
There’s a finished JavaScript proposal, almost in the standard, that provides language-level support for private properties and methods.
Privates should start with  # . They are only accessible from inside the class.
For instance, here’s a private  #waterLimit  property and the water-checking private method  #checkWater :
On the language level,  #  is a special sign that the field is private. We can’t access it from outside or from inheriting classes.
Private fields do not conflict with public ones. We can have both private  #waterAmount  and public  waterAmount  fields at the same time.
For instance, let’s make  waterAmount  an accessor for  #waterAmount :
Unlike protected ones, private fields are enforced by the language itself. That’s a good thing.
But if we inherit from  CoffeeMachine , then we’ll have no direct access to  #waterAmount . We’ll need to rely on  waterAmount  getter/setter:
In many scenarios such limitation is too severe. If we extend a  CoffeeMachine , we may have legitimate reasons to access its internals. That’s why protected fields are used more often, even though they are not supported by the language syntax.
As we know, usually we can access fields using  this[name] :
With private fields that’s impossible:  this['#name']  doesn’t work. That’s a syntax limitation to ensure privacy.
In terms of OOP, delimiting of the internal interface from the external one is called  encapsulation .
Imagine, there’s a team of developers using a coffee machine. It was made by the “Best CoffeeMachine” company, and works fine, but a protective cover was removed. So the internal interface is exposed.
All developers are civilized – they use the coffee machine as intended. But one of them, John, decided that he’s the smartest one, and made some tweaks in the coffee machine internals. So the coffee machine failed two days later.
That’s surely not John’s fault, but rather the person who removed the protective cover and let John do his manipulations.
The same in programming. If a user of a class will change things not intended to be changed from the outside – the consequences are unpredictable.
The situation in programming is more complex than with a real-life coffee machine, because we don’t just buy it once. The code constantly undergoes development and improvement.
If we strictly delimit the internal interface, then the developer of the class can freely change its internal properties and methods, even without informing the users.
If you’re a developer of such class, it’s great to know that private methods can be safely renamed, their parameters can be changed, and even removed, because no external code depends on them.
For users, when a new version comes out, it may be a total overhaul internally, but still simple to upgrade if the external interface is the same.
People adore using things that are simple. At least from outside. What’s inside is a different thing.
It’s always convenient when implementation details are hidden, and a simple, well-documented external interface is available.
To hide an internal interface we use either protected or private properties:
Right now, private fields are not well-supported among browsers, but can be polyfilled.

Private   class fields -  JavaScript  | MDN
Private  and protected properties and methods
GitHub - tc39/proposal- private  -methods:  Private  methods and...
Private   Properties and Methods in  JavaScript  Classes - Ultimate Courses
Private   Members in  JavaScript

             
     
     
       tc39 
     
     / 
   
     proposal-private-methods 
   
  
 


        

      
       littledan 


  

         
           V8 shipping status 
         
    

               
                     No reason to delete 'out', just use mkdirp 
               
          

               
                     No reason to delete 'out', just use mkdirp 
               
          

               
                     Editorial: Fix various minor issues. 
               
          
class   Counter   extends   HTMLElement   { 
   get   x ( )   {   return   this . xValue ;   } 
   set   x ( value )   { 
     this . xValue   =   value ; 
     window . requestAnimationFrame ( this . render . bind ( this ) ) ; 
   } 

   clicked ( )   { 
     this . x ++ ; 
   } 

   constructor ( )   { 
     super ( ) ; 
     this . onclick   =   this . clicked . bind ( this ) ; 
     this . xValue   =   0 ; 
   } 

   connectedCallback ( )   {   this . render ( ) ;   } 

   render ( )   { 
     this . textContent   =   this . x . toString ( ) ; 
   } 
 } 
 window . customElements . define ( 'num-counter' ,   Counter ) ;
class   Counter   extends   HTMLElement   { 
   xValue   =   0 ; 

   get   x ( )   {   return   this . xValue ;   } 
   set   x ( value )   { 
     this . xValue   =   value ; 
     window . requestAnimationFrame ( this . render . bind ( this ) ) ; 
   } 

   clicked ( )   { 
     this . x ++ ; 
     window . requestAnimationFrame ( this . render . bind ( this ) ) ; 
   } 

   constructor ( )   { 
     super ( ) ; 
     this . onclick   =   this . clicked . bind ( this ) ; 
   } 

   connectedCallback ( )   {   this . render ( ) ;   } 

   render ( )   { 
     this . textContent   =   this . x . toString ( ) ; 
   } 
 } 
 window . customElements . define ( 'num-counter' ,   Counter ) ;
class   Counter   extends   HTMLElement   { 
  # xValue   =   0 ; 

   get  # x ( )   {   return  # xValue ;   } 
   set  # x ( value )   { 
     this . # xValue   =   value ; 
     window . requestAnimationFrame ( this . # render . bind ( this ) ) ; 
   } 

  # clicked ( )   { 
     this . # x ++ ; 
   } 

   constructor ( )   { 
     super ( ) ; 
     this . onclick   =   this . # clicked . bind ( this ) ; 
   } 

   connectedCallback ( )   {   this . # render ( ) ;   } 

  # render ( )   { 
     this . textContent   =   this . # x . toString ( ) ; 
   } 
 } 
 window . customElements . define ( 'num-counter' ,   Counter ) ;

       
       
         arai-a.github.io/ecma262-compare/?pr=1668 
       
    

     
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Keeping state and behavior private to a class lets library authors present a clear, stable interface, while changing their code over time behind the scenes. The  class fields  proposal provides private fields for classes and instances, and this proposal builds on that by adding private methods and accessors (getter/setters) to JavaScript. With this proposal, any class element can be private.
For discussion about semantic details, see  DETAILS.md . This document focuses more on the end user experience and intuition.
To define a counter widget which increments when clicked, you can define the following with ES2015:
With the class fields proposal, the above example can be written as
In the above example, you can see a field declared with the syntax  x = 0 . You can also declare a field without an initializer as  x . By declaring fields up-front, class definitions become more self-documenting; instances go through fewer state transitions, as declared fields are always present.
The above example has some implementation details exposed to the world that might be better kept internal. Using ESnext private fields and methods, the definition can be refined to:
To make methods, getter/setters or fields private, just give them a name starting with  # .
With all of its implementation kept internal to the class, this custom element can present an interface which is basically just like a built-in HTML element. Users of the custom element don't have the power to mess around with any of its internals.
Note that this proposal provides private fields and methods only as declared up-front in a field declaration; private fields cannot be created later, ad-hoc, through assigning to them, the way that normal properties can. You also can't declare private fields or methods in object literals; for example, if you implement your class based on object literals, or adding individual methods to the prototype, or using a class framework, you cannot use private methods, fields or accessors.
Decorators aren't part of this proposal, but it's designed to allow decorators to work on top of it. See  unified class features  for an explanation of how they would work together.
This proposal adds only private  instance  methods, omitting private  static  methods, which are under consideration in  the static class features proposal .
This proposal reached  Stage 3  in September 2017. Since that time, there has been extensive thought and lengthy discussion about various alternatives, including:
In considering each proposal, TC39 delegates looked deeply into the motivation, JS developer feedback, and the implications on the future of the language design. In the end, this thought process and continued community engagement led to renewed consensus on the proposal in this repository. Based on that consensus, implementations are moving forward on this proposal.
Several implementations are under development:
You are encouraged to file issues and PRs this repository to
If you have any additional ideas on how to improve JavaScript, see ecma262's  CONTRIBUTING.md  for how to get involved.

      Private methods and getter/setters for ES6 classes
    

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