event

The events module and the EventEmitter pattern are central to Node.js’ event-driven architecture. Let’s dig in.

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📂 1. Importing

const EventEmitter = require("events");

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⚡ 2. Creating an EventEmitter

const EventEmitter = require("events");
 
const emitter = new EventEmitter();
 
// Register a listener
emitter.on("greet", (name) => {
  console.log(`Hello, ${name}!`);
});
 
// Emit (trigger) the event
emitter.emit("greet", "Tyson");

👉 Output:

Hello, Tyson!

• on(event, listener) → listen for an event.

• emit(event, ...args) → trigger the event with optional arguments.

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🔄 3. Multiple Listeners

emitter.on("data", (msg) => console.log("Listener 1:", msg));
emitter.on("data", (msg) => console.log("Listener 2:", msg));
 
emitter.emit("data", "Hello");
// Listener 1: Hello
// Listener 2: Hello

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🧹 4. One-time Listeners

emitter.once("init", () => {
  console.log("This runs only once!");
});
 
emitter.emit("init"); // Runs
emitter.emit("init"); // Ignored

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❌ 5. Removing Listeners

function logMessage(msg) {
  console.log("Message:", msg);
}
 
emitter.on("message", logMessage);
emitter.emit("message", "First");  // Message: First
 
emitter.removeListener("message", logMessage);
emitter.emit("message", "Second"); // (nothing happens)

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📊 6. Inheritance Pattern

Most Node.js core modules (like http.Server, fs.ReadStream) are EventEmitters.
You can extend EventEmitter in your own classes:

class MyServer extends EventEmitter {
  start() {
    console.log("Server started...");
    this.emit("ready");
  }
}
 
const server = new MyServer();
 
server.on("ready", () => console.log("Server is ready!"));
 
server.start();

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⚠️ 7. Best Practices

• Always remove listeners if they’re no longer needed (removeListener or removeAllListeners).

• Watch out for memory leaks: Node warns if you add >10 listeners to one event.

• Prefer .once() for initialization/setup events.

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✅ In Short

• EventEmitter lets you decouple logic using events and listeners.

• Core Node APIs (HTTP, Streams, etc.) are built on this pattern.

• You can use it to build your own async event-driven modules.

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