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More in Classes & Class Syntax
What is a class in JavaScript, and is it a new kind of object model? How does the constructor method work in a class? How do you create a subclass using extends? What are class fields, and how do they differ from constructor-assigned properties? What are static methods and properties, and when do you use them? How do getters and setters work inside a class? How does method overriding with super work in JavaScript classes? What is a static initialization block and why would you use one? Why can classes not be called without 'new', unlike regular functions?
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JavaScript › Classes & Class Syntax

What does the # syntax mean for class members?

Intermediate

Answer

The # prefix marks a truly private field or method, accessible only from inside the class body itself — not even subclasses or external code can read or call it, and accessing it from outside throws a SyntaxError, not just returning undefined.
class BankAccount {
  #balance = 0;
  deposit(amount) { this.#balance += amount; }
  getBalance() { return this.#balance; }
}
const acc = new BankAccount();
// acc.#balance; // SyntaxError outside the class
Real-world example Hiding an account's internal balance so it can only be modified through controlled methods like deposit()/withdraw().

Common follow-ups: How did developers fake privacy before # private fields existed?

Design Patterns in JavaScript
All 10 Classes & Class Syntax questions