How to Make Static Fields Private in a JavaScript Class?

Find out how to create private static fields in a JavaScript class

Starting with ES13/ES2022, you can make static fields/members in a class private by prefixing the field name with # (hash symbol).

For example:

// ES13+
class Foo {
    static PUBLIC_PROP = 321;
    static #PRIVATE_PROP = 123;
}

console.log(Foo.PUBLIC_PROP); // 321

// SyntaxError
console.log(Foo.#PRIVATE_PROP);

Private static fields are only accessible on:

  • On the class itself (e.g. MyClass.staticField), and;
  • When using this in static methods.

One important limitation to understand is that, if a static method in a base class accesses a private static field (or method) within that class using the this context, and you try to access this method from a subclass, a TypeError will be thrown:

class BaseClass {
    static #PRIVATE_STATIC_FIELD;

    static baseStaticMethod1() {
        BaseClass.#PRIVATE_STATIC_FIELD = 'foo';
        return BaseClass.#PRIVATE_STATIC_FIELD;
    }

    static baseStaticMethod2() {
        this.#PRIVATE_STATIC_FIELD = 'foo';
        return this.#PRIVATE_STATIC_FIELD;
    }
}

class SubClass extends BaseClass {}

console.log(SubClass.baseStaticMethod1()); // 'foo'

// TypeError: Cannot write private member #PRIVATE_STATIC_FIELD to an object whose class did not declare it
console.log(SubClass.baseStaticMethod2());

In addition to declaring static fields as private, you can also:


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