Classes

Classes provide strict prototype-based constructor syntax. Methods live on the prototype; fields initialize per instance.

Encapsulated model

class Account {
  #balance = 0;
  constructor(owner) { this.owner = owner; }
  deposit(amount) {
    if (amount <= 0) throw new RangeError("positive amount required");
    this.#balance += amount;
  }
  get balance() { return this.#balance; }
  static from(data) { return new Account(data.owner); }
}

extends and super build inheritance, while # fields enforce privacy. Methods lose their receiver when passed bare, so bind or wrap callbacks. Prefer composition for reusable capabilities and inheritance only for genuine substitutability. Constructors cannot be async; use an async factory when setup requires awaiting.