Observer

type Listener = (quantity: number) => void;

class Cart {
  private quantity = 0;
  private listeners = new Map<number, Listener>();
  private nextId = 1;

  getQuantity(): number {
    return this.quantity;
  }

  subscribe(listener: Listener): number {
    const id = this.nextId++;
    this.listeners.set(id, listener);
    return id;
  }

  unsubscribe(id: number): void {
    this.listeners.delete(id);
  }

  setQuantity(quantity: number): void {
    if (!Number.isInteger(quantity) || quantity < 0 || quantity > 9) return;
    if (quantity === this.quantity) return;
    this.quantity = quantity;
    // Copy the current registrations, then call each one in order.
    for (const listener of [...this.listeners.values()]) listener(quantity);
  }
}

const cart = new Cart();
let badge = cart.getQuantity();
let panel = cart.getQuantity();

cart.subscribe((quantity) => {
  badge = quantity;
});
const panelId = cart.subscribe((quantity) => {
  panel = quantity;
});

cart.setQuantity(1);
console.log(`badge ${badge}, panel ${panel}`);

cart.unsubscribe(panelId); // the panel closes
cart.setQuantity(2);
console.log(`badge ${badge}, panel ${panel}`);