How to fix Error: Listener already called (once event already fired) in Node.js

const { EventEmitter } = require('events');
const emitter = new EventEmitter();

// Using on() - listener persists
emitter.on('data', (value) => {
  console.log('Received:', value);
});

emitter.emit('data', 'first');   // Prints: Received: first
emitter.emit('data', 'second');  // Prints: Received: second
emitter.emit('data', 'third');   // Prints: Received: third

// Using once() - listener fires once then auto-removes
emitter.once('ready', () => {
  console.log('Ready!');
});

emitter.emit('ready');  // Prints: Ready!
emitter.emit('ready');  // Prints nothing (listener already removed)

const { EventEmitter } = require('events');
const emitter = new EventEmitter();

function handleOnce() {
  console.log('One-time handler');
}

// Register as one-time listener
emitter.once('event', handleOnce);

emitter.emit('event');  // Prints: One-time handler

// WRONG: Trying to remove after it's already fired and auto-removed
emitter.removeListener('event', handleOnce);  // No-op, already removed
emitter.listenerCount('event');  // Returns 0

// CORRECT: If you need to control removal, use on() instead
emitter.on('event', handleOnce);
// Then later, explicitly remove when needed:
emitter.removeListener('event', handleOnce);

const { EventEmitter } = require('events');
const server = new EventEmitter();

// Correct use: Server startup happens once
server.once('listening', () => {
  console.log('Server started on port 3000');
});

// Correct use: Handling multiple connections
server.on('connection', (socket) => {
  console.log('Client connected:', socket.id);
});

// DO NOT do this - it defeats the purpose of once()
let shouldHandle = true;
server.once('data', () => {
  if (shouldHandle) {
    console.log('Processing data');
    shouldHandle = false;  // Unnecessary, once() already does this
  }
});

const { EventEmitter } = require('events');
const emitter = new EventEmitter();

function handleReady() {
  console.log('Ready!');
}

emitter.once('ready', handleReady);
console.log('Listeners before emit:', emitter.listenerCount('ready'));  // 1

emitter.emit('ready');
console.log('Listeners after emit:', emitter.listenerCount('ready'));   // 0

// Safe approach: Check before operating on listeners
if (emitter.listenerCount('ready') > 0) {
  // Do something with active listeners
} else {
  console.log('No active listeners for this event');
}

const { EventEmitter } = require('events');
const emitter = new EventEmitter();

// Regular once() - executes in registration order
emitter.once('event', () => console.log('a'));
emitter.on('event', () => console.log('b'));

// Prepend once() - executes before others
emitter.prependOnceListener('event', () => console.log('c'));

emitter.emit('event');
// Prints:
//   c
//   a
//   b

// After first emit, the once() listeners are removed
emitter.emit('event');
// Prints only:
//   b

const { EventEmitter } = require('events');
const emitter = new EventEmitter();

function registerOneTimeHandler() {
  emitter.once('attempt', (result) => {
    console.log('Attempt result:', result);
    if (result === 'retry') {
      // Re-register for next attempt
      registerOneTimeHandler();
    }
  });
}

// Initial registration
registerOneTimeHandler();

emitter.emit('attempt', 'failed');  // Fires, prints: Attempt result: failed
emitter.emit('attempt', 'success');  // Does nothing (listener already removed)

// Need to call registerOneTimeHandler() again for next batch
registerOneTimeHandler();
emitter.emit('attempt', 'success');  // Fires again