EventEmitter কী এবং এটি কীভাবে ব্যবহার করা হয়?

import { EventEmitter } from "events";

const emitter = new EventEmitter();

// SUBSCRIBE: register a listener for an event
emitter.on("order", (order) => {
  console.log(`Processing order ${order.id}`);
});

// PUBLISH: emit the event (with optional data)
emitter.emit("order", { id: 123 }); // → "Processing order 123"

class OrderService extends EventEmitter {
  placeOrder(order) {
    // ...save the order...
    this.emit("order:created", order);  // notify anyone interested
  }
}

const service = new OrderService();
service.on("order:created", (o) => sendEmail(o));   // decoupled listeners
service.on("order:created", (o) => updateAnalytics(o));
service.placeOrder({ id: 1 }); // both listeners run

emitter.once("ready", fn);          // listener runs only ONCE, then auto-removes
emitter.off("order", fn);           // remove a listener (prevent leaks)
emitter.emit("error", new Error()); // "error" is special — throws if no listener!
emitter.listenerCount("order");

⚠️ Memory leaks: forgetting to remove listeners (use off/once). Node warns at >10
   listeners on one event ("possible EventEmitter memory leak").
⚠️ The "error" event: if emitted with no listener, Node THROWS and may crash —
   always attach an error listener to emitters that can error (streams!).

streams (data, end, error events), http.Server (request event),
process (SIGTERM, exit), sockets — all are EventEmitters.