class BankAccount invariant: balance >= 0 ✓
🔒 private · shielded
this.#balance
$100
// only this object's own methods can reach in
✗ direct access
Reach in from outside: account.balance = X. The field is private — this is blocked.
✓ through methods
Go through the front door. The method validates, then updates the shielded field.
console
▸ The left column bounces off the shield — outside code can't touch a private field. The right column runs validation first, so the balance >= 0 invariant always holds.