Learn Private Fields in Classes | Inheritance and Encapsulation

Private fields in JavaScript classes give you a way to hide data from outside code, reinforcing the principle of encapsulation. When you use the # symbol before a field name in a class, you create a private field. This means the field is only accessible within the class body itself—code outside the class cannot read or write to it directly. By restricting access to sensitive data, you prevent accidental modification and help keep your objects' internal state safe. This is especially important in situations where exposing data could lead to bugs or security issues.


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class BankAccount {
  #balance; // private field

  constructor(initialBalance) {
    this.#balance = initialBalance;
  }

  deposit(amount) {
    if (amount > 0) {
      this.#balance += amount;
    }
  }

  withdraw(amount) {
    if (amount > 0 && amount <= this.#balance) {
      this.#balance -= amount;
      return amount;
    }
    return 0;
  }

  getBalance() {
    return this.#balance;
  }
}

const account = new BankAccount(100);
account.deposit(50);
console.log(account.getBalance()); // 150
console.log(account.#balance); // SyntaxError: Private field '#balance' must be declared in an enclosing class

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Section 2. Chapter 7

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class BankAccount {
  #balance; // private field

  constructor(initialBalance) {
    this.#balance = initialBalance;
  }

  deposit(amount) {
    if (amount > 0) {
      this.#balance += amount;
    }
  }

  withdraw(amount) {
    if (amount > 0 && amount <= this.#balance) {
      this.#balance -= amount;
      return amount;
    }
    return 0;
  }

  getBalance() {
    return this.#balance;
  }
}

const account = new BankAccount(100);
account.deposit(50);
console.log(account.getBalance()); // 150
console.log(account.#balance); // SyntaxError: Private field '#balance' must be declared in an enclosing class

Everything was clear?

Thanks for your feedback!

Section 2. Chapter 7