events: fix TypeError in EventEmitter warning

lib/events.js

@@ -257,8 +257,9 @@ function _addListener(target, type, listener, prepend) {
       if (m && m > 0 && existing.length > m) {
         existing.warned = true;
         const w = new Error('Possible EventEmitter memory leak detected. ' +
-                            `${existing.length} ${type} listeners added. ` +
-                            'Use emitter.setMaxListeners() to increase limit');
+                            `${existing.length} ${String(type)} listeners ` +
+                            'added. Use emitter.setMaxListeners() to ' +
+                            'increase limit');
         w.name = 'MaxListenersExceededWarning';
         w.emitter = target;
         w.type = type;

test/parallel/test-event-emitter-check-listener-leaks.js

@@ -1,7 +1,7 @@
 'use strict';
 require('../common');
-var assert = require('assert');
-var events = require('events');
+const assert = require('assert');
+const events = require('events');
 
 var e = new events.EventEmitter();
 
@@ -13,6 +13,14 @@ assert.ok(!e._events['default'].hasOwnProperty('warned'));
 e.on('default', function() {});
 assert.ok(e._events['default'].warned);
 
+// symbol
+const symbol = Symbol('symbol');
+e.setMaxListeners(1);
+e.on(symbol, function() {});
+assert.ok(!e._events[symbol].hasOwnProperty('warned'));
+e.on(symbol, function() {});
+assert.ok(e._events[symbol].hasOwnProperty('warned'));
+
 // specific
 e.setMaxListeners(5);
 for (let i = 0; i < 5; i++) {

test/parallel/test-event-emitter-max-listeners-warning-for-null.js

@@ -0,0 +1,22 @@
+// Flags: --no-warnings
+// The flag suppresses stderr output but the warning event will still emit
+'use strict';
+
+const common = require('../common');
+const events = require('events');
+const assert = require('assert');
+
+const e = new events.EventEmitter();
+e.setMaxListeners(1);
+
+process.on('warning', common.mustCall((warning) => {
+  assert.ok(warning instanceof Error);
+  assert.strictEqual(warning.name, 'MaxListenersExceededWarning');
+  assert.strictEqual(warning.emitter, e);
+  assert.strictEqual(warning.count, 2);
+  assert.strictEqual(warning.type, null);
+  assert.ok(warning.message.includes('2 null listeners added.'));
+}));
+
+e.on(null, function() {});
+e.on(null, function() {});

test/parallel/test-event-emitter-max-listeners-warning-for-symbol.js

@@ -0,0 +1,24 @@
+// Flags: --no-warnings
+// The flag suppresses stderr output but the warning event will still emit
+'use strict';
+
+const common = require('../common');
+const events = require('events');
+const assert = require('assert');
+
+const symbol = Symbol('symbol');
+
+const e = new events.EventEmitter();
+e.setMaxListeners(1);
+
+process.on('warning', common.mustCall((warning) => {
+  assert.ok(warning instanceof Error);
+  assert.strictEqual(warning.name, 'MaxListenersExceededWarning');
+  assert.strictEqual(warning.emitter, e);
+  assert.strictEqual(warning.count, 2);
+  assert.strictEqual(warning.type, symbol);
+  assert.ok(warning.message.includes('2 Symbol(symbol) listeners added.'));
+}));
+
+e.on(symbol, function() {});
+e.on(symbol, function() {});

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

@@ -15,6 +15,7 @@ process.on('warning', common.mustCall((warning) => {
   assert.strictEqual(warning.emitter, e);
   assert.strictEqual(warning.count, 2);
   assert.strictEqual(warning.type, 'event-type');
+  assert.ok(warning.message.includes('2 event-type listeners added.'));
 }));
 
 e.on('event-type', function() {});