Creating and Using Custom Event Emitters in Node.js

<rule>
name: custom_event_emitters_nodejs
description: Enforces best practices for creating and utilizing custom event emitters in Node.js to ensure effective event-driven programming.
filters:
  - type: file_extension
    pattern: "\\.js$|\\.ts$"
  - type: content
    pattern: "EventEmitter"

actions:
  - type: suggest
    message: |
      When implementing custom event emitters in Node.js, adhere to the following best practices:

      1. **Extend EventEmitter Properly**:
         - Use `util.inherits` or `Object.create` to extend `EventEmitter` correctly, ensuring each instance has its own event listeners.
         - **Example**:
           ```javascript
           const EventEmitter = require('events');
           const util = require('util');

           function MyEmitter() {
             EventEmitter.call(this);
           }
           util.inherits(MyEmitter, EventEmitter);
           ```

      2. **Avoid Memory Leaks**:
         - Remove event listeners when they are no longer needed using `removeListener` or `off`.
         - **Example**:
           ```javascript
           const myEmitter = new MyEmitter();
           function handleEvent() {
             console.log('Event occurred');
           }
           myEmitter.on('event', handleEvent);
           // Later, when the listener is no longer needed
           myEmitter.removeListener('event', handleEvent);
           ```

      3. **Handle Errors Gracefully**:
         - Always listen for the `error` event to prevent crashes.
         - **Example**:
           ```javascript
           myEmitter.on('error', (err) => {
             console.error('An error occurred:', err.message);
           });
           ```

      4. **Use Descriptive Event Names**:
         - Choose meaningful and descriptive names for events to enhance code readability and maintainability.
         - **Example**:
           ```javascript
           myEmitter.emit('userLoggedIn', user);
           ```

      5. **Limit the Number of Listeners**:
         - Be cautious of adding too many listeners to a single event to prevent performance degradation.
         - **Example**:
           ```javascript
           myEmitter.setMaxListeners(20);
           ```

      6. **Avoid Emitting Events from Within Listeners**:
         - Emitting events within listeners can lead to complex chains and potential memory leaks. Structure your event flow to avoid this pattern.

      7. **Use Asynchronous Listeners for Long-Running Tasks**:
         - If a listener performs a long-running task, consider making it asynchronous to prevent blocking the event loop.
         - **Example**:
           ```javascript
           myEmitter.on('data', async (data) => {
             await processData(data);
           });
           ```

examples:
  - input: |
      const EventEmitter = require('events');
      class MyEmitter extends EventEmitter {}
      const myEmitter = new MyEmitter();
      myEmitter.on('event', () => {
        console.log('An event occurred!');
      });
      myEmitter.emit('event');
    output: |
      const EventEmitter = require('events');
      const util = require('util');

      function MyEmitter() {
        EventEmitter.call(this);
      }
      util.inherits(MyEmitter, EventEmitter);

      const myEmitter = new MyEmitter();
      myEmitter.on('event', () => {
        console.log('An event occurred!');
      });
      myEmitter.emit('event');
  - input: |
      const EventEmitter = require('events');
      class MyEmitter extends EventEmitter {}
      const myEmitter = new MyEmitter();
      myEmitter.on('event', () => {
        console.log('An event occurred!');
      });
      myEmitter.on('event', () => {
        console.log('Another event occurred!');
      });
      myEmitter.emit('event');
    output: |
      const EventEmitter = require('events');
      const util = require('util');

      function MyEmitter() {
        EventEmitter.call(this);
      }
      util.inherits(MyEmitter, EventEmitter);

      const myEmitter = new MyEmitter();
      myEmitter.on('event', () => {
        console.log('An event occurred!');
      });
      myEmitter.on('event', () => {
        console.log('Another event occurred!');
      });
      myEmitter.emit('event');

metadata:
  priority: high
  version: 1.0
  tags:
    - nodejs
    - event-emitter
    - best-practices
    - memory-management
    - error-handling
</rule>