Encapsulation

Encapsulation hides internal details and exposes a clear interface. It helps prevent accidental misuse and keeps invariants intact.

What is encapsulation?

• Keep internal details private.
• Expose only what callers need.
• Protect invariants (rules your object must always satisfy).

Public vs private data

• Public fields are accessible from anywhere.
• Private fields start with # and are only accessible inside the class.

class BankAccount {
  #balance = 0; // private field
  deposit(amount) {
    if (amount <= 0) throw new Error("Amount must be positive");
    this.#balance += amount;
  }
  getBalance() {
    return this.#balance;
  }
}

Private fields are not enumerable and cannot be accessed outside the class. Note: older environments may need transpilation for #private.

Getters and setters

Use getters/setters to expose computed or validated properties.

class User {
  #email;
  constructor(email) {
    this.email = email; // uses setter
  }
  get email() {
    return this.#email.toLowerCase();
  }
  set email(value) {
    if (!value.includes("@")) throw new Error("Invalid email");
    this.#email = value;
  }
}

Getters compute values on access; setters let you validate or transform input. Use them when you need control, otherwise keep plain fields.