events: type check listeners

Make sure the argument passed is a string. Also use typeof === function
check instead of isArray().

lib/events.js

@@ -288,11 +288,14 @@ EventEmitter.prototype.removeAllListeners = function(type) {
 };
 
 EventEmitter.prototype.listeners = function(type) {
-  if (!this._events || !this._events[type]) return [];
+  if (typeof type !== 'string')
-  if (!isArray(this._events[type])) {
+    throw TypeError('event type must be a string');
+
+  if (!this._events || !this._events[type])
+    return [];
+  if (typeof this._events[type] === 'function')
     return [this._events[type]];
-  }
+  return this._events[type].slice();
-  return this._events[type].slice(0);
 };
 
 EventEmitter.listenerCount = function(emitter, type) {