Node.js 'MaxListenersExceededWarning: Possible EventEmitter memory leak detected. 11 listeners added' - is it a real leak and how do I fix it?

Cause: An EventEmitter warns when more than 10 listeners are attached for one event on one emitter (the default limit). The limit is a guard against leaks, not a hard cap: the listener is still added, and Node prints a warning to stderr. It is a real leak when code adds a listener on every request, retry or render and never removes it. It is a false alarm when many independent consumers legitimately listen to one emitter. Fix (best first): 1. Find the source. Run with node --trace-warnings app.js (or NODE_OPTIONS=--trace-warnings) to get the stack of the code adding the listener. In code, process.on('warning', w => ...) exposes the emitter, event type and count on the warning object. 2. Remove listeners you no longer need. Use emitter.once(...) for single-use handlers, and emitter.off(event, handler) / removeListener with the same function reference when a request, connection or component finishes. Move emitter.on calls out of per-request code paths. 3. Watch for retry loops and libraries that attach a listener to a shared socket, process or stream on each call; pass a single reused handler, or reuse the connection. 4. If many listeners are intended, raise the limit for that one emitter: emitter.setMaxListeners(n). Use emitter.getMaxListeners() to see the current value. Setting 0 or Infinity disables the check, which hides real leaks, so avoid it. 5. Changing events.defaultMaxListeners affects all emitters, including ones already created; a per-instance setMaxListeners value takes precedence over it. Use it sparingly. 6. For EventTarget and AbortSignal, events.setMaxListeners(n, target) and events.getMaxListeners(target) apply; AbortSignal does not warn by default. Verify: rerun with --trace-warnings and exercise the same path repeatedly; the listener count (emitter.listenerCount('event')) should stay flat and the warning should not return. Sources: https://nodejs.org/api/events.html https://nodejs.org/api/cli.html