Private Fields

Private Fields

In JavaScript, class properties are public by default — anyone can read or modify them. Private fields let you restrict access to certain properties, keeping them hidden outside the class. You create them using the # prefix.

class BankAccount {
  #balance = 0;

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

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

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

const account = new BankAccount(1000);
account.deposit(500);
console.log(account.getBalance()); // 1500
console.log(account.#balance); // ❌ SyntaxError!

Why Use Private Fields?

Private fields protect the internal state of your class. Without them, external code could put your object into an invalid state. For example, setting a negative balance on a bank account.

class User {
  #password;

  constructor(username, password) {
    this.username = username;
    this.#password = password;
  }

  checkPassword(guess) {
    return guess === this.#password;
  }
}

const user = new User("alice", "secret123");
console.log(user.username); // "alice" — public
console.log(user.#password); // ❌ Can't access!

Private Methods

You can also make methods private using the # prefix. This is useful for helper methods that shouldn't be called from outside.

class TemperatureConverter {
  #toCelsius(fahrenheit) {
    return (fahrenheit - 32) * 5 / 9;
  }

  convert(fahrenheit) {
    return this.#toCelsius(fahrenheit);
  }
}

const converter = new TemperatureConverter();
console.log(converter.convert(212)); // 100
console.log(converter.#toCelsius(212)); // ❌ Not accessible

Comparison with Underscore Convention

Before private fields, developers used underscore prefixes like _balance to signal "private" properties. But that was just a convention — the property was still accessible. The # prefix is enforced by the language.

class OldStyle {
  constructor() {
    this._secret = "kind of private"; // Just a convention
  }
}

const obj = new OldStyle();
console.log(obj._secret); // Still accessible!

Summary

Private fields (#) give you true encapsulation in JavaScript classes. Use them to protect internal state and expose only what's necessary through public methods.