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JavaScript private class fields use a #-prefixed name, such as #count, to keep class state accessible only from code that declares that private name. They are established syntax, not a newly arriving feature: MDN reports broad browser availability since July 2021. This guide shows how to declare and use private instance and static fields, what the privacy boundary does—and does not—mean, and how to check whether an object carries a private field.
How to use private class fields in JavaScript?
Declare a private field in the class body, then access it from that class’s methods or other class code where the name is in scope. For example:
class Counter {
#count = 0;
increment() {
this.#count++;
}
get value() {
return this.#count;
}
}
const counter = new Counter();
counter.increment();
console.log(counter.value); // 1#count = 0; declares and initializes an instance field. Each Counter instance gets its own field as the instance is initialized. The field is read and updated using this.#count from code within Counter. The public getter provides a controlled way to expose a value without exposing the private field itself.
A private name must be declared in the class body before class code can use it. A direct expression such as counter.#count outside the declaring class is a syntax error. Do not use bracket notation: counter["#count"] refers to an ordinary string key, not the private field.
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What does # mean in a JavaScript class?
A hash-prefixed identifier is a private name recognized by JavaScript’s class syntax. It is not a normal object property whose name happens to begin with a hash. The language enforces access at runtime and at parse time: code that does not have the private name in scope cannot read or write it through ordinary property access.
Private elements do not act like enumerable keys. Ordinary object utilities cannot enumerate, delete, or otherwise interact with them as if they were public properties; they also cannot be proxied as private elements. Operations such as Object.freeze() and Object.seal() do not freeze private elements, and structuredClone() does not copy them. For these details and the syntax rules, see MDN’s Private elements reference.
This is language-enforced encapsulation, not a promise that the containing object, program, or runtime is secure in every other respect. Use a public method or accessor when callers need supported access to a value, and treat that API—not the private field—as the deliberate boundary.
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Can a subclass access a private field?
No. A private name belongs lexically to the class that declares it. A subclass cannot refer to a base class’s private field, even though instances of the subclass also pass through the base class constructor. A subclass may declare the same spelling, but that creates a separate private field:
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#value = "base";
readBaseValue() {
return this.#value;
}
}
class Child extends Base {
#value = "child";
readChildValue() {
return this.#value;
}
}
const item = new Child();
console.log(item.readBaseValue()); // "base"
console.log(item.readChildValue()); // "child"If a subclass needs behavior based on base-class state, expose that behavior through a base-class method or accessor rather than trying to access the base class’s private name directly. Private identifiers in one class also must be unique across its static and instance elements, except for a getter/setter pair; #constructor is invalid.
How do private static fields work?
A private static field belongs to the declaring class constructor rather than to each instance. Declare it with static, and use it from class code:
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class Sequence {
static #nextId = 1;
static nextId() {
return this.#nextId++;
}
}
console.log(Sequence.nextId()); // 1
console.log(Sequence.nextId()); // 2Static private access has an inheritance trap when a method uses this.#value. If that method is inherited and called on a subclass, this is the subclass constructor, which did not declare the base class’s private static field:
class Base {
static #value = 1;
static read() {
return this.#value;
}
}
class Child extends Base {}
Child.read(); // TypeErrorIf the intended owner is specifically the base class, the base class’s method can refer to Base.#value instead. That deliberately reads the declaring class’s field; it is not a mechanism for giving subclasses access to that private slot.
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How do I check whether an object has a private field?
Use the in operator with the private name from within code where that name is declared. It returns a boolean indicating whether the object carries that private element:
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class Counter {
#count = 0;
static hasCount(object) {
return #count in object;
}
}
console.log(Counter.hasCount(new Counter())); // true
console.log(Counter.hasCount({})); // falseThe check must appear in a scope that can refer to #count; outside that class, #count in object is not valid. Directly trying to access a private field on an object that does not carry it throws a TypeError, rather than returning undefined.
What changes when a Proxy wraps an instance?
A proxy is a different object from its target and does not automatically carry the target’s private elements. If a method that reads a private field is called with the proxy as its receiver, private access can fail. When a wrapper must preserve private-field access, arrange for the method to run with the original instance as its receiver—for example, by binding it to the target—while considering the API consequences of doing so. Do not assume a proxy or ordinary object-inspection APIs can expose or forward private elements like public keys.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How do private fields compare with underscore names and WeakMap state?
| Approach | Access control | How state is accessed |
|---|---|---|
#field |
JavaScript-enforced private access within the declaring class’s scope. | Class methods and other code in scope use syntax such as this.#field; ordinary property APIs do not treat it as a key. |
_field |
Naming convention only; callers can still access the ordinary property. | Ordinary property access, including from outside the class. |
Closure or WeakMap |
Alternative encapsulation techniques whose access depends on where the closure or map is held. | Requires separately structured closure or map logic rather than direct private-field syntax. |
These approaches are not interchangeable security guarantees. A leading underscore communicates intent but does not restrict access. Closures and WeakMap can keep data out of ordinary object properties, but use different code organization. The sources cited here do not establish a performance comparison, so choose based on encapsulation needs and maintainability rather than an assumed speed advantage.
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Compatibility and tooling notes
MDN’s compatibility introduction says private elements have been available across browsers since July 2021. That is a broad browser-availability statement, not a guarantee for every older runtime or embedded JavaScript engine. If supporting legacy or embedded environments, check the actual runtime and any build tooling your project targets.
Chrome DevTools documents a console-only relaxation that can make private elements appear inspectable from outside. That behavior is a debugging convenience, not valid application syntax or a change to JavaScript’s private-access rules. For the normative language definition, consult the ECMAScript specification; background on the feature is available in the TC39 class-fields proposal repository.
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