events: optimize condition for optimal scenario

Instead of always checking whether we've already warned about a
possible EventEmitter memory leak, first run the rest of the
code as accessing random properties on an Array is expensive.

In addition, remove an unnecessary truthy check.

PR-URL: #20452
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>

benchmark/events/ee-add-remove.js

@@ -2,7 +2,7 @@
 const common = require('../common.js');
 const events = require('events');
 
-const bench = common.createBenchmark(main, { n: [25e4] });
+const bench = common.createBenchmark(main, { n: [1e6] });
 
 function main({ n }) {
   const ee = new events.EventEmitter();

lib/events.js

@@ -235,22 +235,20 @@ function _addListener(target, type, listener, prepend) {
     }
 
     // Check for listener leak
-    if (!existing.warned) {
-      m = $getMaxListeners(target);
-      if (m && m > 0 && existing.length > m) {
-        existing.warned = true;
-        // No error code for this since it is a Warning
-        // eslint-disable-next-line no-restricted-syntax
-        const w = new Error('Possible EventEmitter memory leak detected. ' +
-                            `${existing.length} ${String(type)} listeners ` +
-                            'added. Use emitter.setMaxListeners() to ' +
-                            'increase limit');
-        w.name = 'MaxListenersExceededWarning';
-        w.emitter = target;
-        w.type = type;
-        w.count = existing.length;
-        process.emitWarning(w);
-      }
+    m = $getMaxListeners(target);
+    if (m > 0 && existing.length > m && !existing.warned) {
+      existing.warned = true;
+      // No error code for this since it is a Warning
+      // eslint-disable-next-line no-restricted-syntax
+      const w = new Error('Possible EventEmitter memory leak detected. ' +
+                          `${existing.length} ${String(type)} listeners ` +
+                          'added. Use emitter.setMaxListeners() to ' +
+                          'increase limit');
+      w.name = 'MaxListenersExceededWarning';
+      w.emitter = target;
+      w.type = type;
+      w.count = existing.length;
+      process.emitWarning(w);
     }
   }