This warning appears when more than 10 event listeners are attached to a single EventEmitter instance, indicating a potential memory leak from listeners not being properly cleaned up.
Node.js EventEmitters have a default limit of 10 listeners per event to help detect memory leaks early. When you exceed this limit, Node.js emits a MaxListenersExceededWarning to alert you that listeners are accumulating on an emitter instance. This is a warning, not an error—your code will continue to run. However, it signals that event listeners are being added repeatedly without being removed, which causes memory consumption to grow over time. Each listener holds references to callback functions and their closures, preventing garbage collection. The warning specifically mentions the number of listeners (e.g., "11 listeners") and suggests using `emitter.setMaxListeners()` to increase the limit. However, simply increasing the limit masks the underlying problem rather than fixing it.
Examine the warning message to see which event has too many listeners:
MaxListenersExceededWarning: Possible EventEmitter memory leak detected.
11 connection listeners added to [RedisClient]The warning shows the event name ("connection") and the emitter type ([RedisClient]). This tells you where to focus your investigation.
Search your codebase for places where you're adding listeners to this event:
// Look for patterns like this
emitter.on('connection', handler);
server.on('error', errorHandler);
socket.on('data', dataHandler);Pay special attention to code inside:
- Request handlers or middleware
- Loops or repeated function calls
- Component lifecycle methods
- Promise chains
If the listener should only fire once, use .once() instead of .on():
// Before (accumulates listeners)
emitter.on('ready', () => {
console.log('Ready!');
});
// After (auto-removes after first trigger)
emitter.once('ready', () => {
console.log('Ready!');
});This is especially important in Promise-based code or initialization logic.
Explicitly remove listeners during cleanup:
function setupHandler() {
const handler = (data) => {
console.log('Received:', data);
};
emitter.on('data', handler);
// Later, when done:
return () => {
emitter.removeListener('data', handler);
};
}
// Or remove all listeners for an event:
emitter.removeAllListeners('data');In React components, remove listeners in cleanup functions:
useEffect(() => {
const handler = () => { /* ... */ };
emitter.on('event', handler);
return () => {
emitter.removeListener('event', handler);
};
}, []);Avoid registering listeners inside code that runs multiple times:
// Bad: Adds a new listener on every request
app.get('/api/data', (req, res) => {
eventBus.on('update', (data) => {
res.json(data);
});
});
// Good: Register listener once at startup
eventBus.once('update', (data) => {
// Handle update
});
app.get('/api/data', (req, res) => {
// Use the data without re-registering
});Run your application and ensure the warning no longer appears:
node your-app.jsMonitor for the MaxListenersExceededWarning in your logs. If it still appears, continue investigating other locations where listeners might be added.
You can also check current listener counts programmatically:
console.log('Listener count:', emitter.listenerCount('eventName'));
console.log('All listeners:', emitter.listeners('eventName'));When increasing the limit is appropriate:
In rare cases, you legitimately need more than 10 listeners (e.g., a message bus with many subscribers). Only then should you increase the limit:
// For a specific emitter instance:
emitter.setMaxListeners(20);
// Globally (use with extreme caution):
require('events').EventEmitter.defaultMaxListeners = 15;
// Unlimited (dangerous, only for debugging):
emitter.setMaxListeners(0);Never use `setMaxListeners(0)` in production—it disables the warning entirely and allows actual memory leaks to go undetected.
HTTP Agent considerations:
When using HTTP/HTTPS agents with keepAlive: true, sockets are reused across requests. If you add error handlers to the socket on each request, they accumulate:
// Problematic pattern
const agent = new https.Agent({ keepAlive: true });
function makeRequest() {
const req = https.request({ agent, ... }, (res) => {
// This adds a new error handler on the same socket each time
res.socket.on('error', handleError);
});
}Solution: Register socket error handlers once, or use .once() for one-time handlers.
Debugging tools:
- Use --trace-warnings flag to see stack traces: node --trace-warnings app.js
- Use process.on('warning', ...) to intercept warnings programmatically
- Tools like clinic, memwatch-next, or Chrome DevTools can help profile memory usage
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