Warning: MaxListenersExceededWarning: Possible EventEmitter memory leak detected

What causes this error

Every EventEmitter (including HTTP requests/responses, streams, and many database clients) defaults to a limit of 10 listeners per event as a leak-detection heuristic. Crossing that limit almost always means a listener is being registered somewhere that runs more than once, rather than the limit being genuinely too low.

The Fix

// Wrong — adds a fresh listener on every call
function handleRequest(req, res) {
  someSharedEmitter.on('data', (chunk) => process(chunk)); // leak!
  // ...
}

// Fix — attach the listener once, outside the hot path
someSharedEmitter.on('data', (chunk) => process(chunk));
function handleRequest(req, res) {
  // ...
}

// If the count really is expected and bounded, raise the limit explicitly
someSharedEmitter.setMaxListeners(20);

Common causes / variations

• Registering a listener inside a request handler, loop, or component that re-runs
• Forgetting to call `.off()`/`.removeListener()` when a resource (a socket, a watched file) is torn down
• A shared singleton emitter being listened to by many short-lived consumers that never clean up

Related errors

See also: listen EADDRINUSE, EventEmitter async iteration errors.