events, doc: check input in defaultMaxListeners

PR-URL: https://github.com/nodejs/node/pull/11938
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: Brian White <mscdex@mscdex.net>

doc/api/events.md

@@ -262,7 +262,8 @@ By default, a maximum of `10` listeners can be registered for any single
 event. This limit can be changed for individual `EventEmitter` instances
 using the [`emitter.setMaxListeners(n)`][] method. To change the default
 for *all* `EventEmitter` instances, the `EventEmitter.defaultMaxListeners`
-property can be used.
+property can be used. If this value is not a positive number, a `TypeError`
+will be thrown.
 
 Take caution when setting the `EventEmitter.defaultMaxListeners` because the
 change effects *all* `EventEmitter` instances, including those created before

lib/events.js

@@ -56,6 +56,10 @@ Object.defineProperty(EventEmitter, 'defaultMaxListeners', {
     // force global console to be compiled.
     // see https://github.com/nodejs/node/issues/4467
     console;
+    // check whether the input is a positive number (whose value is zero or
+    // greater and not a NaN).
+    if (typeof arg !== 'number' || arg < 0 || arg !== arg)
+      throw new TypeError('"defaultMaxListeners" must be a positive number');
     defaultMaxListeners = arg;
   }
 });

test/parallel/test-event-emitter-max-listeners.js

@@ -30,16 +30,13 @@ e.on('maxListeners', common.mustCall(function() {}));
 // Should not corrupt the 'maxListeners' queue.
 e.setMaxListeners(42);
 
-assert.throws(function() {
-  e.setMaxListeners(NaN);
-}, /^TypeError: "n" argument must be a positive number$/);
+const throwsObjs = [NaN, -1, 'and even this'];
 
-assert.throws(function() {
-  e.setMaxListeners(-1);
-}, /^TypeError: "n" argument must be a positive number$/);
-
-assert.throws(function() {
-  e.setMaxListeners('and even this');
-}, /^TypeError: "n" argument must be a positive number$/);
+for (const obj of throwsObjs) {
+  assert.throws(() => e.setMaxListeners(obj),
+                /^TypeError: "n" argument must be a positive number$/);
+  assert.throws(() => events.defaultMaxListeners = obj,
+                /^TypeError: "defaultMaxListeners" must be a positive number$/);
+}
 
 e.emit('maxListeners');