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Implementing JS decorator to wrap class

I'm trying to wrap class constructor and inject to some logic by using class decorator. Everything worked fine until I have tried to extend wrapped class: Extended class don't have methods in prototype.
function logClass(Class) {
// save a reference to the original constructor
const _class = Class;
// proxy constructor
const proxy = function(...args) {
const obj = new _class(...args);
// ... add logic here
return obj
}
// copy prototype so intanceof operator still works
proxy.prototype = _class.prototype;
// return proxy constructor (will override original)
return proxy;
}
#logClass
class Base {
prop = 5;
test() {
console.log("test")
}
}
class Extended extends Base {
test2() {
console.log("test2")
}
}
var base = new Base()
base.test()
var ext = new Extended()
console.log(ext.prop)
ext.test()
ext.test2() // TypeError: ext.test2 is not a function

Okay so I tried to figure out what is "wrong" with your code, but I was not able to make it work because it didn't typecheck. So, as a last resort, I'm posting a partial answer of my attempt, which works (with some quirks) so I can help other users who are more savvy with TypeScript.
First of all, the quirks: class decorators in TS cannot modify the structure of a type, so if you wanted to, for example, add a method to the decorated class, you would be able to do it but you would have to eat up/suppress unavoidable type errors (TS2339) when calling those methods.
There is a work around for this in this other question: Typescript adding methods with decorator type does not exist, but you would lose this current clean syntax for decorators if you do this.
Now, my solution, taken more or less directly from the documentation:
function logClass<T extends { new(...args: any[]): {} }>(constructor: T) {
return class extends constructor {
constructor(...args: any[]) {
super(args);
// ...add programmatic logic here
// (`super` is the decorated class, of type `T`, here)
}
// ...add properties and methods here
log(message: string) { // EXAMPLE
console.log(`${super.constructor.name} says: ${message}`);
}
}
}
#logClass
class Base {
prop = 5;
test() {
console.log("test");
}
constructor() {}
}
class Extended extends Base {
test2() {
console.log("test2");
}
}
var base = new Base();
base.test();
var ext = new Extended();
console.log(ext.prop);
//base.log("Hello"); // unavoidable type error TS2339
ext.test();
ext.test2();

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

ES 6 dynamically work on class after definition

I was previously rolling my own Javascript OOP but now I'm playing with ES6 and want to use the class defined after definition in a generic way.
Note
Any answer with new in it is not what I'm after.
Pseudo code:
// base.js
class Base {
constructor(arg) {
this.arg = arg;
}
// This is the behaviour I'm after
//afterDefined(cls) {
afterExtended(cls) { // probably a better name
console.log(`Class name ${cls.prototype.name}`);
}
}
// frombase.js
class FromBase extends Base {
constructor({ p1='val1', p2='val2'} = {}) {
super(...arguments);
this.p1 = p1;
this.p2 = p2;
}
}
The output in the console should be:
'Class name FromBase'
So far the only solution I have come up with is to have a static method on Base and call it after the class declaration when I define a new class but I will most likely forget to do this more than once.
Just to be really thorough on why I don't like the static solution; it will force me to import Base in every single file.
Example using a static method (which I don't want) https://jsfiddle.net/nL4atqvm/:
// base.js
class Base {
constructor(arg) {
super(...arguments);
this.arg = arg;
}
// This is the behaviour I'm after
static afterExtended(cls) {
console.log(`Class name ${cls.name}`);
}
}
// frombase.js
class FromBase extends Base {
}
// just after defining the FromBase class
Base.afterExtended(FromBase);

There is no javascript built-in trigger that is calling a method on a class when a subclass is defined that extends from it.
Because you're rolling your own library, you could craft some kind of method the creates and returns a new class that extends a given base class. Maybe check out this answer that may help how to define your classes: Instantiate a JavaScript Object Using a String to Define the Class Name
You could also check how other javascript libraries creates (sub)classes. For example, Ext JS has a ClassManager that you could look into.
http://docs.sencha.com/extjs/6.5.1/classic/Ext.ClassManager.html (docs)
http://docs.sencha.com/extjs/6.5.1/classic/src/ClassManager.js.html (source)
When this question would be about instantiation and not about defining classes, I would say:
afterDefined(cls) {
console.log(`Class name ${this.constructor.name}`);
}
Usage:
let x = new FromBase()
x.afterDefined() // --> Class name FromBase
To get the name of the class, use
static afterDefined(cls) {
console.log(`Class name ${this.name}`);
}

Is this what you're looking for?
class Base {
constructor(arg) { this.arg = arg; }
static afterDefined(cls) {
console.log(`Class name ${this.constructor.name}`);
}
}
Base = new Proxy(Base, {
get: function(target, key, receiver) {
if (typeof target == 'function' && key == 'prototype' && target.name == Base.name) {
Reflect.apply(Base.afterDefined, Reflect.construct(class FromBase {}, []), [])
}
return target[key];
}
});
class FromBase extends Base {}
In order for this to load class names, you will have to embed or forward-declare that information inside of the Proxy receiver prior to extending from it (which means you either need to enumerate ahead of time which classes you'll be inheriting to or to have some function call just prior to that class's declaration).
There are a bunch of other neat total hacks in the same vein such as looking at the source (text) of the file that you just loaded JavaScript from and then parsing that text as if it were JavaScript (people have used this to write Scheme interpreters that can accept new code inside of <script> tags).
If you are a library author intending to target Node, there are even more ways to go about doing this.

// base.js
class Base {
constructor(arg) {
this.arg = arg;
}
// This is the behaviour I'm after
afterDefined(cls) {
console.log(`Class name ${cls}`);
}
}
// frombase.js
class FromBase extends Base {
constructor(arg) {
super(arg)
}
}
let f = new FromBase();
f.afterDefined('text');//this you text or object
have to be aware of is. file loading order, super is an instance of the parent class. good luck.

ES6 Classes - Updating Static Properties

I am trying to figure out alternative ways to set a static (or class) property an ES6 Class and then change it after new instances of the class are created.
For example, lets say I have a class called Geo, and I need a static property called all that will give me the array of all instances of the Geo class.
This version works:
class Geo {
constructor(name){
this.name = name;
Geo.all.push(this);
}
}
Geo.all = [];
ruby = new Geo("Ruby");
rocks = new Geo("Rocks");
console.log(Geo.all.length); // => 2
I would prefer to not set the property OUTSIDE of the class definition though. I've tried a few things but can't seem to create a static property within the class that I can update from the constructor.
I should also mention I need to be able to do this in the browser (Chrome) without use of Babel or similar.
Here are examples of some things I've tried:
class Geo {
constructor(name){
this.name = name;
Geo.all.push(this);
}
static get all() {
return [];
}
}
ruby = new Geo("Ruby");
rocks = new Geo("Rocks");
console.log(Geo.all.length); // => 0
And another
class Geo {
constructor(name){
this.name = name;
Geo.all.push(this);
}
static all = [];
}
ruby = new Geo("Ruby");
rocks = new Geo("Rocks");
console.log(Geo.all.length); // => error unexpected "="

There's no such thing as static all = [] in ES6. Class instance and static fields are currently stage 3 proposals which can be used via a transpiler, e.g. Babel. There's already existing implementation in TypeScript that may be incompatible with these proposals in some way, yet static all = [] is valid in TS and ES.Next.
Geo.all = [];
is valid and preferable way to do this in ES6. The alternative is getter/setter pair - or only a getter for read-only property:
class Geo {
static get all() {
if (!this._all)
this._all = [];
return this._all;
}
constructor() { ... }
}
Tracking instances in static property can't generally be considered a good pattern and will lead to uncontrollable memory consumption and leaks (as it was mentioned in comments).

This works for me for static properties.
class NeoGeo {
constructor() {
}
static get topScore () {
if (NeoGeo._topScore===undefined) {
NeoGeo._topScore = 0; // set default here
}
return NeoGeo._topScore;
}
static set topScore (value) {
NeoGeo._topScore = value;
}
}
And your example:
class NeoGeo {
constructor() {
NeoGeo.addInstance(this);
console.log("instance count:" + NeoGeo.all.length);
}
static get all () {
if (NeoGeo._all===undefined) {
NeoGeo._all = [];
}
return NeoGeo._all;
}
static set all (value) {
NeoGeo._all = value;
}
static addInstance(instance) {
// add only if not already added
if (NeoGeo.all.indexOf(instance)==-1) {
NeoGeo.all.push(instance);
}
}
}
Note: In the getter you could also check for the existence of the property using the in keyword or the hasOwnProperty keyword.
static get topScore () {
if (!("_topScore" in NeoGeo)) {
NeoGeo._topScore = 0; // set default here
}
return NeoGeo._topScore;
}
And using hasOwnProperty:
static get topScore () {
if (NeoGeo.hasOwnProperty("_topScore")==false) {
NeoGeo._topScore = 0; // set default here
}
return NeoGeo._topScore;
}

I recently had a similar issue of creating static classes.
I first tried it with constant class variables, but Chrome debugger threw an error.
So I defined the class variables 'static', also the getter methods.
Worked in Chrome.
class TestClass {
//static properties.
static _prop1 = [ 'A', 'B', 'C'];
static _prop2 = true;
static _prop3 = 'some String';
//constructor. Commented out because the class only has static elements.
//constructor () {}
//Getters.
static get prop1 () {
return this._prop1;
}
static get prop2 () {
return this._prop2;
}
static get prop3 () {
return this._prop3;
}
}

The only way to properly add a getter is to extend the class and use that extended class.
class Basic {
get firstGetter() {
return 'firstGetter'
}
}
class ExtendedClass extends Basic {
get firstGetter() {
return 'updatedFirstGetter'
}
}
}

Update your node to the version 12 or up and that's it ;)

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