Optimize event emitter for single listener

src/node.js

@@ -170,37 +170,63 @@ var eventsModule = createInternalModule('events', function (exports) {
   // process.EventEmitter is defined in src/events.cc
   // process.EventEmitter.prototype.emit() is also defined there.
   process.EventEmitter.prototype.addListener = function (type, listener) {
-    if (listener instanceof Function) {
+    if (!(listener instanceof Function)) {
+      throw new Error('addListener only takes instances of Function');
+    }
+
     if (!this._events) this._events = {};
-      if (!this._events.hasOwnProperty(type)) this._events[type] = [];
+
     // To avoid recursion in the case that type == "newListeners"! Before
     // adding it to the listeners, first emit "newListeners".
     this.emit("newListener", type, listener);
+
+    if (!this._events[type]) {
+      // Optimize the case of one listener. Don't need the extra array object.
+      this._events[type] = listener;
+    } else if (this._events[type] instanceof Array) {
+      // If we've already got an array, just append.
       this._events[type].push(listener);
+    } else {
+      // Adding the second element, need to change to array.
+      this._events[type] = [this._events[type], listener];
     }
+
     return this;
   };
 
   process.EventEmitter.prototype.removeListener = function (type, listener) {
-    if (listener instanceof Function) {
+    if (!(listener instanceof Function)) {
+      throw new Error('removeListener only takes instances of Function');
+    }
+
     // does not use listeners(), so no side effect of creating _events[type]
-      if (!this._events || !this._events.hasOwnProperty(type)) return;
+    if (!this._events || !this._events[type]) return this;
+
     var list = this._events[type];
-      if (list.indexOf(listener) < 0) return;
-      list.splice(list.indexOf(listener), 1);
+
+    if (list instanceof Array) {
+      var i = list.indexOf(listener);
+      if (i < 0) return this;
+      list.splice(i, 1);
+    } else {
+      this._events[type] = null;
     }
+
     return this;
   };
 
   process.EventEmitter.prototype.removeAllListeners = function (type) {
     // does not use listeners(), so no side effect of creating _events[type]
-    if (!type || !this._events || !this._events.hasOwnProperty(type)) return this;
-    this._events[type].length = 0;
+    if (!type || !this._events || !this._events[type]) return this;
+    this._events[type] = null;
   };
 
   process.EventEmitter.prototype.listeners = function (type) {
     if (!this._events) this._events = {};
-    if (!this._events.hasOwnProperty(type)) this._events[type] = [];
+    if (!this._events[type]) this._events[type] = [];
+    if (!(this._events[type] instanceof Array)) {
+      this._events[type] = [this._events[type]];
+    }
     return this._events[type];
   };
 

src/node_events.cc

@@ -50,7 +50,22 @@ static bool ReallyEmit(Handle<Object> self,
   Local<Object> events = events_v->ToObject();
 
   Local<Value> listeners_v = events->Get(event);
-  if (!listeners_v->IsArray()) return false;
+  Local<Function> listener;
+
+  if (listeners_v->IsFunction()) {
+    // Optimized one-listener case
+    Local<Function> listener = Local<Function>::Cast(listeners_v);
+
+    TryCatch try_catch;
+
+    listener->Call(self, argc, argv);
+
+    if (try_catch.HasCaught()) {
+      FatalException(try_catch);
+      return false;
+    }
+
+  } else if (listeners_v->IsArray()) {
     Local<Array> listeners = Local<Array>::Cast(listeners_v);
 
     for (unsigned int i = 0; i < listeners->Length(); i++) {
@@ -70,6 +85,10 @@ static bool ReallyEmit(Handle<Object> self,
       }
     }
 
+  } else {
+    return false;
+  }
+
   return true;
 }