Js Private

Js Private class PublicCounter { constructor ( start = 0 ) { let _count = start ; let _init = start ; this . increase = ( x = 1 ) => _count += x ; this . decrease = ( x = 1 ) => _count -= x ; this . reset = ( x = _init ) => _count = x ; this . getCount = ( ) => _count ; } get current ( ) { return this . getCount ( ) ; } } class PrivateCounter { #count ; #init ; constructor ( start = 0 ) { this . #init = start ; this . reset ( start ) ; } increase ( x = 1 ) { this . #count += x ; } decrease ( x = 1 ) { this . #count -= x ; } reset ( x = this . #init ) { this . #count = x ; } get current ( ) { return this . #count ; } } const total = new PrivateCounter ( 7 ) ; console . log ( total . current ) ; // expected output: 7 total . increase ( ) ; // #count now = 8 total . increase ( 5 ) ; // #count now = 13 console . log ( total . current ) ; // expected output: 13 total . reset ( ) ; // #count now = 7 const score = new PrivateCounter ( ) ; // #count and #init are now both 0 console . log ( score . #count ) ; // output: // "Uncaught SyntaxError: Private field '#count' must be declared in an enclosing class" class BadIdea { constructor ( arg ) { this . #init = arg ; // syntax error occurs here #startState = arg ; // syntax error would also occur here } // because private fields weren't defined } // before being referenced class BadIdeas { #firstName ; #firstName ; // syntax error occurs here #lastName ; constructor ( ) { delete this . #lastName ; // also a syntax error } } const planet = { name : 'Terra' , radiusKm : 6371 , radiusMiles : 3959 , } ; const planet = { name : 'Terra' , radiusKm : 6371 , radiusMiles : 3959 , #secret : 'central inner core' , } ; // result: // "Uncaught SyntaxError: Unexpected identifier" class colorMixer { static #red = "rgba(1,0,0,1)" ; static #green = "rgba(0,1,0,1)" ; static #blue = "rgba(0,0,1,1)" ; #mixedColor ; constructor ( ) { // … } } class CustomClick extends HTMLElement { #handleClicked ( ) { // do complicated stuff in here } constructor ( ) { super ( ) ; this . #handleClicked ( ) ; } connectedCallback ( ) { this . addEventListener ( 'click' , this . #handleClicked ) } } customElements . define ( 'chci-interactive' , CustomClick ) ; 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 ++ ; } constructor ( ) { super ( ) ; this . onclick = this . #clicked . bind ( this ) ; } connectedCallback ( ) { this . #render ( ) ; } #render ( ) { this . textContent = this . #x . toString ( ) ; } } customElements . define ( 'num-counter' , Counter ) ; class Scalar { #total = 0 ; constructor ( value ) { this . #total = value || this . #total ; } add ( s ) { // check the passed object defines #total if ( ! ( #total in s ) ) { throw new TypeError ( "Expected an instance of Scalar" ) ; } this . #total += s . #total ; } } const scalar1 = new Scalar ( 1 ) ; scalar1 . add ( scalar1 ) scalar1 . add ( { } ) // throws informative exception Web technology reference for developers Code used to describe document style Protocol for transmitting web resources Interfaces for building web applications Developing extensions for web browsers Web technology reference for developers Learn to structure web content with HTML Learn to run scripts in the browser Learn to make the web accessible to all Frequently asked questions about MDN Plus It's common to want to make fields or methods private, but JavaScript has lacked such a feature since its inception. Conventions have arisen — such as prefixing fields and methods that should be treated as private with an underscore, like _hidden — but these are merely conventions. The underscored features are still fully public. Private class features deliver truly private fields and methods, with that privacy enforced by the language instead of convention. This confers benefits such as avoiding naming collisions between class features and the rest of the code base, and allowing classes to expose a very small interface to the rest of the code. To understand how private fields work, let's first consider a class that has only public fields, but uses the constructor to encapsulate data—a somewhat common technique, even if it is a bit of a hack. The following class creates a basic count that accepts a starting number, allows that number to be increased or decreased, and can be reset to the original starting value or any other value. The idea here is that once a new counter of this type is spawned, its starting value and its current value are not available to code outside the counter. The only way to modify the value of _count is through the defined methods, such as increase() and reset() . Similarly, _init can't be modified, because there are no methods inside the class to do so, and outside code is unable to reach it. Here's the same idea, only this time, we'll use private fields. The "hash mark" ( # ) is what marks a field as being private. It also prevents private fields and property names from ever being in conflict: private names must start with # , whereas property names can never start that way. Having declared the private fields, they act as we saw in the public example. The only way to change the #count value is via the publicly available methods like decrease() , and because (in this example) there are no defined ways to alter it, the #init value is immutable. It's set when a new PrivateCounter is constructed, and can never be changed thereafter. You cannot read a private value directly from code outside the class object. Consider: If you wish to read private data from outside a class, you must first invent a method or other function to return it. We had already done that with the current() getter that returns the current value of #count , but #init is locked away. Unless we add something like a getInit() method to the class, we can't even see the initial value from outside the class, let alone alter it, and the compiler will throw errors if we try. What are the other restrictions around private fields? For one, you can't refer to a private field you didn't previously define. You might be used to inventing new fields on the fly in JavaScript, but that just won't fly with private fields. You can't define the same name twice in a single class, and you can't delete private fields. There is another limitation: you can't declare private fields or methods via object literals . You might be used to something like this: If you try to include a private class feature when doing this, an error will be thrown. On the other hand, you can have static private fields, for things you want to be both private and set in stone at construction. Just like private fields, private methods are marked with a leading # and cannot be accessed from outside their class. They're useful when you have something complex that the class needs to do internally, but it's something that no other part of the code should be allowed to call. For example, imagine creating HTML custom elements that should do something somewhat complicated when clicked/tapped/otherwise activated. Furthermore, the somewhat complicated things that happen when the element is clicked should be restricted to this class, because no other part of the JavaScript will (or should) ever access it. Therefore, something like: This can also be done for getters and setters, which is useful in any situation where you want to only get or set things from within the same class. As with fields and methods, prefix the name of the getter/setter with # . In this case, pretty much every field and method is private to the class. Thus, it presents an interface to the rest of the code that's essentially just like a built-in HTML element. No other part of the JavaScript has the power to affect any of its internals. JavaScript code will throw if you attempt to access a private method or field that does not exist (this differs from a normal/public method, which will return undefined ). If you need to write code to test whether a private feature has been defined you might use try / catch , but it is more compact to use the in operator. This returns true or false depending on whether or not the property is defined. The code below demonstrates the approach using the example of a class for adding Scalar values. The class uses the in operator to check that added objects have the #total private class field, and throws an informative exception message if a different type of object is passed. BCD tables only load in the browser Last modified: Aug 23, 2022 , by MDN contributors Your blueprint for a better internet. Visit Mozilla Corporation’s not-for-profit parent, the Mozilla Foundation . Portions of this content are ©1998– 2022 by individual mozilla.org contributors. Content available under a Creative Commons license . class Circle { #radius; constructor(value) { this. #radius = value; } get area() { return Math.PI * Math.pow(this. #radius, 2); } } let circle = new Circle( 10 ); console .log(circle.area); // 314.1592653589793 class Circle { #radius = 0; constructor(radius) { this.radius = radius; } get area() { return Math.PI * Math.pow(this.radius, 2 ); } set radius(value) { if (typeof value === 'number' && value > 0 ) { this. #radius = value; } else { throw 'The radius must be a positive number' ; } } get radius() { return this. #radius; } } class Cylinder extends Circle { #height; constructor(radius, height) { super(radius); this. #height = height; // cannot access the #radius of the Circle class here } } class Circle { #radius = 0; constructor(radius) { this.radius = radius; } get area() { return Math.PI * Math.pow(this.radius, 2 ); } set radius(value) { if (typeof value === 'number' && value > 0 ) { this. #radius = value; } else { throw 'The radius must be a positive number' ; } } get radius() { return this. #radius; } static hasRadius(circle) { return #radius in circle; } } let circle = new Circle( 10 ); console.log(Circle.hasRadius(circle)); class Circle { #radius = 0; static #count = 0; constructor(radius) { this.radius = radius; Circle. #count++; } get area() { return Math.PI * Math.pow(this.radius, 2 ); } set radius(value) { if (typeof value === 'number' && value > 0 ) { this. #radius = value; } else { throw 'The radius must be a positive number' ; } } get radius() { return this. #radius; } static hasRadius(circle) { return #radius in circle; } static getCount() { return Circle. #count; } } let circles = [ new Circle( 10 ), new Circle( 20 ), new Circle( 30 )]; console.log(Circle.getCount()); static getCount() { return Circle. #count; } let circles = [ new Circle( 10 ), new Circle( 20 ), new Circle( 30 )]; console .log(Circle.getCount()); The JavaScript Tutorial website helps you learn JavaScript programming from scratch quickly and effectively. Home » JavaScript Tutorial » JavaScript Private Fields Summary : in this tutorial, you’ll learn about JavaScript private fields and how to use them effectively. ES2022 allows you to define private fields for a class . To define a private field, you prefix the field name with the # sign. For example, the following defines the Circle class with a private field radius : The following creates a new instance of the Circle class and calculates its area: Because the #radius is a private field, you can only access it inside the Circle class. In other words, the #radius field is invisible outside of the Circle class. The following redefines the Circle class by adding the radius getter and setter to provide access to the #radius private field: Private fields are only accessible inside the class where they’re defined. Also, they’re not accessible from the subclasses. For example, the following defines the Cylinder class that extends the Circle class: If you attempt to access the #radius private field in the Cylinder class, you’ll get a SyntaxError . To check if an object has a private field inside a class, you use the in operator: For example, the following adds the hasRadius() static method to the Circle class that uses the in operator to check if the circle object has the #radius private field: The following example shows how to use use a static private field: First, add a private static field #count to the Circle class and initialize its value to zero: Second, increase the #count by one in the constructor: Third, define a static method that returns the value of the #count private static field: Finally, create three instances of the Circle class and output the count value to the console: Copyright © 2022 by JavaScript Tutorial Website. All Right Reserved. 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Asked 13 years, 11 months ago 42.6k 22 22 gold badges 194 194 silver badges 162 162 bronze badges 7,957 5 5 gold badges 29 29 silver badges 25 25 bronze badges Highest score (default) Trending (recent votes count more) Date modified (newest first) Date created (oldest first) 3,894 1 1 gold badge 17 17 silver badges 26 26 bronze badges 26.8k 13 13 gold badges 65 65 silver badges 85 85 bronze badges 29k 22 22 gold badges 105 105 silver badges 138 138 bronze badges 26.7k 12 12 gold badges 74 74 silver badges 72 72 bronze badges 10k 10 10 gold badges 48 48 silver badges 60 60 bronze badges 2,604 1 1 gold badge 30 30 silver badges 39 39 bronze badges 2,687 4 4 gold badges 18 18 silver badges 26 26 bronze badges 8,322 1 1 gold badge 46 46 silver badges 34 34 bronze badges 4,457 2 2 gold badges 32 32 silver badges 34 34 bronze badges var Restaurant = function() { var totalfoodcount = 0; // Private class property var totalrestroomcount = 0; // Private class property var Restaurant = function(name){ var foodcount = 0; // Private instance property var restroomcount = 0; // Private instance property this.name = name this.incrementFoodCount = function() { foodcount++; totalfoodcount++; this.printStatus(); }; this.incrementRestroomCount = function() { restroomcount++; totalrestroomcount++; this.printStatus(); }; this.getRestroomCount = function() { return restroomcount; }, this.getFoodCount = function() { return foodcount; } }; Restaurant.prototype = { name : '', buy_food : function(){ this.incrementFoodCount(); }, use_restroom : function(){ this.incrementRestroomCount(); }, getTotalRestroomCount : function() { return totalrestroomcount; }, getTotalFoodCount : function() { return totalfoodcount; }, printStatus : function() { document.body.innerHTML += 'Buying food at '+this.name+'

'
+ ''
+ 'Restroom count at ' + this.name + ' : '+ this.getRestroomCount() + '
'
+ 'Food count at ' + this.name + ' : ' + this.getFoodCount() + '
'
+ 'Total restroom count : '+ this.getTotalRestroomCount() + '
'
+ 'Total food count : '+ this.getTotalFoodCount() + '
'
+ '

';
}
};

return { // Singleton public properties
create : function(name) {
return new Restaurant(name);
},
printStatus : function() {
document.body.innerHTML
+= '

' + 'Overview

'
+ ''
+ ''
+ ''
+ ''
+ '

';
}
};
}();

var Wendys = Restaurant.create("Wendy's");
var McDonalds = Restaurant.create("McDonald's");
var KFC = Restaurant.create("KFC");
var BurgerKing = Restaurant.create("Burger King");

Restaurant.printStatus();

Wendys.buy_food();
Wendys.use_restroom();
KFC.use_restroom();
KFC.use_restroom();
Wendys.use_restroom();
McDonalds.buy_food();
BurgerKing.buy_food();

Restaurant.printStatus();

BurgerKing.buy_food();
Wendys.use_restroom();
McDonalds.buy_food();
KFC.buy_food();
Wendys.buy_food();
BurgerKing.buy_food();
McDonalds.buy_food();

Restaurant.printStatus();

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Js Private

' + 'Overview

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