EventEmitter ni nini na jinsi gani inavyotumika?

Question

Accepted Answer

**EventEmitter** ni darasa la asili la Node linalotekeleza muundo wa **publish/subscribe**: vitu hutuma matukio yenye jina, na wasikilizaji waliojisajili kwa matukio hayo huendesha wakati yanapoanzishwa. Sehemu nyingi ya API yenyewe ya Node (streams, HTTP servers, process) imejengwa juu yake.

## Matumizi ya msingi

```js
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"
```

Muundo: `on(event, listener)` inajibu; `emit(event, ...args)` inaanzisha wasikilizaji wote kwa ajili ya matukio hayo kwa usambamano, ikipitisha hoja.

## Kupanua EventEmitter kwa madarasa yako mwenyewe

```js
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
```

Hii inatengana na emitter na wasikilizaji wake — `OrderService` haijui au kukamatia kuhusu nani anayepikiliza, kwa hivyo unaweza kuongeza majibu (barua pepe, uchambuzi, kuingia) bila kuibadilisha.

## Njia muhimu

```js
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");
```

## Mitego miwili muhimu

```text
⚠️ 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!).
```

## Ambapo inatumika katika Node yenyewe

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

## Kwa nini inamaanisha

EventEmitter ni msingi wa usanifu unaotukio wa Node — kuelewa ina maana jinsi streams, servers, na kitu cha process kinavyofanya kazi (wote ni wajumbe), na ni zana ya kawaida kwa mawasiliano yenye tukio lililotenganwa katika koodi yako.

Kujua `on`/`emit`/`once`/`off` API, hatari ya kuvuja kumbukumbu kutoka wasikilizaji wasiozondolewa, na matukio maalum yanayosababisha mgogoro `error` ni muhimu kwa ajili ya matumizi sahihi ya APIs za Node na kujenga mifumo ya matukio inayoweza kuendeshwa.