EventEmitter

Common errors

• TypeError: EventEmitter is not a constructor

  cause Attempting to instantiate `EventEmitter` without correctly importing or accessing it, often due to CommonJS/ESM module mismatches or incorrect global access in the browser.
  fix Ensure you are using the correct import/require syntax for your environment. For modern ESM environments: `import EventEmitter from 'wolfy87-eventemitter';`. For CommonJS: `const EventEmitter = require('wolfy87-eventemitter');`. In browsers (script tag), it might be globally available as `window.EventEmitter`.

• TypeError: Cannot read properties of undefined (reading 'on')

  cause Attempting to call methods like `on` or `emit` on an `EventEmitter` instance that is undefined or null, usually due to incorrect initialization or scope issues.
  fix Verify that your `EventEmitter` instance (`emitter` in examples) is properly initialized with `new EventEmitter()` before attempting to use its methods.

• Uncaught TypeError: Android 2.2 RegExp type check issue

  cause A bug in older Android 2.2 browsers related to `RegExp` type checking prevented proper functionality.
  fix Upgrade to `v4.2.7` or later, which includes a fix for this specific Android 2.2 browser bug. Note that this is for a very old browser version.

Warnings

• breaking The project is abandoned. While `npm` lists `5.2.9`, the GitHub repository has seen no activity since 2016, and `v4.2.11` is the last official release tag. No documentation or migration guides exist for v5.
  Fix: Consider migrating to an actively maintained event emitter library such as `eventemitter3` or Node.js's built-in `events` module if in a Node environment.
• breaking Starting with `v4.2.11`, the project license changed from MIT to The Unlicense. This is a significant shift in licensing terms, moving to public domain dedication.
  Fix: Review The Unlicense terms to ensure compliance with your project's legal requirements.
• gotcha The `EventEmitter` API underwent a significant remapping and redesign in its fourth major rewrite (v4.x). If upgrading from very old versions (pre-4.x), many method names and behaviors will have changed.
  Fix: Refer to the v4.x API documentation for the correct method names and usage patterns, specifically `on`, `emit`, `off`, `once`, `hasListeners`, `getListeners`, `removeAllListeners`.
• gotcha The `removeAllListeners` method was added as an alias to `removeEvent` in `v4.2.4` to align with the Node.js API. While `removeEvent` still functions, `removeAllListeners` is the more consistent and recommended method name.
  Fix: Update calls to `removeEvent(eventName)` to `removeAllListeners(eventName)` for improved clarity and Node.js API conformity.
• gotcha An infinite recursion bug was present in `addOnceListener` in versions prior to `v4.2.3` under specific conditions.
  Fix: Ensure you are using `v4.2.3` or higher to mitigate potential infinite recursion issues with `once` listeners.

Install

• npm install wolfy87-eventemitter npm
• yarn add wolfy87-eventemitter yarn
• pnpm add wolfy87-eventemitter pnpm

Imports

• EventEmitter
  wrong:

  const EventEmitter = require('wolfy87-eventemitter');

  correct:

  import EventEmitter from 'wolfy87-eventemitter';

  ESM import for modern bundlers or Node.js. Older versions and environments typically use CommonJS `require` or global access.
• EventEmitter
  wrong:

  import EventEmitter from 'wolfy87-eventemitter';

  correct:

  const EventEmitter = require('wolfy87-eventemitter');

  CommonJS `require` for Node.js (prior to ESM) or older bundlers. The package might expose a global `EventEmitter` in browser environments when included via a script tag.
• typeof EventEmitter

  import type EventEmitter from 'wolfy87-eventemitter';

  TypeScript type import. The library ships its own type definitions.

Quickstart

Demonstrates basic event handling: registering, emitting, one-time listeners, and removing listeners.

import EventEmitter from 'wolfy87-eventemitter';

const emitter = new EventEmitter();

// Register a listener for the 'data' event
emitter.on('data', (payload: string) => {
  console.log('Received data:', payload);
});

// Register a 'once' listener for the 'init' event
emitter.once('init', () => {
  console.log('Application initialized.');
});

// Emit the 'init' event
emitter.emit('init');
// This will not log again as 'once' listeners are removed after first emission
emitter.emit('init');

// Emit the 'data' event with a payload
emitter.emit('data', 'Hello Event World!');

// Remove a specific listener
const anotherListener = (num: number) => console.log('Number received:', num);
emitter.on('number', anotherListener);
emitter.emit('number', 42);
emitter.off('number', anotherListener);
emitter.emit('number', 100); // This will not log

// Remove all listeners for a specific event
emitter.on('cleanup', () => console.log('Cleaning up 1'));
emitter.on('cleanup', () => console.log('Cleaning up 2'));
emitter.emit('cleanup');
emitter.removeAllListeners('cleanup');
emitter.emit('cleanup'); // No output