EventEmitter and event arguments

Model answer. The events module provides `EventEmitter`, the observer pattern built into Node. Many core objects (streams, HTTP servers, sockets) are emitters.

import { EventEmitter } from 'node:events';

class OrderService extends EventEmitter {
  place(order) {
    // ...save the order...
    this.emit('placed', order.id, order.total);   // extra arguments go to listeners
  }
}

const orders = new OrderService();
orders.on('placed', (id, total) => console.log(`order ${id}: ₹${total}`));
orders.once('placed', () => console.log('first order of the day!'));
orders.place({ id: 17, total: 499 });

• on adds a listener, once listens a single time, off removes one.
• Listeners run synchronously, in the order they were added, during emit.
• Event arguments are whatever values you pass to emit after the event name.
• An 'error' event with no listener throws, and can crash the process, so always listen for 'error' on emitters you create or use.
• Adding more than 10 listeners to one event prints a memory-leak warning, which usually means listeners are added repeatedly without being removed.