Node.js "MaxListenersExceededWarning" - Fix EventEmitter Leaks in CI

Node warns when an EventEmitter accumulates more than 10 listeners for one event - usually a sign that listeners are added in a loop or per request without ever being removed.

What this error means

A noisy MaxListenersExceededWarning appears in logs (and a stack trace if --trace-warnings is on). In long test runs it can precede slow growth in memory. The warning itself does not crash, but it points at a real leak.

Node output

(node:1) MaxListenersExceededWarning: Possible EventEmitter memory leak
detected. 11 SIGTERM listeners added to [process]. Use emitter.setMaxListeners()
to increase limit

Common causes

Listeners added without removal

A function called repeatedly (per request, per test, in a loop) attaches a listener each time but never calls removeListener/off. The count climbs past the default limit of 10.

A genuinely high but bounded listener count

Some legitimate designs attach more than 10 listeners to one emitter on purpose. Here the warning is a false positive and the limit should be raised intentionally.

How to fix it

Find and remove the leaked listeners

Run with --trace-warnings to get the stack where the listener is added, then ensure each on() has a matching off() (or use once()).

Terminal / app.mjs

node --trace-warnings app.mjs
# in code: remove what you add
const onTerm = () => shutdown();
process.on('SIGTERM', onTerm);
// later
process.off('SIGTERM', onTerm);

Raise the limit only when the count is legitimate

If many listeners are correct by design, raise the threshold on that specific emitter - not globally as a way to silence a real leak.

app.mjs

emitter.setMaxListeners(50);

How to prevent it

Pair every on() with an off(), or use once() for one-shot handlers.
Run with --trace-warnings in CI so the leak source is visible.
Only call setMaxListeners when a high count is intentional and bounded.