What is the Observer pattern, and how does Node.js's EventEmitter class implement it natively?

The Observer pattern lets an object (the subject) maintain a list of dependents (observers) and notify them automatically of state changes, without the subject needing to know any specifics about the observers -- Node.js's built-in EventEmitter class is a direct, first-class implementation of this pattern: any object can extend EventEmitter, emit named events, and any number of listeners can subscribe to those events independently.

const EventEmitter = require('node:events');

class OrderProcessor extends EventEmitter {
  processOrder(order) {
    // ... process order ...
    this.emit('orderCompleted', order);
  }
}

const processor = new OrderProcessor();
processor.on('orderCompleted', (order) => sendConfirmationEmail(order));
processor.on('orderCompleted', (order) => updateInventory(order));