JavaScript private methods - javascript

To make a JavaScript class with a public method I'd do something like:
function Restaurant() {}
Restaurant.prototype.buy_food = function(){
// something here
}
Restaurant.prototype.use_restroom = function(){
// something here
}
That way users of my class can:
var restaurant = new Restaurant();
restaurant.buy_food();
restaurant.use_restroom();
How do I create a private method that can be called by the buy_food and use_restroom methods but not externally by users of the class?
In other words, I want my method implementation to be able to do:
Restaurant.prototype.use_restroom = function() {
this.private_stuff();
}
But this shouldn't work:
var r = new Restaurant();
r.private_stuff();
How do I define private_stuff as a private method so both of these hold true?
I've read Doug Crockford's writeup a few times but it doesn't seem like "private" methods can be called by public methods and "privileged" methods can be called externally.

You can do it, but the downside is that it can't be part of the prototype:
function Restaurant() {
var myPrivateVar;
var private_stuff = function() { // Only visible inside Restaurant()
myPrivateVar = "I can set this here!";
}
this.use_restroom = function() { // use_restroom is visible to all
private_stuff();
}
this.buy_food = function() { // buy_food is visible to all
private_stuff();
}
}

Using self invoking function and call
JavaScript uses prototypes and does't have classes (or methods for that matter) like Object Oriented languages. A JavaScript developer need to think in JavaScript.
Wikipedia quote:
Unlike many object-oriented languages, there is no distinction between
a function definition and a method definition. Rather, the distinction
occurs during function calling; when a function is called as a method
of an object, the function's local this keyword is bound to that
object for that invocation.
Solution using a self invoking function and the call function to call the private "method" :
var MyObject = (function () {
// Constructor
function MyObject(foo) {
this._foo = foo;
}
function privateFun(prefix) {
return prefix + this._foo;
}
MyObject.prototype.publicFun = function () {
return privateFun.call(this, ">>");
}
return MyObject;
}());
var myObject = new MyObject("bar");
myObject.publicFun(); // Returns ">>bar"
myObject.privateFun(">>"); // ReferenceError: private is not defined
The call function allows us to call the private function with the appropriate context (this).
Simpler with Node.js
If you are using Node.js, you don't need the IIFE because you can take advantage of the module loading system:
function MyObject(foo) {
this._foo = foo;
}
function privateFun(prefix) {
return prefix + this._foo;
}
MyObject.prototype.publicFun = function () {
return privateFun.call(this, ">>");
}
module.exports= MyObject;
Load the file:
var MyObject = require("./MyObject");
var myObject = new MyObject("bar");
myObject.publicFun(); // Returns ">>bar"
myObject.privateFun(">>"); // ReferenceError: private is not defined
(new!) Native private methods in future JavaScript versions
TC39 private methods and getter/setters for JavaScript classes proposal is stage 3. That means any time soon, JavaScript will implement private methods natively!
Note that JavaScript private class fields already exists in modern JavaScript versions.
Here is an example of how it is used:
class MyObject {
// Private field
#foo;
constructor(foo) {
this.#foo = foo;
}
#privateFun(prefix) {
return prefix + this.#foo;
}
publicFun() {
return this.#privateFun(">>");
}
}
You may need a JavaScript transpiler/compiler to run this code on old JavaScript engines.
PS: If you wonder why the # prefix, read this.
(deprecated) ES7 with the Bind Operator
Warning: The bind operator TC39 proposition is near dead https://github.com/tc39/proposal-bind-operator/issues/53#issuecomment-374271822
The bind operator :: is an ECMAScript proposal and is implemented in Babel (stage 0).
export default class MyObject {
constructor (foo) {
this._foo = foo;
}
publicFun () {
return this::privateFun(">>");
}
}
function privateFun (prefix) {
return prefix + this._foo;
}

You can simulate private methods like this:
function Restaurant() {
}
Restaurant.prototype = (function() {
var private_stuff = function() {
// Private code here
};
return {
constructor:Restaurant,
use_restroom:function() {
private_stuff();
}
};
})();
var r = new Restaurant();
// This will work:
r.use_restroom();
// This will cause an error:
r.private_stuff();
More information on this technique here: http://webreflection.blogspot.com/2008/04/natural-javascript-private-methods.html

In these situations when you have a public API, and you would like private and public methods/properties, I always use the Module Pattern. This pattern was made popular within the YUI library, and the details can be found here:
http://yuiblog.com/blog/2007/06/12/module-pattern/
It is really straightforward, and easy for other developers to comprehend. For a simple example:
var MYLIB = function() {
var aPrivateProperty = true;
var aPrivateMethod = function() {
// some code here...
};
return {
aPublicMethod : function() {
aPrivateMethod(); // okay
// some code here...
},
aPublicProperty : true
};
}();
MYLIB.aPrivateMethod() // not okay
MYLIB.aPublicMethod() // okay

Here is the class which I created to understand what Douglas Crockford's has suggested in his site Private Members in JavaScript
function Employee(id, name) { //Constructor
//Public member variables
this.id = id;
this.name = name;
//Private member variables
var fName;
var lName;
var that = this;
//By convention, we create a private variable 'that'. This is used to
//make the object available to the private methods.
//Private function
function setFName(pfname) {
fName = pfname;
alert('setFName called');
}
//Privileged function
this.setLName = function (plName, pfname) {
lName = plName; //Has access to private variables
setFName(pfname); //Has access to private function
alert('setLName called ' + this.id); //Has access to member variables
}
//Another privileged member has access to both member variables and private variables
//Note access of this.dataOfBirth created by public member setDateOfBirth
this.toString = function () {
return 'toString called ' + this.id + ' ' + this.name + ' ' + fName + ' ' + lName + ' ' + this.dataOfBirth;
}
}
//Public function has access to member variable and can create on too but does not have access to private variable
Employee.prototype.setDateOfBirth = function (dob) {
alert('setDateOfBirth called ' + this.id);
this.dataOfBirth = dob; //Creates new public member note this is accessed by toString
//alert(fName); //Does not have access to private member
}
$(document).ready()
{
var employee = new Employee(5, 'Shyam'); //Create a new object and initialize it with constructor
employee.setLName('Bhaskar', 'Ram'); //Call privileged function
employee.setDateOfBirth('1/1/2000'); //Call public function
employee.id = 9; //Set up member value
//employee.setFName('Ram'); //can not call Private Privileged method
alert(employee.toString()); //See the changed object
}

ES12 Private Methods
You can do this now with es12 private methods. You just need to add a # before the method name.
class ClassWithPrivateMethod {
#privateMethod() {
return 'hello world';
}
getPrivateMessage() {
return #privateMethod();
}
}

I conjured up this: EDIT: Actually, someone has linked to a identical solution. Duh!
var Car = function() {
}
Car.prototype = (function() {
var hotWire = function() {
// Private code *with* access to public properties through 'this'
alert( this.drive() ); // Alerts 'Vroom!'
}
return {
steal: function() {
hotWire.call( this ); // Call a private method
},
drive: function() {
return 'Vroom!';
}
};
})();
var getAwayVechile = new Car();
hotWire(); // Not allowed
getAwayVechile.hotWire(); // Not allowed
getAwayVechile.steal(); // Alerts 'Vroom!'

ES2021 / ES12 - Private Methods
Private method names start with a hash # prefix and can be accessed only inside the class where it is defined.
class Restaurant {
// private method
#private_stuff() {
console.log("private stuff");
}
// public method
buy_food() {
this.#private_stuff();
}
};
const restaurant = new Restaurant();
restaurant.buy_food(); // "private stuff";
restaurant.private_stuff(); // Uncaught TypeError: restaurant.private_stuff is not a function

I think such questions come up again and again because of the lack of understanding of the closures. Сlosures is most important thing in JS. Every JS programmer have to feel the essence of it.
1. First of all we need to make separate scope (closure).
function () {
}
2. In this area, we can do whatever we want. And no one will know about it.
function () {
var name,
secretSkills = {
pizza: function () { return new Pizza() },
sushi: function () { return new Sushi() }
}
function Restaurant(_name) {
name = _name
}
Restaurant.prototype.getFood = function (name) {
return name in secretSkills ? secretSkills[name]() : null
}
}
3. For the world to know about our restaurant class,
we have to return it from the closure.
var Restaurant = (function () {
// Restaurant definition
return Restaurant
})()
4. At the end, we have:
var Restaurant = (function () {
var name,
secretSkills = {
pizza: function () { return new Pizza() },
sushi: function () { return new Sushi() }
}
function Restaurant(_name) {
name = _name
}
Restaurant.prototype.getFood = function (name) {
return name in secretSkills ? secretSkills[name]() : null
}
return Restaurant
})()
5. Also, this approach has potential for inheritance and templating
// Abstract class
function AbstractRestaurant(skills) {
var name
function Restaurant(_name) {
name = _name
}
Restaurant.prototype.getFood = function (name) {
return skills && name in skills ? skills[name]() : null
}
return Restaurant
}
// Concrete classes
SushiRestaurant = AbstractRestaurant({
sushi: function() { return new Sushi() }
})
PizzaRestaurant = AbstractRestaurant({
pizza: function() { return new Pizza() }
})
var r1 = new SushiRestaurant('Yo! Sushi'),
r2 = new PizzaRestaurant('Dominos Pizza')
r1.getFood('sushi')
r2.getFood('pizza')
I hope this helps someone better understand this subject

Personally, I prefer the following pattern for creating classes in JavaScript :
var myClass = (function() {
// Private class properties go here
var blueprint = function() {
// Private instance properties go here
...
};
blueprint.prototype = {
// Public class properties go here
...
};
return {
// Public class properties go here
create : function() { return new blueprint(); }
...
};
})();
As you can see, it allows you to define both class properties and instance properties, each of which can be public and private.
Demo
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
+= '<h3>Buying food at '+this.name+'</h3>'
+ '<ul>'
+ '<li>Restroom count at ' + this.name + ' : '+ this.getRestroomCount() + '</li>'
+ '<li>Food count at ' + this.name + ' : ' + this.getFoodCount() + '</li>'
+ '<li>Total restroom count : '+ this.getTotalRestroomCount() + '</li>'
+ '<li>Total food count : '+ this.getTotalFoodCount() + '</li>'
+ '</ul>';
}
};
return { // Singleton public properties
create : function(name) {
return new Restaurant(name);
},
printStatus : function() {
document.body.innerHTML
+= '<hr />'
+ '<h3>Overview</h3>'
+ '<ul>'
+ '<li>Total restroom count : '+ Restaurant.prototype.getTotalRestroomCount() + '</li>'
+ '<li>Total food count : '+ Restaurant.prototype.getTotalFoodCount() + '</li>'
+ '</ul>'
+ '<hr />';
}
};
}();
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();
See also this Fiddle.

All of this closure will cost you. Make sure you test the speed implications especially in IE. You will find you are better off with a naming convention. There are still a lot of corporate web users out there that are forced to use IE6...

Don't be so verbose. It's Javascript. Use a Naming Convention.
After years of working in es6 classes, I recently started work on an es5 project (using requireJS which is already very verbose-looking). I've been over and over all the strategies mentioned here and it all basically boils down to use a naming convention:
Javascript doesn't have scope keywords like private. Other developers entering Javascript will know this upfront. Therefore, a simple naming convention is more than sufficient. A simple naming convention of prefixing with an underscore solves the problem of both private properties and private methods.
Let's take advantage of the Prototype for speed reasons, but lets not get anymore verbose than that. Let's try to keep the es5 "class" looking as closely to what we might expect in other backend languages (and treat every file as a class, even if we don't need to return an instance).
Let's demonstrate with a more realistic module situation (we'll use old es5 and old requireJs).
my-tooltip.js
define([
'tooltip'
],
function(
tooltip
){
function MyTooltip() {
// Later, if needed, we can remove the underscore on some
// of these (make public) and allow clients of our class
// to set them.
this._selector = "#my-tooltip"
this._template = 'Hello from inside my tooltip!';
this._initTooltip();
}
MyTooltip.prototype = {
constructor: MyTooltip,
_initTooltip: function () {
new tooltip.tooltip(this._selector, {
content: this._template,
closeOnClick: true,
closeButton: true
});
}
}
return {
init: function init() {
new MyTooltip(); // <-- Our constructor adds our tooltip to the DOM so not much we need to do after instantiation.
}
// You could instead return a new instantiation,
// if later you do more with this class.
/*
create: function create() {
return new MyTooltip();
}
*/
}
});

Take any of the solutions that follow Crockford's private or priviledged pattern. For example:
function Foo(x) {
var y = 5;
var bar = function() {
return y * x;
};
this.public = function(z) {
return bar() + x * z;
};
}
In any case where the attacker has no "execute" right on the JS context he has no way of accessing any "public" or "private" fields or methods. In case the attacker does have that access he can execute this one-liner:
eval("Foo = " + Foo.toString().replace(
/{/, "{ this.eval = function(code) { return eval(code); }; "
));
Note that the above code is generic to all constructor-type-privacy. It will fail with some of the solutions here but it should be clear that pretty much all of the closure based solutions can be broken like this with different replace() parameters.
After this is executed any object created with new Foo() is going to have an eval method which can be called to return or change values or methods defined in the constructor's closure, e.g.:
f = new Foo(99);
f.eval("x");
f.eval("y");
f.eval("x = 8");
The only problem I can see with this that it won't work for cases where there is only one instance and it's created on load. But then there is no reason to actually define a prototype and in that case the attacker can simply recreate the object instead of the constructor as long as he has a way of passing the same parameters (e.g. they are constant or calculated from available values).
In my opinion, this pretty much makes Crockford's solution useless. Since the "privacy" is easily broken the downsides of his solution (reduced readability & maintainability, decreased performance, increased memory) makes the "no privacy" prototype based method the better choice.
I do usually use leading underscores to mark __private and _protected methods and fields (Perl style), but the idea of having privacy in JavaScript just shows how it's a misunderstood language.
Therefore I disagree with Crockford except for his first sentence.
So how do you get real privacy in JS? Put everything that is required to be private on the server side and use JS to do AJAX calls.

The apotheosis of the Module Pattern: The Revealing Module Pattern
A neat little extension to a very robust pattern.

If you want the full range of public and private functions with the ability for public functions to access private functions, layout code for an object like this:
function MyObject(arg1, arg2, ...) {
//constructor code using constructor arguments...
//create/access public variables as
// this.var1 = foo;
//private variables
var v1;
var v2;
//private functions
function privateOne() {
}
function privateTwon() {
}
//public functions
MyObject.prototype.publicOne = function () {
};
MyObject.prototype.publicTwo = function () {
};
}

var TestClass = function( ) {
var privateProperty = 42;
function privateMethod( ) {
alert( "privateMethod, " + privateProperty );
}
this.public = {
constructor: TestClass,
publicProperty: 88,
publicMethod: function( ) {
alert( "publicMethod" );
privateMethod( );
}
};
};
TestClass.prototype = new TestClass( ).public;
var myTestClass = new TestClass( );
alert( myTestClass.publicProperty );
myTestClass.publicMethod( );
alert( myTestClass.privateMethod || "no privateMethod" );
Similar to georgebrock but a little less verbose (IMHO)
Any problems with doing it this way? (I haven't seen it anywhere)
edit: I realised this is kinda useless since every independent instantiation has its own copy of the public methods, thus undermining the use of the prototype.

Here's what i enjoyed the most so far regarding private/public methods/members and instantiation in javascript:
here is the article: http://www.sefol.com/?p=1090
and here is the example:
var Person = (function () {
//Immediately returns an anonymous function which builds our modules
return function (name, location) {
alert("createPerson called with " + name);
var localPrivateVar = name;
var localPublicVar = "A public variable";
var localPublicFunction = function () {
alert("PUBLIC Func called, private var is :" + localPrivateVar)
};
var localPrivateFunction = function () {
alert("PRIVATE Func called ")
};
var setName = function (name) {
localPrivateVar = name;
}
return {
publicVar: localPublicVar,
location: location,
publicFunction: localPublicFunction,
setName: setName
}
}
})();
//Request a Person instance - should print "createPerson called with ben"
var x = Person("ben", "germany");
//Request a Person instance - should print "createPerson called with candide"
var y = Person("candide", "belgium");
//Prints "ben"
x.publicFunction();
//Prints "candide"
y.publicFunction();
//Now call a public function which sets the value of a private variable in the x instance
x.setName("Ben 2");
//Shouldn't have changed this : prints "candide"
y.publicFunction();
//Should have changed this : prints "Ben 2"
x.publicFunction();
JSFiddle: http://jsfiddle.net/northkildonan/kopj3dt3/1/

The module pattern is right in most cases. But if you have thousands of instances, classes save memory. If saving memory is a concern and your objects contain a small amount of private data, but have a lot of public functions, then you'll want all public functions to live in the .prototype to save memory.
This is what I came up with:
var MyClass = (function () {
var secret = {}; // You can only getPriv() if you know this
function MyClass() {
var that = this, priv = {
foo: 0 // ... and other private values
};
that.getPriv = function (proof) {
return (proof === secret) && priv;
};
}
MyClass.prototype.inc = function () {
var priv = this.getPriv(secret);
priv.foo += 1;
return priv.foo;
};
return MyClass;
}());
var x = new MyClass();
x.inc(); // 1
x.inc(); // 2
The object priv contains private properties. It is accessible through the public function getPriv(), but this function returns false unless you pass it the secret, and this is only known inside the main closure.

What about this?
var Restaurant = (function() {
var _id = 0;
var privateVars = [];
function Restaurant(name) {
this.id = ++_id;
this.name = name;
privateVars[this.id] = {
cooked: []
};
}
Restaurant.prototype.cook = function (food) {
privateVars[this.id].cooked.push(food);
}
return Restaurant;
})();
Private variable lookup is impossible outside of the scope of the immediate function.
There is no duplication of functions, saving memory.
The downside is that the lookup of private variables is clunky privateVars[this.id].cooked is ridiculous to type. There is also an extra "id" variable.

Wrap all code in Anonymous Function: Then , all functions will be private ,ONLY functions attached to window object :
(function(w,nameSpacePrivate){
w.Person=function(name){
this.name=name;
return this;
};
w.Person.prototype.profilePublic=function(){
return nameSpacePrivate.profile.call(this);
};
nameSpacePrivate.profile=function(){
return 'My name is '+this.name;
};
})(window,{});
Use this :
var abdennour=new Person('Abdennour');
abdennour.profilePublic();
FIDDLE

I prefer to store private data in an associated WeakMap. This allows you to keep your public methods on the prototype where they belong. This seems to be the most efficient way to handle this problem for large numbers of objects.
const data = new WeakMap();
function Foo(value) {
data.set(this, {value});
}
// public method accessing private value
Foo.prototype.accessValue = function() {
return data.get(this).value;
}
// private 'method' accessing private value
function accessValue(foo) {
return data.get(foo).value;
}
export {Foo};

2021 HERE!
This polyfill effectively hides your private properties and methods returning undefined when you try to read your private property and a TypeError when you try to execute your private method thus effectively making them both PRIVATE to the outside but giving you access to them by using your public methods.
If you check it you will see it is very easy to implement. For the most part you don't need to do anything quirky like using Proxy objects, underscore functions (_myprivate), getters or setters. None of that. The only thing required is to place in your constructor that like snippet of code that is aimed to let you expose your public interface to the outside world.
((self) => ({
pubProp: self.pubProp,
// More public properties to export HERE
// ...
pubMethod: self.pubMethod.bind(self)
// More public mehods to export HERE
// Be sure bind each of them to self!!!
// ...
}))(self);
The above code is where the magic happens. It is an IIFE that returns an object with just the properties and methods you want to exposed and bound to the context of the object that was first instantiated.
You can still access your hidden properties and methods but only through your public methods just the way OOP should do.
Consider that part of the code as your module.exports
BTW, this is without using the latest ECMAScript 2022 # addition to the language.
'use strict';
class MyClass {
constructor(pubProp) {
let self = this;
self.pubProp = pubProp;
self.privProp = "I'm a private property!";
return ((self) => ({
pubProp: self.pubProp,
// More public properties to export HERE
// ...
pubMethod: self.pubMethod.bind(self)
// More public mehods to export HERE
// Be sure to bind each of them to self!!!
// ...
}))(self);
}
pubMethod() {
console.log("I'm a public method!");
console.log(this.pubProp);
return this.privMethod();
}
privMethod() {
console.log("I'm a private method!");
return this.privProp
}
}
const myObj = new MyClass("I'm a public property!");
console.log("***DUMPING MY NEW INSTANCE***");
console.dir(myObj);
console.log("");
console.log("***TESTING ACCESS TO PUBLIC PROPERTIES***");
console.log(myObj.pubProp);
console.log("");
console.log("***TESTING ACCESS TO PRIVATE PROPERTIES***");
console.log(myObj.privProp);
console.log("");
console.log("***TESTING ACCESS TO PUBLIC METHODS***");
console.log("1. pubMethod access pubProp ");
console.log("2. pubMethod calls privMethod");
console.log("3. privMethod access privProp");
console.log("")
console.log(myObj.pubMethod());
console.log("");
console.log("***TESTING ACCESS TO PRIVATE METHODS***");
console.log(myObj.privMethod());
Check my gist

Private functions cannot access the public variables using module pattern

Since everybody was posting here his own code, I'm gonna do that too...
I like Crockford because he introduced real object oriented patterns in Javascript. But he also came up with a new misunderstanding, the "that" one.
So why is he using "that = this"? It has nothing to do with private functions at all. It has to do with inner functions!
Because according to Crockford this is buggy code:
Function Foo( ) {
this.bar = 0;
var foobar=function( ) {
alert(this.bar);
}
}
So he suggested doing this:
Function Foo( ) {
this.bar = 0;
that = this;
var foobar=function( ) {
alert(that.bar);
}
}
So as I said, I'm quite sure that Crockford was wrong his explanation about that and this (but his code is certainly correct). Or was he just fooling the Javascript world, to know who is copying his code? I dunno...I'm no browser geek ;D
EDIT
Ah, that's what is all about: What does 'var that = this;' mean in JavaScript?
So Crockie was really wrong with his explanation....but right with his code, so he's still a great guy. :))

In general I added the private Object _ temporarily to the object.
You have to open the privacy exlipcitly in the "Power-constructor" for the method.
If you call the method from the prototype, you will
be able to overwrite the prototype-method
Make a public method accessible in the "Power-constructor": (ctx is the object context)
ctx.test = GD.Fabric.open('test', GD.Test.prototype, ctx, _); // is a private object
Now I have this openPrivacy:
GD.Fabric.openPrivacy = function(func, clss, ctx, _) {
return function() {
ctx._ = _;
var res = clss[func].apply(ctx, arguments);
ctx._ = null;
return res;
};
};

This is what I worked out:
Needs one class of sugar code that you can find here. Also supports protected, inheritance, virtual, static stuff...
;( function class_Restaurant( namespace )
{
'use strict';
if( namespace[ "Restaurant" ] ) return // protect against double inclusions
namespace.Restaurant = Restaurant
var Static = TidBits.OoJs.setupClass( namespace, "Restaurant" )
// constructor
//
function Restaurant()
{
this.toilets = 3
this.Private( private_stuff )
return this.Public( buy_food, use_restroom )
}
function private_stuff(){ console.log( "There are", this.toilets, "toilets available") }
function buy_food (){ return "food" }
function use_restroom (){ this.private_stuff() }
})( window )
var chinese = new Restaurant
console.log( chinese.buy_food() ); // output: food
console.log( chinese.use_restroom() ); // output: There are 3 toilets available
console.log( chinese.toilets ); // output: undefined
console.log( chinese.private_stuff() ); // output: undefined
// and throws: TypeError: Object #<Restaurant> has no method 'private_stuff'

Class({
Namespace:ABC,
Name:"ClassL2",
Bases:[ABC.ClassTop],
Private:{
m_var:2
},
Protected:{
proval:2,
fight:Property(function(){
this.m_var--;
console.log("ClassL2::fight (m_var)" +this.m_var);
},[Property.Type.Virtual])
},
Public:{
Fight:function(){
console.log("ClassL2::Fight (m_var)"+this.m_var);
this.fight();
}
}
});
https://github.com/nooning/JSClass

I have created a new tool to allow you to have true private methods on the prototype
https://github.com/TremayneChrist/ProtectJS
Example:
var MyObject = (function () {
// Create the object
function MyObject() {}
// Add methods to the prototype
MyObject.prototype = {
// This is our public method
public: function () {
console.log('PUBLIC method has been called');
},
// This is our private method, using (_)
_private: function () {
console.log('PRIVATE method has been called');
}
}
return protect(MyObject);
})();
// Create an instance of the object
var mo = new MyObject();
// Call its methods
mo.public(); // Pass
mo._private(); // Fail

You have to put a closure around your actual constructor-function, where you can define your private methods.
To change data of the instances through these private methods, you have to give them "this" with them, either as an function argument or by calling this function with .apply(this) :
var Restaurant = (function(){
var private_buy_food = function(that){
that.data.soldFood = true;
}
var private_take_a_shit = function(){
this.data.isdirty = true;
}
// New Closure
function restaurant()
{
this.data = {
isdirty : false,
soldFood: false,
};
}
restaurant.prototype.buy_food = function()
{
private_buy_food(this);
}
restaurant.prototype.use_restroom = function()
{
private_take_a_shit.call(this);
}
return restaurant;
})()
// TEST:
var McDonalds = new Restaurant();
McDonalds.buy_food();
McDonalds.use_restroom();
console.log(McDonalds);
console.log(McDonalds.__proto__);

I know it's a bit too late but how about this?
var obj = function(){
var pr = "private";
var prt = Object.getPrototypeOf(this);
if(!prt.hasOwnProperty("showPrivate")){
prt.showPrivate = function(){
console.log(pr);
}
}
}
var i = new obj();
i.showPrivate();
console.log(i.hasOwnProperty("pr"));

Related

How can I make an object variable in javascript private? [duplicate]

Is it possible to create private properties in ES6 classes?
Here's an example.
How can I prevent access to instance.property?
class Something {
constructor(){
this.property = "test";
}
}
var instance = new Something();
console.log(instance.property); //=> "test"
Private class features is now supported by the majority of browsers.
class Something {
#property;
constructor(){
this.#property = "test";
}
#privateMethod() {
return 'hello world';
}
getPrivateMessage() {
return this.#property;
}
}
const instance = new Something();
console.log(instance.property); //=> undefined
console.log(instance.privateMethod); //=> undefined
console.log(instance.getPrivateMessage()); //=> test
console.log(instance.#property); //=> Syntax error
Update: See others answer, this is outdated.
Short answer, no, there is no native support for private properties with ES6 classes.
But you could mimic that behaviour by not attaching the new properties to the object, but keeping them inside a class constructor, and use getters and setters to reach the hidden properties. Note that the getters and setters gets redefine on each new instance of the class.
ES6
class Person {
constructor(name) {
var _name = name
this.setName = function(name) { _name = name; }
this.getName = function() { return _name; }
}
}
ES5
function Person(name) {
var _name = name
this.setName = function(name) { _name = name; }
this.getName = function() { return _name; }
}
Yes, prefix the name with # and include it in the class definition, not just the constructor.
MDN Docs
Real private properties were finally added in ES2022. As of 2023-01-01, private properties (fields and methods) have been supported in all major browsers for at least a year, but 5-10% of users are still on older browsers [Can I Use].
Example:
class Person {
#age
constructor(name) {
this.name = name; // this is public
this.#age = 20; // this is private
}
greet() {
// here we can access both name and age
console.log(`name: ${this.name}, age: ${this.#age}`);
}
}
let joe = new Person('Joe');
joe.greet();
// here we can access name but not age
Following are methods for keeping properties private in pre-ES2022 environments, with various tradeoffs.
Scoped variables
The approach here is to use the scope of the constructor function, which is private, to store private data. For methods to have access to this private data they must be created within the constructor as well, meaning you're recreating them with every instance. This is a performance and memory penalty, but it may be acceptable. The penalty can be avoided for methods that do not need access to private data by declaring them in the normal way.
Example:
class Person {
constructor(name) {
let age = 20; // this is private
this.name = name; // this is public
this.greet = () => {
// here we can access both name and age
console.log(`name: ${this.name}, age: ${age}`);
};
}
anotherMethod() {
// here we can access name but not age
}
}
let joe = new Person('Joe');
joe.greet();
// here we can access name but not age
Scoped WeakMap
A WeakMap can be used to improve the performance of the above approach, in exchange for even more clutter. WeakMaps associate data with Objects (here, class instances) in such a way that it can only be accessed using that WeakMap. So, we use the scoped variables method to create a private WeakMap, then use that WeakMap to retrieve private data associated with this. This is faster than the scoped variables method because all your instances can share a single WeakMap, so you don't need to recreate methods just to make them access their own WeakMaps.
Example:
let Person = (function () {
let privateProps = new WeakMap();
return class Person {
constructor(name) {
this.name = name; // this is public
privateProps.set(this, {age: 20}); // this is private
}
greet() {
// Here we can access both name and age
console.log(`name: ${this.name}, age: ${privateProps.get(this).age}`);
}
};
})();
let joe = new Person('Joe');
joe.greet();
// here we can access name but not age
This example uses a WeakMap with Object keys to use one WeakMap for multiple private properties; you could also use multiple WeakMaps and use them like privateAge.set(this, 20), or write a small wrapper and use it another way, like privateProps.set(this, 'age', 0).
The privacy of this approach could theoretically be breached by tampering with the global WeakMap object. That said, all JavaScript can be broken by mangled globals.
(This method could also be done with Map, but WeakMap is better because Map will create memory leaks unless you're very careful, and for this purpose the two aren't otherwise different.)
Half-Answer: Scoped Symbols
A Symbol is a type of primitive value that can serve as a property name instead of a string. You can use the scoped variable method to create a private Symbol, then store private data at this[mySymbol].
The privacy of this method can be breached using Object.getOwnPropertySymbols, but is somewhat awkward to do.
Example:
let Person = (() => {
let ageKey = Symbol();
return class Person {
constructor(name) {
this.name = name; // this is public
this[ageKey] = 20; // this is intended to be private
}
greet() {
// Here we can access both name and age
console.log(`name: ${this.name}, age: ${this[ageKey]}`);
}
}
})();
let joe = new Person('Joe');
joe.greet();
// Here we can access joe's name and, with a little effort, age. We can’t
// access ageKey directly, but we can obtain it by listing all Symbol
// properties on `joe` with `Object.getOwnPropertySymbols(joe)`.
Note that making a property non-enumerable using Object.defineProperty does not prevent it from being included in Object.getOwnPropertySymbols.
Half-Answer: Underscores
The old convention is to just use a public property with an underscore prefix. This does not keep it private, but it does do a good job of communicating to readers that they should treat it as private, which often gets the job done. In exchange for this, we get an approach that's easier to read, easier to type, and faster than the other workarounds.
Example:
class Person {
constructor(name) {
this.name = name; // this is public
this._age = 20; // this is intended to be private
}
greet() {
// Here we can access both name and age
console.log(`name: ${this.name}, age: ${this._age}`);
}
}
let joe = new Person('Joe');
joe.greet();
// Here we can access both joe's name and age. But we know we aren't
// supposed to access his age, which just might stop us.
Summary
ES2022: great but not yet supported by all visitors
Scoped variables: private, slower, awkward
Scoped WeakMaps: hackable, awkward
Scoped Symbols: enumerable and hackable, somewhat awkward
Underscores: just a request for privacy, no other downsides
Update: A proposal with nicer syntax is on its way. Contributions are welcome.
Yes, there is - for scoped access in objects - ES6 introduces Symbols.
Symbols are unique, you can't gain access to one from the outside except with reflection (like privates in Java/C#) but anyone who has access to a symbol on the inside can use it for key access:
var property = Symbol();
class Something {
constructor(){
this[property] = "test";
}
}
var instance = new Something();
console.log(instance.property); //=> undefined, can only access with access to the Symbol
The answer is "No". But you can create private access to properties like this:
Use modules. Everything in a module is private unless it's made public by using the export keyword.
Inside modules, use function closure: http://www.kirupa.com/html5/closures_in_javascript.htm
(The suggestion that Symbols could be used to ensure privacy was true in an earlier version of the ES6 spec but is no longer the case:https://mail.mozilla.org/pipermail/es-discuss/2014-January/035604.html and https://stackoverflow.com/a/22280202/1282216. For a longer discussion about Symbols and privacy see: https://curiosity-driven.org/private-properties-in-javascript)
The only way to get true privacy in JS is through scoping, so there is no way to have a property that is a member of this that will be accessible only inside the component. The best way to store truly private data in ES6 is with a WeakMap.
const privateProp1 = new WeakMap();
const privateProp2 = new WeakMap();
class SomeClass {
constructor() {
privateProp1.set(this, "I am Private1");
privateProp2.set(this, "I am Private2");
this.publicVar = "I am public";
this.publicMethod = () => {
console.log(privateProp1.get(this), privateProp2.get(this))
};
}
printPrivate() {
console.log(privateProp1.get(this));
}
}
Obviously this is a probably slow, and definitely ugly, but it does provide privacy.
Keep in mind that EVEN THIS isn't perfect, because Javascript is so dynamic. Someone could still do
var oldSet = WeakMap.prototype.set;
WeakMap.prototype.set = function(key, value){
// Store 'this', 'key', and 'value'
return oldSet.call(this, key, value);
};
to catch values as they are stored, so if you wanted to be extra careful, you'd need to capture a local reference to .set and .get to use explicitly instead of relying on the overridable prototype.
const {set: WMSet, get: WMGet} = WeakMap.prototype;
const privateProp1 = new WeakMap();
const privateProp2 = new WeakMap();
class SomeClass {
constructor() {
WMSet.call(privateProp1, this, "I am Private1");
WMSet.call(privateProp2, this, "I am Private2");
this.publicVar = "I am public";
this.publicMethod = () => {
console.log(WMGet.call(privateProp1, this), WMGet.call(privateProp2, this))
};
}
printPrivate() {
console.log(WMGet.call(privateProp1, this));
}
}
For future reference of other on lookers, I'm hearing now that the recommendation is to use WeakMaps to hold private data.
Here is a more clear, working example:
function storePrivateProperties(a, b, c, d) {
let privateData = new WeakMap;
// unique object as key, weak map can only accept object as key, when key is no longer referened, garbage collector claims the key-value
let keyA = {}, keyB = {}, keyC = {}, keyD = {};
privateData.set(keyA, a);
privateData.set(keyB, b);
privateData.set(keyC, c);
privateData.set(keyD, d);
return {
logPrivateKey(key) {
switch(key) {
case "a":
console.log(privateData.get(keyA));
break;
case "b":
console.log(privateData.get(keyB));
break;
case "c":
console.log(privateData.get(keyC));
break;
case "d":
console.log(privateData.set(keyD));
break;
default:
console.log(`There is no value for ${key}`)
}
}
}
}
Depends on whom you ask :-)
No private property modifier is included in the Maximally minimal classes proposal which seems to have made it into the current draft.
However, there might be support for private names, which does allow private properties - and they probably could be used in class definitions as well.
Using ES6 modules (initially proposed by #d13) works well for me. It doesn't mimic private properties perfectly, but at least you can be confident that properties that should be private won't leak outside of your class. Here's an example:
something.js
let _message = null;
const _greet = name => {
console.log('Hello ' + name);
};
export default class Something {
constructor(message) {
_message = message;
}
say() {
console.log(_message);
_greet('Bob');
}
};
Then the consuming code can look like this:
import Something from './something.js';
const something = new Something('Sunny day!');
something.say();
something._message; // undefined
something._greet(); // exception
Update (Important):
As #DanyalAytekin outlined in the comments, these private properties are static, so therefore global in scope. They will work well when working with Singletons, but care must be taken for Transient objects. Extending the example above:
import Something from './something.js';
import Something2 from './something.js';
const a = new Something('a');
a.say(); // a
const b = new Something('b');
b.say(); // b
const c = new Something2('c');
c.say(); // c
a.say(); // c
b.say(); // c
c.say(); // c
Yes - you can create encapsulated property, but it's not been done with access modifiers (public|private) at least not with ES6.
Here is a simple example how it can be done with ES6:
1 Create class using class word
2 Inside it's constructor declare block-scoped variable using let OR const reserved words -> since they are block-scope they cannot be accessed from outside (encapsulated)
3 To allow some access control (setters|getters) to those variables you can declare instance method inside it's constructor using: this.methodName=function(){} syntax
"use strict";
class Something{
constructor(){
//private property
let property="test";
//private final (immutable) property
const property2="test2";
//public getter
this.getProperty2=function(){
return property2;
}
//public getter
this.getProperty=function(){
return property;
}
//public setter
this.setProperty=function(prop){
property=prop;
}
}
}
Now lets check it:
var s=new Something();
console.log(typeof s.property);//undefined
s.setProperty("another");//set to encapsulated `property`
console.log(s.getProperty());//get encapsulated `property` value
console.log(s.getProperty2());//get encapsulated immutable `property2` value
Completing #d13 and the comments by #johnny-oshika and #DanyalAytekin:
I guess in the example provided by #johnny-oshika we could use normal functions instead of arrow functions and then .bind them with the current object plus a _privates object as a curried parameter:
something.js
function _greet(_privates) {
return 'Hello ' + _privates.message;
}
function _updateMessage(_privates, newMessage) {
_privates.message = newMessage;
}
export default class Something {
constructor(message) {
const _privates = {
message
};
this.say = _greet.bind(this, _privates);
this.updateMessage = _updateMessage.bind(this, _privates);
}
}
main.js
import Something from './something.js';
const something = new Something('Sunny day!');
const message1 = something.say();
something.updateMessage('Cloudy day!');
const message2 = something.say();
console.log(message1 === 'Hello Sunny day!'); // true
console.log(message2 === 'Hello Cloudy day!'); // true
// the followings are not public
console.log(something._greet === undefined); // true
console.log(something._privates === undefined); // true
console.log(something._updateMessage === undefined); // true
// another instance which doesn't share the _privates
const something2 = new Something('another Sunny day!');
const message3 = something2.say();
console.log(message3 === 'Hello another Sunny day!'); // true
Benefits I can think of:
we can have private methods (_greet and _updateMessage act like private methods as long as we don't export the references)
although they're not on the prototype, the above mentioned methods will save memory because the instances are created once, outside the class (as opposed to defining them in the constructor)
we don't leak any globals since we're inside a module
we can also have private properties using the binded _privates object
Some drawbacks I can think of:
less intuitive
mixed usage of class syntax and old school patterns (object bindings, module/function scoped variables)
hard bindings - we can't rebind the public methods (although we can improve this by using soft bindings (https://github.com/getify/You-Dont-Know-JS/blob/master/this%20%26%20object%20prototypes/ch2.md#softening-binding))
A running snippet can be found here: http://www.webpackbin.com/NJgI5J8lZ
A different approach to "private"
Instead of fighting against the fact that private visibility is currently unavailable in ES6, I decided to take a more practical approach that does just fine if your IDE supports JSDoc (e.g., Webstorm). The idea is to use the #private tag. As far as development goes, the IDE will prevent you from accessing any private member from outside its class. Works pretty well for me and it's been really useful for hiding internal methods so the auto-complete feature shows me just what the class really meant to expose. Here's an example:
Oh, so many exotic solutions! I usually don't care about privacy so I use "pseudo privacy" as it's said here. But if do care (if there are some special requirements for that) I use something like in this example:
class jobImpl{
// public
constructor(name){
this.name = name;
}
// public
do(time){
console.log(`${this.name} started at ${time}`);
this.prepare();
this.execute();
}
//public
stop(time){
this.finish();
console.log(`${this.name} finished at ${time}`);
}
// private
prepare(){ console.log('prepare..'); }
// private
execute(){ console.log('execute..'); }
// private
finish(){ console.log('finish..'); }
}
function Job(name){
var impl = new jobImpl(name);
return {
do: time => impl.do(time),
stop: time => impl.stop(time)
};
}
// Test:
// create class "Job"
var j = new Job("Digging a ditch");
// call public members..
j.do("08:00am");
j.stop("06:00pm");
// try to call private members or fields..
console.log(j.name); // undefined
j.execute(); // error
Another possible implementation of function (constructor) Job:
function Job(name){
var impl = new jobImpl(name);
this.do = time => impl.do(time),
this.stop = time => impl.stop(time)
}
WeakMap
supported in IE11 (Symbols are not)
hard-private (props using Symbols are soft-private due to Object.getOwnPropertySymbols)
can look really clean (unlike closures which require all props and methods in the constructor)
First, define a function to wrap WeakMap:
function Private() {
const map = new WeakMap();
return obj => {
let props = map.get(obj);
if (!props) {
props = {};
map.set(obj, props);
}
return props;
};
}
Then, construct a reference outside your class:
const p = new Private();
class Person {
constructor(name, age) {
this.name = name;
p(this).age = age; // it's easy to set a private variable
}
getAge() {
return p(this).age; // and get a private variable
}
}
Note: class isn't supported by IE11, but it looks cleaner in the example.
I came across this post when looking for the best practice for "private data for classes". It was mentioned that a few of the patterns would have performance issues.
I put together a few jsperf tests based on the 4 main patterns from the online book "Exploring ES6":
http://exploringjs.com/es6/ch_classes.html#sec_private-data-for-classes
The tests can be found here:
https://jsperf.com/private-data-for-classes
In Chrome 63.0.3239 / Mac OS X 10.11.6, the best performing patterns were "Private data via constructor environments" and "Private data via a naming convention". For me Safari performed well for WeakMap but Chrome not so well.
I don't know the memory impact, but the pattern for "constructor environments" which some had warned would be a performance issue was very performant.
The 4 basic patterns are:
Private data via constructor environments
class Countdown {
constructor(counter, action) {
Object.assign(this, {
dec() {
if (counter < 1) return;
counter--;
if (counter === 0) {
action();
}
}
});
}
}
const c = new Countdown(2, () => {});
c.dec();
c.dec();
Private data via constructor environments 2
class Countdown {
constructor(counter, action) {
this.dec = function dec() {
if (counter < 1) return;
counter--;
if (counter === 0) {
action();
}
}
}
}
const c = new Countdown(2, () => {});
c.dec();
c.dec();
Private data via a naming convention
class Countdown {
constructor(counter, action) {
this._counter = counter;
this._action = action;
}
dec() {
if (this._counter < 1) return;
this._counter--;
if (this._counter === 0) {
this._action();
}
}
}
const c = new Countdown(2, () => {});
c.dec();
c.dec();
Private data via WeakMaps
const _counter = new WeakMap();
const _action = new WeakMap();
class Countdown {
constructor(counter, action) {
_counter.set(this, counter);
_action.set(this, action);
}
dec() {
let counter = _counter.get(this);
if (counter < 1) return;
counter--;
_counter.set(this, counter);
if (counter === 0) {
_action.get(this)();
}
}
}
const c = new Countdown(2, () => {});
c.dec();
c.dec();
Private data via symbols
const _counter = Symbol('counter');
const _action = Symbol('action');
class Countdown {
constructor(counter, action) {
this[_counter] = counter;
this[_action] = action;
}
dec() {
if (this[_counter] < 1) return;
this[_counter]--;
if (this[_counter] === 0) {
this[_action]();
}
}
}
const c = new Countdown(2, () => {});
c.dec();
c.dec();
Personally I like the proposal of the bind operator :: and would then combine it with the solution #d13 mentioned but for now stick with #d13 's answer where you use the export keyword for your class and put the private functions in the module.
there is one more solution tough which hasn't been mentioned here that follows are more functional approach and would allow it to have all the private props/methods within the class.
Private.js
export const get = state => key => state[key];
export const set = state => (key,value) => { state[key] = value; }
Test.js
import { get, set } from './utils/Private'
export default class Test {
constructor(initialState = {}) {
const _set = this.set = set(initialState);
const _get = this.get = get(initialState);
this.set('privateMethod', () => _get('propValue'));
}
showProp() {
return this.get('privateMethod')();
}
}
let one = new Test({ propValue: 5});
let two = new Test({ propValue: 8});
two.showProp(); // 8
one.showProp(); // 5
comments on it would be appreciated.
I think Benjamin's answer is probably the best for most cases until the language natively supports explicitly private variables.
However, if for some reason you need to prevent access with Object.getOwnPropertySymbols(), a method I've considered using is attaching a unique, non-configurable, non-enumerable, non-writable property that can be used as a property identifier to each object on construction (such as a unique Symbol, if you don't already have some other unique property like an id). Then just keep a map of each object's 'private' variables using that identifier.
const privateVars = {};
class Something {
constructor(){
Object.defineProperty(this, '_sym', {
configurable: false,
enumerable: false,
writable: false,
value: Symbol()
});
var myPrivateVars = {
privateProperty: "I'm hidden"
};
privateVars[this._sym] = myPrivateVars;
this.property = "I'm public";
}
getPrivateProperty() {
return privateVars[this._sym].privateProperty;
}
// A clean up method of some kind is necessary since the
// variables won't be cleaned up from memory automatically
// when the object is garbage collected
destroy() {
delete privateVars[this._sym];
}
}
var instance = new Something();
console.log(instance.property); //=> "I'm public"
console.log(instance.privateProperty); //=> undefined
console.log(instance.getPrivateProperty()); //=> "I'm hidden"
The potential advantage of this approach over using a WeakMap is faster access time if performance becomes a concern.
I believe it is possible to get 'best of both worlds' using closures inside constructors. There are two variations:
All data members are private
function myFunc() {
console.log('Value of x: ' + this.x);
this.myPrivateFunc();
}
function myPrivateFunc() {
console.log('Enhanced value of x: ' + (this.x + 1));
}
class Test {
constructor() {
let internal = {
x : 2,
};
internal.myPrivateFunc = myPrivateFunc.bind(internal);
this.myFunc = myFunc.bind(internal);
}
};
Some members are private
NOTE: This is admittedly ugly. If you know a better solution, please edit this response.
function myFunc(priv, pub) {
pub.y = 3; // The Test object now gets a member 'y' with value 3.
console.log('Value of x: ' + priv.x);
this.myPrivateFunc();
}
function myPrivateFunc() {
pub.z = 5; // The Test object now gets a member 'z' with value 3.
console.log('Enhanced value of x: ' + (priv.x + 1));
}
class Test {
constructor() {
let self = this;
let internal = {
x : 2,
};
internal.myPrivateFunc = myPrivateFunc.bind(null, internal, self);
this.myFunc = myFunc.bind(null, internal, self);
}
};
In fact it is possible using Symbols and Proxies. You use the symbols in the class scope and set two traps in a proxy: one for the class prototype so that the Reflect.ownKeys(instance) or Object.getOwnPropertySymbols doesn't give your symbols away, the other one is for the constructor itself so when new ClassName(attrs) is called, the instance returned will be intercepted and have the own properties symbols blocked.
Here's the code:
const Human = (function() {
const pet = Symbol();
const greet = Symbol();
const Human = privatizeSymbolsInFn(function(name) {
this.name = name; // public
this[pet] = 'dog'; // private
});
Human.prototype = privatizeSymbolsInObj({
[greet]() { // private
return 'Hi there!';
},
revealSecrets() {
console.log(this[greet]() + ` The pet is a ${this[pet]}`);
}
});
return Human;
})();
const bob = new Human('Bob');
console.assert(bob instanceof Human);
console.assert(Reflect.ownKeys(bob).length === 1) // only ['name']
console.assert(Reflect.ownKeys(Human.prototype).length === 1 ) // only ['revealSecrets']
// Setting up the traps inside proxies:
function privatizeSymbolsInObj(target) {
return new Proxy(target, { ownKeys: Object.getOwnPropertyNames });
}
function privatizeSymbolsInFn(Class) {
function construct(TargetClass, argsList) {
const instance = new TargetClass(...argsList);
return privatizeSymbolsInObj(instance);
}
return new Proxy(Class, { construct });
}
Reflect.ownKeys() works like so: Object.getOwnPropertyNames(myObj).concat(Object.getOwnPropertySymbols(myObj)) that's why we need a trap for these objects.
Even Typescript can't do it. From their documentation:
When a member is marked private, it cannot be accessed from outside of its containing class. For example:
class Animal {
private name: string;
constructor(theName: string) { this.name = theName; }
}
new Animal("Cat").name; // Error: 'name' is private;
But transpiled on their playground this gives:
var Animal = (function () {
function Animal(theName) {
this.name = theName;
}
return Animal;
}());
console.log(new Animal("Cat").name);
So their "private" keyword is ineffective.
Coming very late to this party but I hit the OP question in a search so...
Yes, you can have private properties by wrapping the class declaration in a closure
There is an example of how I have private methods in this codepen. In the snippet below, the Subscribable class has two 'private' functions process and processCallbacks. Any properties can be added in this manner and they are kept private through the use of the closure. IMO Privacy is a rare need if concerns are well separated and Javascript does not need to become bloated by adding more syntax when a closure neatly does the job.
const Subscribable = (function(){
const process = (self, eventName, args) => {
self.processing.set(eventName, setTimeout(() => processCallbacks(self, eventName, args)))};
const processCallbacks = (self, eventName, args) => {
if (self.callingBack.get(eventName).length > 0){
const [nextCallback, ...callingBack] = self.callingBack.get(eventName);
self.callingBack.set(eventName, callingBack);
process(self, eventName, args);
nextCallback(...args)}
else {
delete self.processing.delete(eventName)}};
return class {
constructor(){
this.callingBack = new Map();
this.processing = new Map();
this.toCallbacks = new Map()}
subscribe(eventName, callback){
const callbacks = this.unsubscribe(eventName, callback);
this.toCallbacks.set(eventName, [...callbacks, callback]);
return () => this.unsubscribe(eventName, callback)} // callable to unsubscribe for convenience
unsubscribe(eventName, callback){
let callbacks = this.toCallbacks.get(eventName) || [];
callbacks = callbacks.filter(subscribedCallback => subscribedCallback !== callback);
if (callbacks.length > 0) {
this.toCallbacks.set(eventName, callbacks)}
else {
this.toCallbacks.delete(eventName)}
return callbacks}
emit(eventName, ...args){
this.callingBack.set(eventName, this.toCallbacks.get(eventName) || []);
if (!this.processing.has(eventName)){
process(this, eventName, args)}}}})();
I like this approach because it separates concerns nicely and keeps things truly private. The only downside is the need to use 'self' (or something similar) to refer to 'this' in the private content.
Yes totally can, and pretty easily too. This is done by exposing your private variables and functions by returning the prototype object graph in the constructor. This is nothing new, but take a bit of js foo to understand the elegance of it. This way does not use global scoped, or weakmaps. It is a form of reflection built into the language. Depending on how you leverage this; one can either force an exception which interrupts the call stack, or bury the exception as an undefined. This is demonstarted below, and can read more about these features here
class Clazz {
constructor() {
var _level = 1
function _private(x) {
return _level * x;
}
return {
level: _level,
public: this.private,
public2: function(x) {
return _private(x);
},
public3: function(x) {
return _private(x) * this.public(x);
},
};
}
private(x) {
return x * x;
}
}
var clazz = new Clazz();
console.log(clazz._level); //undefined
console.log(clazz._private); // undefined
console.log(clazz.level); // 1
console.log(clazz.public(1)); //1
console.log(clazz.public2(2)); //2
console.log(clazz.public3(3)); //27
console.log(clazz.private(0)); //error
class Something {
constructor(){
var _property = "test";
Object.defineProperty(this, "property", {
get: function(){ return _property}
});
}
}
var instance = new Something();
console.log(instance.property); //=> "test"
instance.property = "can read from outside, but can't write";
console.log(instance.property); //=> "test"
Another way similar to the last two posted
class Example {
constructor(foo) {
// privates
const self = this;
this.foo = foo;
// public interface
return self.public;
}
public = {
// empty data
nodata: { data: [] },
// noop
noop: () => {},
}
// everything else private
bar = 10
}
const test = new Example('FOO');
console.log(test.foo); // undefined
console.log(test.noop); // { data: [] }
console.log(test.bar); // undefined
I found a very simple solution, just use Object.freeze(). Of course the problem is you can't add nothing to the object later.
class Cat {
constructor(name ,age) {
this.name = name
this.age = age
Object.freeze(this)
}
}
let cat = new Cat('Garfield', 5)
cat.age = 6 // doesn't work, even throws an error in strict mode
This code demonstrates private and public, static and non-static, instance and class-level, variables, methods, and properties.
https://codesandbox.io/s/class-demo-837bj
class Animal {
static count = 0 // class static public
static #ClassPriVar = 3 // class static private
constructor(kind) {
this.kind = kind // instance public property
Animal.count++
let InstancePriVar = 'InstancePriVar: ' + kind // instance private constructor-var
log(InstancePriVar)
Animal.#ClassPriVar += 3
this.adhoc = 'adhoc' // instance public property w/out constructor- parameter
}
#PawCount = 4 // instance private var
set Paws(newPawCount) {
// instance public prop
this.#PawCount = newPawCount
}
get Paws() {
// instance public prop
return this.#PawCount
}
get GetPriVar() {
// instance public prop
return Animal.#ClassPriVar
}
static get GetPriVarStat() {
// class public prop
return Animal.#ClassPriVar
}
PrintKind() {
// instance public method
log('kind: ' + this.kind)
}
ReturnKind() {
// instance public function
return this.kind
}
/* May be unsupported
get #PrivMeth(){ // instance private prop
return Animal.#ClassPriVar + ' Private Method'
}
static get #PrivMeth(){ // class private prop
return Animal.#ClassPriVar + ' Private Method'
}
*/
}
function log(str) {
console.log(str)
}
// TESTING
log(Animal.count) // static, avail w/out instance
log(Animal.GetPriVarStat) // static, avail w/out instance
let A = new Animal('Cat')
log(Animal.count + ': ' + A.kind)
log(A.GetPriVar)
A.PrintKind()
A.Paws = 6
log('Paws: ' + A.Paws)
log('ReturnKind: ' + A.ReturnKind())
log(A.adhoc)
let B = new Animal('Dog')
log(Animal.count + ': ' + B.kind)
log(B.GetPriVar)
log(A.GetPriVar) // returns same as B.GetPriVar. Acts like a class-level property, but called like an instance-level property. It's cuz non-stat fx requires instance.
log('class: ' + Animal.GetPriVarStat)
// undefined
log('instance: ' + B.GetPriVarStat) // static class fx
log(Animal.GetPriVar) // non-stat instance fx
log(A.InstancePriVar) // private
log(Animal.InstancePriVar) // private instance var
log('PawCount: ' + A.PawCount) // private. Use getter
/* log('PawCount: ' + A.#PawCount) // private. Use getter
log('PawCount: ' + Animal.#PawCount) // Instance and private. Use getter */
Reading the previous answer i thought that this example can summarise the above solutions
const friend = Symbol('friend');
const ClassName = ((hidden, hiddenShared = 0) => {
class ClassName {
constructor(hiddenPropertyValue, prop){
this[hidden] = hiddenPropertyValue * ++hiddenShared;
this.prop = prop
}
get hidden(){
console.log('getting hidden');
return this[hidden];
}
set [friend](v){
console.log('setting hiddenShared');
hiddenShared = v;
}
get counter(){
console.log('getting hiddenShared');
return hiddenShared;
}
get privileged(){
console.log('calling privileged method');
return privileged.bind(this);
}
}
function privileged(value){
return this[hidden] + value;
}
return ClassName;
})(Symbol('hidden'), 0);
const OtherClass = (() => class OtherClass extends ClassName {
constructor(v){
super(v, 100);
this[friend] = this.counter - 1;
}
})();
UPDATE
now is it possible to make true private properties and methods (at least on chrome based browsers for now).
The syntax is pretty neat
class MyClass {
#privateProperty = 1
#privateMethod() { return 2 }
static #privateStatic = 3
static #privateStaticMethod(){return 4}
static get #privateStaticGetter(){return 5}
// also using is quite straightforward
method(){
return (
this.#privateMethod() +
this.#privateProperty +
MyClass.#privateStatic +
MyClass.#privateStaticMethod() +
MyClass.#privateStaticGetter
)
}
}
new MyClass().method()
// returns 15
Note that for retrieving static references you wouldn't use this.constructor.#private, because it would brake its subclasses. You must use a reference to the proper class in order to retrieve its static private references (that are available only inside the methods of that class), ie MyClass.#private.
Most answers either say it's impossible, or require you to use a WeakMap or Symbol, which are ES6 features that would probably require polyfills. There's however another way! Check out this out:
// 1. Create closure
var SomeClass = function() {
// 2. Create `key` inside a closure
var key = {};
// Function to create private storage
var private = function() {
var obj = {};
// return Function to access private storage using `key`
return function(testkey) {
if(key === testkey) return obj;
// If `key` is wrong, then storage cannot be accessed
console.error('Cannot access private properties');
return undefined;
};
};
var SomeClass = function() {
// 3. Create private storage
this._ = private();
// 4. Access private storage using the `key`
this._(key).priv_prop = 200;
};
SomeClass.prototype.test = function() {
console.log(this._(key).priv_prop); // Using property from prototype
};
return SomeClass;
}();
// Can access private property from within prototype
var instance = new SomeClass();
instance.test(); // `200` logged
// Cannot access private property from outside of the closure
var wrong_key = {};
instance._(wrong_key); // undefined; error logged
I call this method accessor pattern. The essential idea is that we have a closure, a key inside the closure, and we create a private object (in the constructor) that can only be accessed if you have the key.
If you are interested, you can read more about this in my article. Using this method, you can create per object properties that cannot be accessed outside of the closure. Therefore, you can use them in constructor or prototype, but not anywhere else. I haven't seen this method used anywhere, but I think it's really powerful.
See this answer for a a clean & simple 'class' solution with a private and public interface and support for composition
I use this pattern and it's always worked for me
class Test {
constructor(data) {
class Public {
constructor(prv) {
// public function (must be in constructor on order to access "prv" variable)
connectToDb(ip) {
prv._db(ip, prv._err);
}
}
// public function w/o access to "prv" variable
log() {
console.log("I'm logging");
}
}
// private variables
this._data = data;
this._err = function(ip) {
console.log("could not connect to "+ip);
}
}
// private function
_db(ip, err) {
if(!!ip) {
console.log("connected to "+ip+", sending data '"+this.data+"'");
return true;
}
else err(ip);
}
}
var test = new Test(10),
ip = "185.167.210.49";
test.connectToDb(ip); // true
test.log(); // I'm logging
test._err(ip); // undefined
test._db(ip, function() { console.log("You have got hacked!"); }); // undefined

Declare a public "static" function inside a JavaScript class

Suppose I need to declare some private static members to use it in some public static methods...
// ***** Variant I *****
function supports() {
var impl = document.implementation; // private, non-static
this.SVG = function () { // public, non-static
return impl.hasFeature("http://www.w3.org/TR/SVG11/feature#Image", "1.1");
};
}
// ***** Variant II *****
function supports() { }
supports.prototype.impl = document.implementation; // public, non-static
supports.SVG = function () { // public, static
return impl.hasFeature("http://www.w3.org/TR/SVG11/feature#Image", "1.1");
};
I know there are some difficulties in JavaScript with the 'static' OOP concept, so my question about is:
Can I declare public static methods inside the "declaration body" of the 'object' (like in the "Variant I" above)?
There is no private, static or public in JavaScript. There is only local and global, in-scope and out-of-scope. Use an IIFE to capture a variable and return a closure; this should work equivalently to a static method.
var supports = (function supportsWrapper() {
var impl = document.implementation; // "static"-ish.
return function supports() {
this.SVG = function () {
return impl.hasFeature("http://www.w3.org/TR/SVG11/feature#Image", "1.1");
};
}
})();
impl will be initialised only once, and will be readable only by supports, but will persist between calls to it.
In your Variant II the function that is supposedly private and static is neither private nor static.
You are correct that it is quite difficult to define a private static member of the class, but it is possible if you exploit JavaScript's scope.
var Person = (function(){
var privateStaticProperty = true;
function privateStatic() {
return privateStaticProperty;
}
function Person(){
// class declarations
}
Person.prototype.example = function() {
return privateStatic();
}
return Person;
}());
var p = new Person();
console.log(p.example());
Note, however, that if you extend the prototype outside of the closure, the supposedly private static members will not be available.
In JavaScript, there is no term or keyword static, but we can put such data directly into function object (like in any other object).
Static methods
Static methods, just like variables, are attached to functions. They are used mostly for objects:
function Animal(name) {
arguments.callee.count = ++arguments.callee.count || 1
this.name = name
}
Animal.showCount = function() {
alert( Animal.count )
}
var mouse = new Animal("Mouse")
var elephant = new Animal("elephant")
Animal.showCount() // 2
The only solution I see after analyzing some answers is this one:
var Person = (function () {
var _privateStatic = 'private static member';
function privateStatic() {
return _privateStatic + ' + private-s-function';
}
function Person() {
this.publicNonStatic = 'public, non-static member';
this.publicFunction = function () {
return this.publicNonStatic + '+ public non-static-function';
}
var _privateNonStatic = 'private, non-static member';
function privateNonStatic() {
return _privateNonStatic + " + private-ns-function"
}
this.publicStatic = Person.publicStatic = 'public static member';
this.publicStaticFunction = Person.publicStaticFunction = function () {
return Person.publicStatic + ' + public static function';
}
// Accessible internal
var test = _privateNonStatic;
test = _privateStatic;
test = privateStatic();
test = privateNonStatic();
// other class declarations
}
return Person;
}());
// Accessible external members:
var p = new Person();
console.log(p.publicFunction());
console.log(p.publicNonStatic);
console.log(p.publicStatic);
console.log(p.publicStaticFunction());
console.log(Person.publicStatic);
console.log(Person.publicStaticFunction());
PS.
I remarked however that publicStaticFunction() becames accesible only after a new Person() declaration... so, before it they are not available:
// Accessible external members:
console.log(Person.publicStatic); // NO WAY
console.log(Person.publicStaticFunction()); // NO WAY
var p = new Person(); // lazy declaration
console.log(Person.publicStatic); // IS OK!
console.log(Person.publicStaticFunction()); // IS OK!
, so I suppose there is no way to achieve it INSIDE the function Person body, only inside the anonymous function that wraps the Person...
So the initial SVG sample should be like this:
var supports = (function(){
function supports() { this.SVG = supports.SVG; }
supports.SVG = function () {
return document.implementation.hasFeature("http://shortened", "1.1");
};
return supports;
}());
// usage of public, static method
if (supports.SVG()) {return 'supports SVG!';}

Private properties in JavaScript ES6 classes

Is it possible to create private properties in ES6 classes?
Here's an example.
How can I prevent access to instance.property?
class Something {
constructor(){
this.property = "test";
}
}
var instance = new Something();
console.log(instance.property); //=> "test"
Private class features is now supported by the majority of browsers.
class Something {
#property;
constructor(){
this.#property = "test";
}
#privateMethod() {
return 'hello world';
}
getPrivateMessage() {
return this.#property;
}
}
const instance = new Something();
console.log(instance.property); //=> undefined
console.log(instance.privateMethod); //=> undefined
console.log(instance.getPrivateMessage()); //=> test
console.log(instance.#property); //=> Syntax error
Update: See others answer, this is outdated.
Short answer, no, there is no native support for private properties with ES6 classes.
But you could mimic that behaviour by not attaching the new properties to the object, but keeping them inside a class constructor, and use getters and setters to reach the hidden properties. Note that the getters and setters gets redefine on each new instance of the class.
ES6
class Person {
constructor(name) {
var _name = name
this.setName = function(name) { _name = name; }
this.getName = function() { return _name; }
}
}
ES5
function Person(name) {
var _name = name
this.setName = function(name) { _name = name; }
this.getName = function() { return _name; }
}
Yes, prefix the name with # and include it in the class definition, not just the constructor.
MDN Docs
Real private properties were finally added in ES2022. As of 2023-01-01, private properties (fields and methods) have been supported in all major browsers for at least a year, but 5-10% of users are still on older browsers [Can I Use].
Example:
class Person {
#age
constructor(name) {
this.name = name; // this is public
this.#age = 20; // this is private
}
greet() {
// here we can access both name and age
console.log(`name: ${this.name}, age: ${this.#age}`);
}
}
let joe = new Person('Joe');
joe.greet();
// here we can access name but not age
Following are methods for keeping properties private in pre-ES2022 environments, with various tradeoffs.
Scoped variables
The approach here is to use the scope of the constructor function, which is private, to store private data. For methods to have access to this private data they must be created within the constructor as well, meaning you're recreating them with every instance. This is a performance and memory penalty, but it may be acceptable. The penalty can be avoided for methods that do not need access to private data by declaring them in the normal way.
Example:
class Person {
constructor(name) {
let age = 20; // this is private
this.name = name; // this is public
this.greet = () => {
// here we can access both name and age
console.log(`name: ${this.name}, age: ${age}`);
};
}
anotherMethod() {
// here we can access name but not age
}
}
let joe = new Person('Joe');
joe.greet();
// here we can access name but not age
Scoped WeakMap
A WeakMap can be used to improve the performance of the above approach, in exchange for even more clutter. WeakMaps associate data with Objects (here, class instances) in such a way that it can only be accessed using that WeakMap. So, we use the scoped variables method to create a private WeakMap, then use that WeakMap to retrieve private data associated with this. This is faster than the scoped variables method because all your instances can share a single WeakMap, so you don't need to recreate methods just to make them access their own WeakMaps.
Example:
let Person = (function () {
let privateProps = new WeakMap();
return class Person {
constructor(name) {
this.name = name; // this is public
privateProps.set(this, {age: 20}); // this is private
}
greet() {
// Here we can access both name and age
console.log(`name: ${this.name}, age: ${privateProps.get(this).age}`);
}
};
})();
let joe = new Person('Joe');
joe.greet();
// here we can access name but not age
This example uses a WeakMap with Object keys to use one WeakMap for multiple private properties; you could also use multiple WeakMaps and use them like privateAge.set(this, 20), or write a small wrapper and use it another way, like privateProps.set(this, 'age', 0).
The privacy of this approach could theoretically be breached by tampering with the global WeakMap object. That said, all JavaScript can be broken by mangled globals.
(This method could also be done with Map, but WeakMap is better because Map will create memory leaks unless you're very careful, and for this purpose the two aren't otherwise different.)
Half-Answer: Scoped Symbols
A Symbol is a type of primitive value that can serve as a property name instead of a string. You can use the scoped variable method to create a private Symbol, then store private data at this[mySymbol].
The privacy of this method can be breached using Object.getOwnPropertySymbols, but is somewhat awkward to do.
Example:
let Person = (() => {
let ageKey = Symbol();
return class Person {
constructor(name) {
this.name = name; // this is public
this[ageKey] = 20; // this is intended to be private
}
greet() {
// Here we can access both name and age
console.log(`name: ${this.name}, age: ${this[ageKey]}`);
}
}
})();
let joe = new Person('Joe');
joe.greet();
// Here we can access joe's name and, with a little effort, age. We can’t
// access ageKey directly, but we can obtain it by listing all Symbol
// properties on `joe` with `Object.getOwnPropertySymbols(joe)`.
Note that making a property non-enumerable using Object.defineProperty does not prevent it from being included in Object.getOwnPropertySymbols.
Half-Answer: Underscores
The old convention is to just use a public property with an underscore prefix. This does not keep it private, but it does do a good job of communicating to readers that they should treat it as private, which often gets the job done. In exchange for this, we get an approach that's easier to read, easier to type, and faster than the other workarounds.
Example:
class Person {
constructor(name) {
this.name = name; // this is public
this._age = 20; // this is intended to be private
}
greet() {
// Here we can access both name and age
console.log(`name: ${this.name}, age: ${this._age}`);
}
}
let joe = new Person('Joe');
joe.greet();
// Here we can access both joe's name and age. But we know we aren't
// supposed to access his age, which just might stop us.
Summary
ES2022: great but not yet supported by all visitors
Scoped variables: private, slower, awkward
Scoped WeakMaps: hackable, awkward
Scoped Symbols: enumerable and hackable, somewhat awkward
Underscores: just a request for privacy, no other downsides
Update: A proposal with nicer syntax is on its way. Contributions are welcome.
Yes, there is - for scoped access in objects - ES6 introduces Symbols.
Symbols are unique, you can't gain access to one from the outside except with reflection (like privates in Java/C#) but anyone who has access to a symbol on the inside can use it for key access:
var property = Symbol();
class Something {
constructor(){
this[property] = "test";
}
}
var instance = new Something();
console.log(instance.property); //=> undefined, can only access with access to the Symbol
The answer is "No". But you can create private access to properties like this:
Use modules. Everything in a module is private unless it's made public by using the export keyword.
Inside modules, use function closure: http://www.kirupa.com/html5/closures_in_javascript.htm
(The suggestion that Symbols could be used to ensure privacy was true in an earlier version of the ES6 spec but is no longer the case:https://mail.mozilla.org/pipermail/es-discuss/2014-January/035604.html and https://stackoverflow.com/a/22280202/1282216. For a longer discussion about Symbols and privacy see: https://curiosity-driven.org/private-properties-in-javascript)
The only way to get true privacy in JS is through scoping, so there is no way to have a property that is a member of this that will be accessible only inside the component. The best way to store truly private data in ES6 is with a WeakMap.
const privateProp1 = new WeakMap();
const privateProp2 = new WeakMap();
class SomeClass {
constructor() {
privateProp1.set(this, "I am Private1");
privateProp2.set(this, "I am Private2");
this.publicVar = "I am public";
this.publicMethod = () => {
console.log(privateProp1.get(this), privateProp2.get(this))
};
}
printPrivate() {
console.log(privateProp1.get(this));
}
}
Obviously this is a probably slow, and definitely ugly, but it does provide privacy.
Keep in mind that EVEN THIS isn't perfect, because Javascript is so dynamic. Someone could still do
var oldSet = WeakMap.prototype.set;
WeakMap.prototype.set = function(key, value){
// Store 'this', 'key', and 'value'
return oldSet.call(this, key, value);
};
to catch values as they are stored, so if you wanted to be extra careful, you'd need to capture a local reference to .set and .get to use explicitly instead of relying on the overridable prototype.
const {set: WMSet, get: WMGet} = WeakMap.prototype;
const privateProp1 = new WeakMap();
const privateProp2 = new WeakMap();
class SomeClass {
constructor() {
WMSet.call(privateProp1, this, "I am Private1");
WMSet.call(privateProp2, this, "I am Private2");
this.publicVar = "I am public";
this.publicMethod = () => {
console.log(WMGet.call(privateProp1, this), WMGet.call(privateProp2, this))
};
}
printPrivate() {
console.log(WMGet.call(privateProp1, this));
}
}
For future reference of other on lookers, I'm hearing now that the recommendation is to use WeakMaps to hold private data.
Here is a more clear, working example:
function storePrivateProperties(a, b, c, d) {
let privateData = new WeakMap;
// unique object as key, weak map can only accept object as key, when key is no longer referened, garbage collector claims the key-value
let keyA = {}, keyB = {}, keyC = {}, keyD = {};
privateData.set(keyA, a);
privateData.set(keyB, b);
privateData.set(keyC, c);
privateData.set(keyD, d);
return {
logPrivateKey(key) {
switch(key) {
case "a":
console.log(privateData.get(keyA));
break;
case "b":
console.log(privateData.get(keyB));
break;
case "c":
console.log(privateData.get(keyC));
break;
case "d":
console.log(privateData.set(keyD));
break;
default:
console.log(`There is no value for ${key}`)
}
}
}
}
Depends on whom you ask :-)
No private property modifier is included in the Maximally minimal classes proposal which seems to have made it into the current draft.
However, there might be support for private names, which does allow private properties - and they probably could be used in class definitions as well.
Using ES6 modules (initially proposed by #d13) works well for me. It doesn't mimic private properties perfectly, but at least you can be confident that properties that should be private won't leak outside of your class. Here's an example:
something.js
let _message = null;
const _greet = name => {
console.log('Hello ' + name);
};
export default class Something {
constructor(message) {
_message = message;
}
say() {
console.log(_message);
_greet('Bob');
}
};
Then the consuming code can look like this:
import Something from './something.js';
const something = new Something('Sunny day!');
something.say();
something._message; // undefined
something._greet(); // exception
Update (Important):
As #DanyalAytekin outlined in the comments, these private properties are static, so therefore global in scope. They will work well when working with Singletons, but care must be taken for Transient objects. Extending the example above:
import Something from './something.js';
import Something2 from './something.js';
const a = new Something('a');
a.say(); // a
const b = new Something('b');
b.say(); // b
const c = new Something2('c');
c.say(); // c
a.say(); // c
b.say(); // c
c.say(); // c
Yes - you can create encapsulated property, but it's not been done with access modifiers (public|private) at least not with ES6.
Here is a simple example how it can be done with ES6:
1 Create class using class word
2 Inside it's constructor declare block-scoped variable using let OR const reserved words -> since they are block-scope they cannot be accessed from outside (encapsulated)
3 To allow some access control (setters|getters) to those variables you can declare instance method inside it's constructor using: this.methodName=function(){} syntax
"use strict";
class Something{
constructor(){
//private property
let property="test";
//private final (immutable) property
const property2="test2";
//public getter
this.getProperty2=function(){
return property2;
}
//public getter
this.getProperty=function(){
return property;
}
//public setter
this.setProperty=function(prop){
property=prop;
}
}
}
Now lets check it:
var s=new Something();
console.log(typeof s.property);//undefined
s.setProperty("another");//set to encapsulated `property`
console.log(s.getProperty());//get encapsulated `property` value
console.log(s.getProperty2());//get encapsulated immutable `property2` value
Completing #d13 and the comments by #johnny-oshika and #DanyalAytekin:
I guess in the example provided by #johnny-oshika we could use normal functions instead of arrow functions and then .bind them with the current object plus a _privates object as a curried parameter:
something.js
function _greet(_privates) {
return 'Hello ' + _privates.message;
}
function _updateMessage(_privates, newMessage) {
_privates.message = newMessage;
}
export default class Something {
constructor(message) {
const _privates = {
message
};
this.say = _greet.bind(this, _privates);
this.updateMessage = _updateMessage.bind(this, _privates);
}
}
main.js
import Something from './something.js';
const something = new Something('Sunny day!');
const message1 = something.say();
something.updateMessage('Cloudy day!');
const message2 = something.say();
console.log(message1 === 'Hello Sunny day!'); // true
console.log(message2 === 'Hello Cloudy day!'); // true
// the followings are not public
console.log(something._greet === undefined); // true
console.log(something._privates === undefined); // true
console.log(something._updateMessage === undefined); // true
// another instance which doesn't share the _privates
const something2 = new Something('another Sunny day!');
const message3 = something2.say();
console.log(message3 === 'Hello another Sunny day!'); // true
Benefits I can think of:
we can have private methods (_greet and _updateMessage act like private methods as long as we don't export the references)
although they're not on the prototype, the above mentioned methods will save memory because the instances are created once, outside the class (as opposed to defining them in the constructor)
we don't leak any globals since we're inside a module
we can also have private properties using the binded _privates object
Some drawbacks I can think of:
less intuitive
mixed usage of class syntax and old school patterns (object bindings, module/function scoped variables)
hard bindings - we can't rebind the public methods (although we can improve this by using soft bindings (https://github.com/getify/You-Dont-Know-JS/blob/master/this%20%26%20object%20prototypes/ch2.md#softening-binding))
A running snippet can be found here: http://www.webpackbin.com/NJgI5J8lZ
A different approach to "private"
Instead of fighting against the fact that private visibility is currently unavailable in ES6, I decided to take a more practical approach that does just fine if your IDE supports JSDoc (e.g., Webstorm). The idea is to use the #private tag. As far as development goes, the IDE will prevent you from accessing any private member from outside its class. Works pretty well for me and it's been really useful for hiding internal methods so the auto-complete feature shows me just what the class really meant to expose. Here's an example:
Oh, so many exotic solutions! I usually don't care about privacy so I use "pseudo privacy" as it's said here. But if do care (if there are some special requirements for that) I use something like in this example:
class jobImpl{
// public
constructor(name){
this.name = name;
}
// public
do(time){
console.log(`${this.name} started at ${time}`);
this.prepare();
this.execute();
}
//public
stop(time){
this.finish();
console.log(`${this.name} finished at ${time}`);
}
// private
prepare(){ console.log('prepare..'); }
// private
execute(){ console.log('execute..'); }
// private
finish(){ console.log('finish..'); }
}
function Job(name){
var impl = new jobImpl(name);
return {
do: time => impl.do(time),
stop: time => impl.stop(time)
};
}
// Test:
// create class "Job"
var j = new Job("Digging a ditch");
// call public members..
j.do("08:00am");
j.stop("06:00pm");
// try to call private members or fields..
console.log(j.name); // undefined
j.execute(); // error
Another possible implementation of function (constructor) Job:
function Job(name){
var impl = new jobImpl(name);
this.do = time => impl.do(time),
this.stop = time => impl.stop(time)
}
WeakMap
supported in IE11 (Symbols are not)
hard-private (props using Symbols are soft-private due to Object.getOwnPropertySymbols)
can look really clean (unlike closures which require all props and methods in the constructor)
First, define a function to wrap WeakMap:
function Private() {
const map = new WeakMap();
return obj => {
let props = map.get(obj);
if (!props) {
props = {};
map.set(obj, props);
}
return props;
};
}
Then, construct a reference outside your class:
const p = new Private();
class Person {
constructor(name, age) {
this.name = name;
p(this).age = age; // it's easy to set a private variable
}
getAge() {
return p(this).age; // and get a private variable
}
}
Note: class isn't supported by IE11, but it looks cleaner in the example.
I came across this post when looking for the best practice for "private data for classes". It was mentioned that a few of the patterns would have performance issues.
I put together a few jsperf tests based on the 4 main patterns from the online book "Exploring ES6":
http://exploringjs.com/es6/ch_classes.html#sec_private-data-for-classes
The tests can be found here:
https://jsperf.com/private-data-for-classes
In Chrome 63.0.3239 / Mac OS X 10.11.6, the best performing patterns were "Private data via constructor environments" and "Private data via a naming convention". For me Safari performed well for WeakMap but Chrome not so well.
I don't know the memory impact, but the pattern for "constructor environments" which some had warned would be a performance issue was very performant.
The 4 basic patterns are:
Private data via constructor environments
class Countdown {
constructor(counter, action) {
Object.assign(this, {
dec() {
if (counter < 1) return;
counter--;
if (counter === 0) {
action();
}
}
});
}
}
const c = new Countdown(2, () => {});
c.dec();
c.dec();
Private data via constructor environments 2
class Countdown {
constructor(counter, action) {
this.dec = function dec() {
if (counter < 1) return;
counter--;
if (counter === 0) {
action();
}
}
}
}
const c = new Countdown(2, () => {});
c.dec();
c.dec();
Private data via a naming convention
class Countdown {
constructor(counter, action) {
this._counter = counter;
this._action = action;
}
dec() {
if (this._counter < 1) return;
this._counter--;
if (this._counter === 0) {
this._action();
}
}
}
const c = new Countdown(2, () => {});
c.dec();
c.dec();
Private data via WeakMaps
const _counter = new WeakMap();
const _action = new WeakMap();
class Countdown {
constructor(counter, action) {
_counter.set(this, counter);
_action.set(this, action);
}
dec() {
let counter = _counter.get(this);
if (counter < 1) return;
counter--;
_counter.set(this, counter);
if (counter === 0) {
_action.get(this)();
}
}
}
const c = new Countdown(2, () => {});
c.dec();
c.dec();
Private data via symbols
const _counter = Symbol('counter');
const _action = Symbol('action');
class Countdown {
constructor(counter, action) {
this[_counter] = counter;
this[_action] = action;
}
dec() {
if (this[_counter] < 1) return;
this[_counter]--;
if (this[_counter] === 0) {
this[_action]();
}
}
}
const c = new Countdown(2, () => {});
c.dec();
c.dec();
Personally I like the proposal of the bind operator :: and would then combine it with the solution #d13 mentioned but for now stick with #d13 's answer where you use the export keyword for your class and put the private functions in the module.
there is one more solution tough which hasn't been mentioned here that follows are more functional approach and would allow it to have all the private props/methods within the class.
Private.js
export const get = state => key => state[key];
export const set = state => (key,value) => { state[key] = value; }
Test.js
import { get, set } from './utils/Private'
export default class Test {
constructor(initialState = {}) {
const _set = this.set = set(initialState);
const _get = this.get = get(initialState);
this.set('privateMethod', () => _get('propValue'));
}
showProp() {
return this.get('privateMethod')();
}
}
let one = new Test({ propValue: 5});
let two = new Test({ propValue: 8});
two.showProp(); // 8
one.showProp(); // 5
comments on it would be appreciated.
I think Benjamin's answer is probably the best for most cases until the language natively supports explicitly private variables.
However, if for some reason you need to prevent access with Object.getOwnPropertySymbols(), a method I've considered using is attaching a unique, non-configurable, non-enumerable, non-writable property that can be used as a property identifier to each object on construction (such as a unique Symbol, if you don't already have some other unique property like an id). Then just keep a map of each object's 'private' variables using that identifier.
const privateVars = {};
class Something {
constructor(){
Object.defineProperty(this, '_sym', {
configurable: false,
enumerable: false,
writable: false,
value: Symbol()
});
var myPrivateVars = {
privateProperty: "I'm hidden"
};
privateVars[this._sym] = myPrivateVars;
this.property = "I'm public";
}
getPrivateProperty() {
return privateVars[this._sym].privateProperty;
}
// A clean up method of some kind is necessary since the
// variables won't be cleaned up from memory automatically
// when the object is garbage collected
destroy() {
delete privateVars[this._sym];
}
}
var instance = new Something();
console.log(instance.property); //=> "I'm public"
console.log(instance.privateProperty); //=> undefined
console.log(instance.getPrivateProperty()); //=> "I'm hidden"
The potential advantage of this approach over using a WeakMap is faster access time if performance becomes a concern.
I believe it is possible to get 'best of both worlds' using closures inside constructors. There are two variations:
All data members are private
function myFunc() {
console.log('Value of x: ' + this.x);
this.myPrivateFunc();
}
function myPrivateFunc() {
console.log('Enhanced value of x: ' + (this.x + 1));
}
class Test {
constructor() {
let internal = {
x : 2,
};
internal.myPrivateFunc = myPrivateFunc.bind(internal);
this.myFunc = myFunc.bind(internal);
}
};
Some members are private
NOTE: This is admittedly ugly. If you know a better solution, please edit this response.
function myFunc(priv, pub) {
pub.y = 3; // The Test object now gets a member 'y' with value 3.
console.log('Value of x: ' + priv.x);
this.myPrivateFunc();
}
function myPrivateFunc() {
pub.z = 5; // The Test object now gets a member 'z' with value 3.
console.log('Enhanced value of x: ' + (priv.x + 1));
}
class Test {
constructor() {
let self = this;
let internal = {
x : 2,
};
internal.myPrivateFunc = myPrivateFunc.bind(null, internal, self);
this.myFunc = myFunc.bind(null, internal, self);
}
};
In fact it is possible using Symbols and Proxies. You use the symbols in the class scope and set two traps in a proxy: one for the class prototype so that the Reflect.ownKeys(instance) or Object.getOwnPropertySymbols doesn't give your symbols away, the other one is for the constructor itself so when new ClassName(attrs) is called, the instance returned will be intercepted and have the own properties symbols blocked.
Here's the code:
const Human = (function() {
const pet = Symbol();
const greet = Symbol();
const Human = privatizeSymbolsInFn(function(name) {
this.name = name; // public
this[pet] = 'dog'; // private
});
Human.prototype = privatizeSymbolsInObj({
[greet]() { // private
return 'Hi there!';
},
revealSecrets() {
console.log(this[greet]() + ` The pet is a ${this[pet]}`);
}
});
return Human;
})();
const bob = new Human('Bob');
console.assert(bob instanceof Human);
console.assert(Reflect.ownKeys(bob).length === 1) // only ['name']
console.assert(Reflect.ownKeys(Human.prototype).length === 1 ) // only ['revealSecrets']
// Setting up the traps inside proxies:
function privatizeSymbolsInObj(target) {
return new Proxy(target, { ownKeys: Object.getOwnPropertyNames });
}
function privatizeSymbolsInFn(Class) {
function construct(TargetClass, argsList) {
const instance = new TargetClass(...argsList);
return privatizeSymbolsInObj(instance);
}
return new Proxy(Class, { construct });
}
Reflect.ownKeys() works like so: Object.getOwnPropertyNames(myObj).concat(Object.getOwnPropertySymbols(myObj)) that's why we need a trap for these objects.
Even Typescript can't do it. From their documentation:
When a member is marked private, it cannot be accessed from outside of its containing class. For example:
class Animal {
private name: string;
constructor(theName: string) { this.name = theName; }
}
new Animal("Cat").name; // Error: 'name' is private;
But transpiled on their playground this gives:
var Animal = (function () {
function Animal(theName) {
this.name = theName;
}
return Animal;
}());
console.log(new Animal("Cat").name);
So their "private" keyword is ineffective.
Coming very late to this party but I hit the OP question in a search so...
Yes, you can have private properties by wrapping the class declaration in a closure
There is an example of how I have private methods in this codepen. In the snippet below, the Subscribable class has two 'private' functions process and processCallbacks. Any properties can be added in this manner and they are kept private through the use of the closure. IMO Privacy is a rare need if concerns are well separated and Javascript does not need to become bloated by adding more syntax when a closure neatly does the job.
const Subscribable = (function(){
const process = (self, eventName, args) => {
self.processing.set(eventName, setTimeout(() => processCallbacks(self, eventName, args)))};
const processCallbacks = (self, eventName, args) => {
if (self.callingBack.get(eventName).length > 0){
const [nextCallback, ...callingBack] = self.callingBack.get(eventName);
self.callingBack.set(eventName, callingBack);
process(self, eventName, args);
nextCallback(...args)}
else {
delete self.processing.delete(eventName)}};
return class {
constructor(){
this.callingBack = new Map();
this.processing = new Map();
this.toCallbacks = new Map()}
subscribe(eventName, callback){
const callbacks = this.unsubscribe(eventName, callback);
this.toCallbacks.set(eventName, [...callbacks, callback]);
return () => this.unsubscribe(eventName, callback)} // callable to unsubscribe for convenience
unsubscribe(eventName, callback){
let callbacks = this.toCallbacks.get(eventName) || [];
callbacks = callbacks.filter(subscribedCallback => subscribedCallback !== callback);
if (callbacks.length > 0) {
this.toCallbacks.set(eventName, callbacks)}
else {
this.toCallbacks.delete(eventName)}
return callbacks}
emit(eventName, ...args){
this.callingBack.set(eventName, this.toCallbacks.get(eventName) || []);
if (!this.processing.has(eventName)){
process(this, eventName, args)}}}})();
I like this approach because it separates concerns nicely and keeps things truly private. The only downside is the need to use 'self' (or something similar) to refer to 'this' in the private content.
Yes totally can, and pretty easily too. This is done by exposing your private variables and functions by returning the prototype object graph in the constructor. This is nothing new, but take a bit of js foo to understand the elegance of it. This way does not use global scoped, or weakmaps. It is a form of reflection built into the language. Depending on how you leverage this; one can either force an exception which interrupts the call stack, or bury the exception as an undefined. This is demonstarted below, and can read more about these features here
class Clazz {
constructor() {
var _level = 1
function _private(x) {
return _level * x;
}
return {
level: _level,
public: this.private,
public2: function(x) {
return _private(x);
},
public3: function(x) {
return _private(x) * this.public(x);
},
};
}
private(x) {
return x * x;
}
}
var clazz = new Clazz();
console.log(clazz._level); //undefined
console.log(clazz._private); // undefined
console.log(clazz.level); // 1
console.log(clazz.public(1)); //1
console.log(clazz.public2(2)); //2
console.log(clazz.public3(3)); //27
console.log(clazz.private(0)); //error
class Something {
constructor(){
var _property = "test";
Object.defineProperty(this, "property", {
get: function(){ return _property}
});
}
}
var instance = new Something();
console.log(instance.property); //=> "test"
instance.property = "can read from outside, but can't write";
console.log(instance.property); //=> "test"
Another way similar to the last two posted
class Example {
constructor(foo) {
// privates
const self = this;
this.foo = foo;
// public interface
return self.public;
}
public = {
// empty data
nodata: { data: [] },
// noop
noop: () => {},
}
// everything else private
bar = 10
}
const test = new Example('FOO');
console.log(test.foo); // undefined
console.log(test.noop); // { data: [] }
console.log(test.bar); // undefined
I found a very simple solution, just use Object.freeze(). Of course the problem is you can't add nothing to the object later.
class Cat {
constructor(name ,age) {
this.name = name
this.age = age
Object.freeze(this)
}
}
let cat = new Cat('Garfield', 5)
cat.age = 6 // doesn't work, even throws an error in strict mode
This code demonstrates private and public, static and non-static, instance and class-level, variables, methods, and properties.
https://codesandbox.io/s/class-demo-837bj
class Animal {
static count = 0 // class static public
static #ClassPriVar = 3 // class static private
constructor(kind) {
this.kind = kind // instance public property
Animal.count++
let InstancePriVar = 'InstancePriVar: ' + kind // instance private constructor-var
log(InstancePriVar)
Animal.#ClassPriVar += 3
this.adhoc = 'adhoc' // instance public property w/out constructor- parameter
}
#PawCount = 4 // instance private var
set Paws(newPawCount) {
// instance public prop
this.#PawCount = newPawCount
}
get Paws() {
// instance public prop
return this.#PawCount
}
get GetPriVar() {
// instance public prop
return Animal.#ClassPriVar
}
static get GetPriVarStat() {
// class public prop
return Animal.#ClassPriVar
}
PrintKind() {
// instance public method
log('kind: ' + this.kind)
}
ReturnKind() {
// instance public function
return this.kind
}
/* May be unsupported
get #PrivMeth(){ // instance private prop
return Animal.#ClassPriVar + ' Private Method'
}
static get #PrivMeth(){ // class private prop
return Animal.#ClassPriVar + ' Private Method'
}
*/
}
function log(str) {
console.log(str)
}
// TESTING
log(Animal.count) // static, avail w/out instance
log(Animal.GetPriVarStat) // static, avail w/out instance
let A = new Animal('Cat')
log(Animal.count + ': ' + A.kind)
log(A.GetPriVar)
A.PrintKind()
A.Paws = 6
log('Paws: ' + A.Paws)
log('ReturnKind: ' + A.ReturnKind())
log(A.adhoc)
let B = new Animal('Dog')
log(Animal.count + ': ' + B.kind)
log(B.GetPriVar)
log(A.GetPriVar) // returns same as B.GetPriVar. Acts like a class-level property, but called like an instance-level property. It's cuz non-stat fx requires instance.
log('class: ' + Animal.GetPriVarStat)
// undefined
log('instance: ' + B.GetPriVarStat) // static class fx
log(Animal.GetPriVar) // non-stat instance fx
log(A.InstancePriVar) // private
log(Animal.InstancePriVar) // private instance var
log('PawCount: ' + A.PawCount) // private. Use getter
/* log('PawCount: ' + A.#PawCount) // private. Use getter
log('PawCount: ' + Animal.#PawCount) // Instance and private. Use getter */
Reading the previous answer i thought that this example can summarise the above solutions
const friend = Symbol('friend');
const ClassName = ((hidden, hiddenShared = 0) => {
class ClassName {
constructor(hiddenPropertyValue, prop){
this[hidden] = hiddenPropertyValue * ++hiddenShared;
this.prop = prop
}
get hidden(){
console.log('getting hidden');
return this[hidden];
}
set [friend](v){
console.log('setting hiddenShared');
hiddenShared = v;
}
get counter(){
console.log('getting hiddenShared');
return hiddenShared;
}
get privileged(){
console.log('calling privileged method');
return privileged.bind(this);
}
}
function privileged(value){
return this[hidden] + value;
}
return ClassName;
})(Symbol('hidden'), 0);
const OtherClass = (() => class OtherClass extends ClassName {
constructor(v){
super(v, 100);
this[friend] = this.counter - 1;
}
})();
UPDATE
now is it possible to make true private properties and methods (at least on chrome based browsers for now).
The syntax is pretty neat
class MyClass {
#privateProperty = 1
#privateMethod() { return 2 }
static #privateStatic = 3
static #privateStaticMethod(){return 4}
static get #privateStaticGetter(){return 5}
// also using is quite straightforward
method(){
return (
this.#privateMethod() +
this.#privateProperty +
MyClass.#privateStatic +
MyClass.#privateStaticMethod() +
MyClass.#privateStaticGetter
)
}
}
new MyClass().method()
// returns 15
Note that for retrieving static references you wouldn't use this.constructor.#private, because it would brake its subclasses. You must use a reference to the proper class in order to retrieve its static private references (that are available only inside the methods of that class), ie MyClass.#private.
Most answers either say it's impossible, or require you to use a WeakMap or Symbol, which are ES6 features that would probably require polyfills. There's however another way! Check out this out:
// 1. Create closure
var SomeClass = function() {
// 2. Create `key` inside a closure
var key = {};
// Function to create private storage
var private = function() {
var obj = {};
// return Function to access private storage using `key`
return function(testkey) {
if(key === testkey) return obj;
// If `key` is wrong, then storage cannot be accessed
console.error('Cannot access private properties');
return undefined;
};
};
var SomeClass = function() {
// 3. Create private storage
this._ = private();
// 4. Access private storage using the `key`
this._(key).priv_prop = 200;
};
SomeClass.prototype.test = function() {
console.log(this._(key).priv_prop); // Using property from prototype
};
return SomeClass;
}();
// Can access private property from within prototype
var instance = new SomeClass();
instance.test(); // `200` logged
// Cannot access private property from outside of the closure
var wrong_key = {};
instance._(wrong_key); // undefined; error logged
I call this method accessor pattern. The essential idea is that we have a closure, a key inside the closure, and we create a private object (in the constructor) that can only be accessed if you have the key.
If you are interested, you can read more about this in my article. Using this method, you can create per object properties that cannot be accessed outside of the closure. Therefore, you can use them in constructor or prototype, but not anywhere else. I haven't seen this method used anywhere, but I think it's really powerful.
See this answer for a a clean & simple 'class' solution with a private and public interface and support for composition
I use this pattern and it's always worked for me
class Test {
constructor(data) {
class Public {
constructor(prv) {
// public function (must be in constructor on order to access "prv" variable)
connectToDb(ip) {
prv._db(ip, prv._err);
}
}
// public function w/o access to "prv" variable
log() {
console.log("I'm logging");
}
}
// private variables
this._data = data;
this._err = function(ip) {
console.log("could not connect to "+ip);
}
}
// private function
_db(ip, err) {
if(!!ip) {
console.log("connected to "+ip+", sending data '"+this.data+"'");
return true;
}
else err(ip);
}
}
var test = new Test(10),
ip = "185.167.210.49";
test.connectToDb(ip); // true
test.log(); // I'm logging
test._err(ip); // undefined
test._db(ip, function() { console.log("You have got hacked!"); }); // undefined

Converting Typescript to Javascript Best Practices

How would you recommend I convert this JavaScript to TypeScript?
JAVASCRIPT:
function MyClass() {
var self = this,
var1 = "abc",
var2 = "xyz";
// Public
self.method1 = function () {
return "something " + self.var1;
};
self.method2 = function (parm1) {
var x = self.method1();
return x + " something";
};
// Private
function func1(parm1, parm2) {
return parm1 + parm2;
}
}
This is what I would like to do in TypeScript, or at least how I would expect to write it:
module MyNamespace {
export class MyClass {
private var1: string = "abc";
private var2: string = "xyz";
constructor() {
}
// Public
public method1() {
return "something " + self.var1;
}
public method2(parm1) {
var x = this.method1();
return x + " something";
}
// Private
private func1(parm1, parm2) {
return parm1 + parm2;
}
}
}
But the generated JavaScript puts everything on the prototype and I will have some "this" scoping issues:
var MyNamespace;
(function (MyNamespace) {
var MyClass = (function () {
function MyClass() {
this.var1 = "abc";
this.var2 = "xyz";
}
// Public
MyClass.prototype.method1 = function () {
return "something " + this.var1;
};
MyClass.prototype.method2 = function (parm1) {
var x = this.method1();
return x + " something";
};
// Private
MyClass.prototype.func1 = function (parm1, parm2) {
return parm1 + parm2;
};
return MyClass;
})();
MyNamespace.MyClass = MyClass;
})(MyNamespace || (MyNamespace = {}))
I know I can declare the methods within the constructor, use lambda functions to get "_this" instead of "this", etc. but what are the best practices of "porting to TypeScript"?
But the generated Javascript puts everything on the prototype
I think that's what you want to happen. The only time it creates a "this" issue is when you want to pass the function as a callback. That's a known concern when passing callbacks and when doing so is the right time to worry about it. To me it feels like the JavaScript way and it's better to embrace the system rather than fight it.
If every instance has a unique function closure you're using a lot more memory or at a minimum creating way more work for the optimizer.
I think the porting you have there looks right. You should consider letting class methods stay on the prototype for memory, performance, and reusability reasons. But if you have some methods you really want to be on the instance for the purposes of keeping this, you can use the member initialization + lambda syntax:
class MyClass {
greeting = 'hello, ';
/** someFunc is safe to use in callback positions */
someFunc = (msg: string) => {
console.log(this.greeting + msg);
}
}
JavaScript determines the value of this when calling/invoking the method, not when you declare it. https://stackoverflow.com/a/16063711/1641941 (under "The this variable")
To set the right context you can use =>
class MyClass {
greeting = 'hello, ';
someFunc(msg: string) {
console.log(this.greeting + msg);
}
}
var m = new MyClass();
setTimeout(() => {m.someFunc("hello")},100);
setTimeout will be passed a closure that is created in the current scope.
If you were to set an interval or event handler and the current scope has a very large irrelevant variable(s) in it then it's better to declare a function that returns the closure somewhere outside of the current scope and pass it the only the variables that are relevant for this closure.
Code that will return a closure function for you and limit scope:
class MyClass {
greeting = 'hello, ';
someFunc(msg: string) {
console.log(this.greeting + msg);
}
public static closures ={
someFunc(me:MyClass,msg:string){
return function(){
me.someFunc(msg);
};
}
}
}
var m = new MyClass();
setTimeout(MyClass.closures.someFunc(m,"hello"),100);

Nested class in javascript, inheritance of private methods

i'm quite a newbie in javascript, and i'm spending some time trying to create namespaced objects in js.
Now, that's what i'm trying to do:
MainObject = function() {
var privateVariable = "i'm private";
var privateMethod = function() {
// doSomething
}
this.publicMethod = function() {
// doPublicSomething
}
}
MainObject.prototype.nested = function() {
this.publicNestedMethod = function() {
// that's not working at all
this.privateMethod(privateVariable);
}
}
MyObject = new MainObject();
MyObject.publicMethod();
MyObject.publicNestedMethod();
I tried to include the nested class inside the first one, but it's not working also if i try:
this.nested = function() {
var mainObject = this;
return {
publicNestedMethod = function() {
mainObject.privateMethod();
}
}
}();
Someone can help me please? i'm gonna loose my mind on this.
Phaedra.
Closures are a lexical feature, not a semantic one. If the object is outside the lexical scope of another, it can no longer be "nested" and access the former's local variables. In the code of your nested function/class, there's no such thing as this.privateMethod, because privateMethod is never made to be a property of MainObject. It's simply a local variable inside a function.
There's no such things as "private properties", "private methods" or "private members" in JavaScript. Hell, there's no such thing as a "class". Some people like to emulate private members using local variables as above, but doing so results in cases like this, where the discrepancy between the two concepts comes and bites one in the behind.
To conclude, it is a bad idea to write Java code, with all its OO techniques in JS, just as it is a bad idea to write C code, with all its pointers and unbounded buffers, in C#. Sure, in both cases you can do it, but you would be failing to appreciate and exploit the language's features this way.
And now that I'm done with the rant, you can do something like this to get "namespaced" functions:
MainObject = function() {
var privateVariable = "I'm private";
var privateMethod = function() {
alert('Private');
}
this.publicMethod = function() {
alert('Public');
}
this.nested = {
publicNestedMethod: function() {
privateMethod();
}
};
// or
this.nested = (function() {
var nestedPrivate = 5;
return {
publicNestedMethod: function() {
alert(nestedPrivate);
privateMethod();
}
};
})();
}
MyObject = new MainObject();
MyObject.publicMethod();
MyObject.nested.publicNestedMethod();​
Using the convention of underscore for "private" methods is a reasonable way to keep things organized.
MainObject = function() {
this._privateVariable = "i'm private";
this._privateMethod = function() {
// doSomething
}
this.publicMethod = function() {
// doPublicSomething
}
}
Well to provide the benefit of prototypal inheritance where all "subclasses" share a single instance of the method in prototype, but to ALSO provide the feature of inheriting private instances...
I came up with:
function Person(name,latentPower){
var privatesForChildren = { password:"xyz"
,latentPower:"invisibility"}
this.inherit = function(){
for(v in privatesForChildren){
eval("var " + v + "=privatesForChildren['" + v + "'];");
}
}
this.name = name;
this.revealName = function(){ alert("My name is" + this.name + "."); }
this.revealPowers = function(){ alert("I'm normal."); }
}
function Mutant(name,latentPower,fuel){
this.inherit.call(this); // Inherit private instance variables
var fuel = fuel;
this.name = name;
this.revealPowers = function(){
alert("I manifest the powers of " + latentPower + " when I " + fuel + ".");
}
}
Mutant.prototype = new Person;
Mutant.prototype.constructor = Mutant;
bob = new Person("Bob","telekenesis");
jim = new Mutant("Jim","nausea","eat pizza");
buford = new Mutant("Buford","Teflon Man","breathe");
jim.revealName(); //Inherited properly from prototype
bob.revealPowers();
jim.revealPowers();
buford.revealPowers(); //distinct from Jim's so is an "instance var"
alert(bob.latentPower); //returns undefined
alert(buford.latentPower); //returns undefined, so is "private".
How useful is that?
JavaScript Classes and Inheritance (ES6)
According to ES6, you can use JavaScript classes and inheritance to accomplish what you need.
JavaScript classes, introduced in ECMAScript 2015, are primarily syntactical sugar over JavaScript's existing prototype-based inheritance.
Reference: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Classes
I'm showing the examples below with variables, but it can be applied also to functions.
Inheritance (1st Approach)
This solution can only be used with getters for your private variables, otherwise your subclass will not get access to them.
class Main {
constructor() {
let privateVariable = "private";
this.publicVariable = "public";
this.getPrivateVariable = () => {
return privateVariable;
}
}
}
Main.Sub = class Sub extends Main {
getAllVariables() {
return this.publicVariable + "-" + this.getPrivateVariable();
}
}
// Testing...
let main = new Main();
let sub = new Main.Sub();
console.log(main.privateVariable); // undefined
console.log(main.publicVariable); // "public"
console.log(sub.privateVariable); // undefined
console.log(sub.publicVariable); // "public"
console.log(main.getPrivateVariable()); // "private"
console.log(sub.getPrivateVariable()); // "private"
console.log(sub.getAllVariables()) // "public-private"
Nesting (2nd Approach)
Maybe this solution is better for you because it doesn't expose your private variables outside the Main and Nested classes.
class Main {
constructor() {
let privateVariable = "private";
this.publicVariable = "public";
Main.Nested = class Nested extends Main {
getAllVariables() {
return this.publicVariable + "-" + privateVariable;
}
}
}
}
// Testing...
let main = new Main();
let nested = new Main.Nested();
console.log(main.privateVariable); // undefined
console.log(main.publicVariable); // "public"
console.log(nested.privateVariable); // undefined
console.log(nested.publicVariable); // "public"
console.log(main.getPrivateVariable); // undefined
console.log(nested.getPrivateVariable); // undefined
console.log(nested.getAllVariables()) // "public-private"
What OO system lets you inherit private methods? Part of being private is being unaccessible from other objects.
In JS in particular, "private members" are really just local variables of the function where they are declared. JS doesn't have typical OO notions of "class", "inheritance", "public", and "private", so you can't expect to copy your OOP techniques verbatim from other OOP languages.
It is a convention. You can imitate OO Java techniques like private members but that's not recommended. You can imitate in this way:
MyFunction = function(options){
var private = {};
//to reference MyFunction as a context
var that = this;
function privateFunctionThatCallPublicMethod(){
that.publicFunction("hello");
}
this.publicFunction = function(params){
alert(params + " " + private);
}
...
}
var instance = new MyFunction({oneOption:'fsdfsad'});
This is the bests approach i found to emulate OO Java Techniques...
But there is a problem, is very inefficient...
You must use prototype instead, because otherwise it would create one object per function per instance of the "class".
MyFunction = function(options){
this._private = {};
}
MyFunction.prototype._privateFunctionThatCallPublicMethod = function(){
this.publicFunction("hello");
}
MyFunction.prototype.publicFunction = function(params){
alert(params + " " + this._private);
}
Like you think private members are (in this way) a convention.
Also, there is another thing you must know...
When you pass a function of an object as a parameter to another function you must bind the context of the function...
function bind(fnThis, fn) {
return function(){
return fn.apply(fnThis, arguments);
};
}
function makeSomething(callback){
callback("hello");
}
var instance = new MyFunction();
makeSomething(bind(instance, instance.publicFunction));
This is because you must bind "this" as instance in the body of the publicFunction, otherwise is gonna be "window" instead.

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