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 ClassWithPrivateField {
  #privateField;
}

class ClassWithPrivateMethod {
  #privateMethod() {
    return 'hello world';
  }
}

class ClassWithPrivateStaticField {
  static #PRIVATE_STATIC_FIELD;
}

class ClassWithPrivateStaticMethod {
  static #privateStaticMethod() {
    return 'hello world';
  }
}

Examples

Private fields

Private fields include private instance fields and private static fields.

Private instance fields

Private instance fields are declared with # names (pronounced "hash names"), which are identifiers prefixed with #. The # is a part of the name itself. Private fields are accessible on the class constructor from inside the class declaration itself. They are used for declaration of field names as well as for accessing a field's value.

It is a syntax error to refer to # names from out of scope. It is also a syntax error to refer to private fields that were not declared before they were called, or to attempt to remove declared fields with delete. 

class ClassWithPrivateField {
  #privateField;

  constructor() {
    this.#privateField = 42;
    delete this.#privateField;   // Syntax error
    this.#undeclaredField = 444; // Syntax error
  }
}

const instance = new ClassWithPrivateField()
instance.#privateField === 42;   // Syntax error

Note: Use the in operator to check for potentially missing private fields (or private methods). This will return true if the private field or method exists, and false otherwise.

Like public fields, private fields are added at construction time in a base class, or at the point where super() is invoked in a subclass.

class ClassWithPrivateField {
  #privateField;

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

class SubClass extends ClassWithPrivateField {
  #subPrivateField;

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

new SubClass();
// SubClass {#subPrivateField: 23}

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

Private static fields

Private static fields are added to the class constructor at class evaluation time. Like their public counterparts, private static fields are only accessible on the class itself or on the this context of static methods, but not on the this context of instance methods.

class ClassWithPrivateStaticField {
  static #PRIVATE_STATIC_FIELD;

  static publicStaticMethod() {
    ClassWithPrivateStaticField.#PRIVATE_STATIC_FIELD = 42;
    return ClassWithPrivateStaticField.#PRIVATE_STATIC_FIELD;
  }

  publicInstanceMethod() {
    ClassWithPrivateStaticField.#PRIVATE_STATIC_FIELD = 42;
    return ClassWithPrivateStaticField.#PRIVATE_STATIC_FIELD;
  }
}

console.log(ClassWithPrivateStaticField.publicStaticMethod()); // 42
console.log(new ClassWithPrivateStaticField().publicInstanceMethod()); // 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 BaseClassWithPrivateStaticField class) when we try to call SubClass.basePublicStaticMethod(), and so causes a TypeError. 

class BaseClassWithPrivateStaticField {
  static #PRIVATE_STATIC_FIELD;

  static basePublicStaticMethod() {
    this.#PRIVATE_STATIC_FIELD = 42;
    return this.#PRIVATE_STATIC_FIELD;
  }
}

class SubClass extends BaseClassWithPrivateStaticField { };

let error = null;

try {
  SubClass.basePublicStaticMethod();
} catch (e) {
  error = e;
}

console.log(error instanceof TypeError);
// true
console.log(error);
// TypeError: Cannot write private member #PRIVATE_STATIC_FIELD
// to an object whose class did not declare it

Private methods

Private instance methods

Private instance methods are methods available on class instances whose access is restricted in the same manner as private instance fields. 

class ClassWithPrivateMethod {
  #privateMethod() {
    return 'hello world';
  }

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

const instance = new ClassWithPrivateMethod();
console.log(instance.getPrivateMessage());
// hello world

Private instance methods may be generator, async, or async generator functions. Private getters and setters are also possible, although not in generator, async, or async generator forms. 

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🛑

Private static methods

Like their public equivalent, private static methods are called on the class itself, not instances of the class. Like private static fields, they are only accessible from inside the class declaration. 

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

  static publicStaticMethod1() {
    return ClassWithPrivateStaticMethod.#privateStaticMethod();
  }

  static publicStaticMethod2() {
    return this.#privateStaticMethod();
  }
}

console.log(ClassWithPrivateStaticMethod.publicStaticMethod1() === 42);
// true
console.log(ClassWithPrivateStaticMethod.publicStaticMethod2() === 42);
// true

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 Derived.publicStaticMethod2(), this refers to the Derived class (not the Base class) and so causes a TypeError. 

class Base {
  static #privateStaticMethod() {
    return 42;
  }
  static publicStaticMethod1() {
    return Base.#privateStaticMethod();
  }
  static publicStaticMethod2() {
    return this.#privateStaticMethod();
  }
}

class Derived extends Base {}

console.log(Derived.publicStaticMethod1());
// 42
console.log(Derived.publicStaticMethod2());
// TypeError: Cannot read private member #privateStaticMethod
// from an object whose class did not declare it

Specifications

Specification
ECMAScript Language Specification
# prod-PrivateIdentifier

Browser compatibility

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See also