Private class features

Class fields are public by default, but private class members can be created by using a hash # prefix. The privacy encapsulation of these class features is enforced by JavaScript itself.

Private members are not native to the language before this syntax existed. In prototypical inheritance, its behavior may be emulated with WeakMap objects or closures, but they can't compare to the # syntax in terms of ergonomics.

Syntax

class ClassWithPrivate {
  #privateField;
  #privateFieldWithInitializer = 42;

  #privateMethod() {
    // …
  }

  static #privateStaticField;
  static #privateStaticFieldWithInitializer = 42;

  static #privateStaticMethod() {
    // …
  }
}

There are some additional syntax restrictions:

All private identifiers declared within a class must be unique. The namespace is shared between static and instance properties. The only exception is when the two declarations define a getter-setter pair.
The private identifier cannot be #constructor.

Description

Most class features have their private counterparts:

Private fields
Private methods
Private static fields
Private static methods
Private getters
Private setters
Private static getters
Private static setters

These features are collectively called private properties. However, constructors cannot be private in JavaScript. To prevent classes from being constructed outside of the class, you have to use a private flag.

Private properties are declared with # names (pronounced "hash names"), which are identifiers prefixed with #. The hash prefix is an inherent part of the property name — you can draw relationship with the old underscore prefix convention _privateField — but it's not an ordinary string property, so you can't dynamically access it with the bracket notation.

It is a syntax error to refer to # names from outside of the class. It is also a syntax error to refer to private properties that were not declared in the class body, or to attempt to remove declared properties with delete.

class ClassWithPrivateField {
  #privateField;

  constructor() {;
    delete this.#privateField; // Syntax error
    this.#undeclaredField = 42; // Syntax error
  }
}

const instance = new ClassWithPrivateField();
instance.#privateField; // Syntax error

JavaScript, being a dynamic language, is able to perform this compile-time check because of the special hash identifier syntax, making it different from normal properties on the syntax level.

Note: Code run in the Chrome console can access private properties outside the class. This is a DevTools-only relaxation of the JavaScript syntax restriction.

If you access a private property from an object that doesn't have the property, a TypeError is thrown, instead of returning undefined as normal properties do.

class C {
  #x;

  static getX(obj) {
    return obj.#x;
  }
}

console.log(C.getX(new C())); // undefined
console.log(C.getX({})); // TypeError: Cannot read private member #x from an object whose class did not declare it

This example also illustrates that you can access private properties within static functions too, and on externally defined instances of the class.

You can use the in operator to check whether an externally defined object possesses a private property. This will return true if the private field or method exists, and false otherwise.

class C {
  #x;
  constructor(x) {
    this.#x = x;
  }
  static getX(obj) {
    if (#x in obj) return obj.#x;

    return "obj must be an instance of C";
  }
}
console.log(C.getX(new C("foo"))); // "foo"
console.log(C.getX(new C(0.196))); // 0.196
console.log(C.getX(new C(new Date()))); // the current date and time
console.log(C.getX({})); // "obj must be an instance of C"

Note a corollary of private names being always pre-declared and non-deletable: if you found that an object possesses one private property of the current class (either from a try...catch or an in check), it must possess all other private properties. An object possessing the private properties of a class generally means it was constructed by that class (although not always).

Private properties are not part of the prototypical inheritance model since they can only be accessed within the current class's body and aren't inherited by subclasses. Private properties with the same name within different classes are entirely different and do not interoperate with each other. See them as external metadata attached to each instance, managed by the class. For this reason, Object.freeze and Object.seal have no effect on private properties.

For more information on how and when private fields are initialized, see public class fields.

Examples

Private fields

Private fields include private instance fields and private static fields. Private fields are only accessible from inside the class declaration.

Private instance fields

Like their public counterparts, private instance fields:

are added before the constructor runs in a base class, or immediately after super() is invoked in a subclass, and
are only available on instances of the class.

class ClassWithPrivateField {
  #privateField;

  constructor() {
    this.#privateField = 42;
  }
}

class Subclass extends ClassWithPrivateField {
  #subPrivateField;

  constructor() {
    super();
    this.#subPrivateField = 23;
  }
}

new Subclass(); // In some dev tools, it shows Subclass {#privateField: 42, #subPrivateField: 23}

Note: #privateField from the ClassWithPrivateField base class is private to ClassWithPrivateField and is not accessible from the derived Subclass.

Returning overriding object

A class's constructor can return a different object, which will be used as the new this for the derived class constructor. The derived class may then define private fields on that returned object — meaning it is possible to "stamp" private fields onto unrelated objects.

class Stamper extends class {
  // A base class whose constructor returns the object it's given
  constructor(obj) {
    return obj;
  }
} {
  // This declaration will "stamp" the private field onto the object
  // returned by the base class constructor
  #stamp = 42;
  static getStamp(obj) {
    return obj.#stamp;
  }
}

const obj = {};
new Stamper(obj);
// `Stamper` calls `Base`, which returns `obj`, so `obj` is
// now the `this` value. `Stamper` then defines `#stamp` on `obj`

console.log(obj); // In some dev tools, it shows {#stamp: 42}
console.log(Stamper.getStamp(obj)); // 42
console.log(obj instanceof Stamper); // false

// You cannot stamp private properties twice
new Stamper(obj); // Error: Initializing an object twice is an error with private fields

Warning: This is a potentially very confusing thing to do. You are generally advised to avoid returning anything from the constructor — especially something unrelated to this.

Private static fields

Like their public counterparts, private static fields:

are added to the class constructor at class evaluation time, and
are only available on the class itself.

class ClassWithPrivateStaticField {
  static #privateStaticField = 42;

  static publicStaticMethod() {
    return ClassWithPrivateStaticField.#privateStaticField;
  }
}

console.log(ClassWithPrivateStaticField.publicStaticMethod()); // 42

There is a restriction on private static fields: only the class which defines the private static field can access the field. This can lead to unexpected behavior when using this. In the following example, this refers to the Subclass class (not the ClassWithPrivateStaticField class) when we try to call Subclass.publicStaticMethod(), and so causes a TypeError.

class ClassWithPrivateStaticField {
  static #privateStaticField = 42;

  static publicStaticMethod() {
    return this.#privateStaticField;
  }
}

class Subclass extends ClassWithPrivateStaticField {}

Subclass.publicStaticMethod(); // TypeError: Cannot read private member #privateStaticField from an object whose class did not declare it

This is the same if you call the method with super, because super methods are not called with the super class as this.

class ClassWithPrivateStaticField {
  static #privateStaticField = 42;

  static publicStaticMethod() {
    // When invoked through super, `this` still refers to Subclass
    return this.#privateStaticField;
  }
}

class Subclass extends ClassWithPrivateStaticField {
  static callSuperMethod() {
    return super.publicStaticMethod();
  }
}

Subclass.callSuperMethod(); // TypeError: Cannot read private member #privateStaticField from an object whose class did not declare it

You are advised to always access private static fields through the class name, not through this, so inheritance doesn't break the method.

Private methods

Private methods include private instance methods and private static methods. Private methods are only accessible from inside the class declaration.

Private instance methods

Unlike their public counterparts, private instance methods:

are installed immediately before the instance fields are installed, and
are only available on instances of the class, not on its .prototype property.

class ClassWithPrivateMethod {
  #privateMethod() {
    return 42;
  }

  publicMethod() {
    return this.#privateMethod();
  }
}

const instance = new ClassWithPrivateMethod();
console.log(instance.publicMethod()); // 42

Private instance methods may be generator, async, or async generator functions. Private getters and setters are also possible, and follow the same syntax requirements as their public getter and setter counterparts.

class ClassWithPrivateAccessor {
  #message;

  get #decoratedMessage() {
    return `🎬${this.#message}🛑`;
  }
  set #decoratedMessage(msg) {
    this.#message = msg;
  }

  constructor() {
    this.#decoratedMessage = "hello world";
    console.log(this.#decoratedMessage);
  }
}

new ClassWithPrivateAccessor(); // 🎬hello world🛑

Unlike public methods, private methods are not accessible on the .prototype property of their class.

class C {
  #method() {}

  static getMethod(x) {
    return x.#method;
  }
}

console.log(C.getMethod(new C())); // [Function: #method]
console.log(C.getMethod(C.prototype)); // TypeError: Receiver must be an instance of class C

Private static methods

Like their public counterparts, private static methods:

are added to the class constructor at class evaluation time, and
are only available on the class itself.

class ClassWithPrivateStaticMethod {
  static #privateStaticMethod() {
    return 42;
  }

  static publicStaticMethod() {
    return ClassWithPrivateStaticMethod.#privateStaticMethod();
  }
}

console.log(ClassWithPrivateStaticMethod.publicStaticMethod()); // 42

Private static methods may be generator, async, and async generator functions.

The same restriction previously mentioned for private static fields holds for private static methods, and similarly can lead to unexpected behavior when using this. In the following example, when we try to call Subclass.publicStaticMethod(), this refers to the Subclass class (not the ClassWithPrivateStaticMethod class) and so causes a TypeError.

class ClassWithPrivateStaticMethod {
  static #privateStaticMethod() {
    return 42;
  }

  static publicStaticMethod() {
    return this.#privateStaticMethod();
  }
}

class Subclass extends ClassWithPrivateStaticMethod {}

console.log(Subclass.publicStaticMethod()); // TypeError: Cannot read private member #privateStaticMethod from an object whose class did not declare it

Simulating private constructors

Many other languages include the capability to mark a constructor as private, which prevents the class from being instantiated outside of the class itself — you can only use static factory methods that create instances, or not be able to create instances at all. JavaScript does not have a native way to do this, but it can be accomplished by using a private static flag.

class PrivateConstructor {
  static #isInternalConstructing = false;

  constructor() {
    if (!PrivateConstructor.#isInternalConstructing) {
      throw new TypeError("PrivateConstructor is not constructable");
    }
    PrivateConstructor.#isInternalConstructing = false;
    // More initialization logic
  }

  static create() {
    PrivateConstructor.#isInternalConstructing = true;
    const instance = new PrivateConstructor();
    return instance;
  }
}

new PrivateConstructor(); // TypeError: PrivateConstructor is not constructable
PrivateConstructor.create(); // PrivateConstructor {}