How to fix SyntaxError – Private field '#x' must be declared in an enclosing class in JavaScript?

In JavaScript "SyntaxError : Private field '#x' must be declared in an enclosing class" error occurs when code tries to access or use a private field that has not been declared within the class it is being used, private fields in JavaScript are declared using a # prefix and can only be accessed within the class they are defined, in the given article we will delve into this error, its causes, and how to resolve it with examples.

Understanding an error

A private field called ‘#x’ must be declared in an enclosing class error signifies that the code is attempting to use a private field that has not been declared within the class scope and if you want to attempt access on the same, one has to state its name at the beginning of the method.

These are the following cases where this type of error occurs:

Case 1: Accessing an Undeclared Private Field

Error Cause:

This error is all about what you will get when you try to access a private field that has not been declared in the class.

Example: This code throws a SyntaxError because the private field #x is used inside the class constructor without being declared.

class MyClass {
  constructor() {
    console.log(this.#x); // Trying to access #x which is not declared
  }
}

const instance = new MyClass();

Output:

SyntaxError: Private field '#x' must be declared in an enclosing class

Resolution of error:

To avoid such kind of error you should use the # symbol to declare the private field within the class.

class MyClass {
  #x; // Declare the private field

  constructor() {
    this.#x = 10;
    console.log(this.#x); // Accessing the declared private field
  }
}

const instance = new MyClass(); // Outputs: 10

10

Case 2: Using a Private Field Outside Its Declaring Class

Error Cause:

You will see this error when you are attempting access of a private field from outside of its class declaration.

Example: This code throws a SyntaxError because private fields (like #x) are only accessible within the class that defines them. Attempting to access #x outside the class results in this error.

class MyClass {
  #x = 10;

  getX() {
    return this.#x;
  }
}

const instance = new MyClass();
console.log(instance.#x); // Trying to access #x outside of its class

Output:

SyntaxError: Private field '#x' must be declared in an enclosing class

Resolution of error :

Only use methods defined in this class if you want to get into the private field.

class MyClass {
  #x = 10;

  getX() {
    return this.#x;
  }
}

const instance = new MyClass();
console.log(instance.getX()); // Outputs: 10

10

Case 3: Incorrect Class Instantiation

Error Cause:

If you are trying to access a private field directly through a class without proper instantiation or within non-class context then there will be an error.

Example: This code throws a SyntaxError because private fields (like #x) are not accessible on the class itself. They can only be accessed within an instance of the class. Attempting to access #x directly from the class results in this error.

class MyClass {
  #x = 10;

  constructor() {
    console.log(this.#x);
  }
}

console.log(MyClass.#x); // Trying to access #x on the class itself

Output:

SyntaxError: Private field '#x' must be declared in an enclosing class

Resolution of error:

Instead we can only access it via an instance of that particular class.

class MyClass {
  #x = 10;

  constructor() {
    console.log(this.#x);
  }
}

const instance = new MyClass(); // Outputs: 10

10

Conclusion

In conclusion to prevent occurrence of “Private field ‘#x’ must be declared in an enclosing class” error in JavaScript, make sure that private fields are defined only within the class scope with # prefix and they are accessed just by methods which belong to this class, by following these private field syntax rules as per JavaScript makes sure correct approach to encapsulation and access control are adopted.