Events in Node.js

Node.js provides the EventEmitter class for event-driven programming outside the browser. Many Node.js core modules (streams, HTTP servers, file system watchers) are built on EventEmitter.

The EventEmitter Class

Import EventEmitter from the events module:

const EventEmitter = require("events");
// or in ES modules:
// import { EventEmitter } from 'events';

const emitter = new EventEmitter();

Basic Usage

Register listeners with on and trigger events with emit:

const EventEmitter = require("events");
const emitter = new EventEmitter();

// Register a listener
emitter.on("greet", (name) => {
  console.log(`Hello, ${name}!`);
});

// Emit the event
emitter.emit("greet", "Alice");  // Hello, Alice!
emitter.emit("greet", "Bob");  // Hello, Bob!

Passing Multiple Arguments

emit can pass multiple arguments to listeners:

emitter.on("userCreated", (id, name, email) => {
  console.log(`New user: ${name} (${email}) with ID ${id}`);
});

emitter.emit("userCreated", 1, "Alice", "alice@example.com");

Once: Single-Use Listeners

Use once for listeners that should run only once:

emitter.once("ready", () => {
  console.log("Application is ready!");
});

emitter.emit("ready");  // Logs message
emitter.emit("ready");  // Nothing happens

Removing Listeners

Remove specific listeners with off or removeListener:

function handleData(data) {
  console.log("Received:", data);
}

emitter.on("data", handleData);
emitter.off("data", handleData);

// Or remove all listeners for an event
emitter.removeAllListeners("data");

Creating Custom Event Emitters

Extend EventEmitter to create your own event-driven classes. This is a common pattern in Node.js:

const EventEmitter = require("events");

class TaskQueue extends EventEmitter {
  constructor() {
    super();
    this.tasks = [];
  }

  addTask(task) {
    this.tasks.push(task);
    this.emit("taskAdded", task);
  }

  processNext() {
    if (this.tasks.length === 0) {
      this.emit("empty");
      return;
    }

    const task = this.tasks.shift();
    this.emit("taskStarted", task);

    // Simulate async work
    setTimeout(() => {
      this.emit("taskCompleted", task);
    }, 1000);
  }
}

// Usage
const queue = new TaskQueue();

queue.on("taskAdded", (task) => console.log("Added:", task));
queue.on("taskStarted", (task) => console.log("Started:", task));
queue.on("taskCompleted", (task) => console.log("Completed:", task));
queue.on("empty", () => console.log("Queue is empty"));

queue.addTask("Send email");
queue.addTask("Generate report");
queue.processNext();

Error Handling with error Events

EventEmitter has special handling for error events. If an error event is emitted with no listeners, Node.js throws an exception:

const emitter = new EventEmitter();

// This will crash the process!
emitter.emit("error", new Error("Something went wrong"));

// Always add an error listener
emitter.on("error", (err) => {
  console.error("Error occurred:", err.message);
});

Practical Example: File Watcher

const EventEmitter = require("events");
const fs = require("fs");

class FileWatcher extends EventEmitter {
  constructor(filepath) {
    super();
    this.filepath = filepath;
  }

  start() {
    fs.watch(this.filepath, (eventType, filename) => {
      if (eventType === "change") {
        this.emit("change", filename);
      } else if (eventType === "rename") {
        this.emit("rename", filename);
      }
    });

    this.emit("started", this.filepath);
  }
}

const watcher = new FileWatcher("./data.json");

watcher.on("started", (path) => console.log(`Watching: ${path}`));
watcher.on("change", (file) => console.log(`File changed: ${file}`));
watcher.on("rename", (file) => console.log(`File renamed: ${file}`));

watcher.start();

Useful EventEmitter Methods

Browser vs Node.js Comparison