var self = this isn't needed here

You only need to do that when you're passing the value of this to
another function. Also remove wrapper function from `FluentSender.emit`

lib/sender.js

@@ -29,6 +29,7 @@ function FluentSender(tag_prefix, options){
   this._sendQueue  = [];    // queue for items waiting for being sent.
   this._sendQueueTail = -1;
   this._eventEmitter = new EventEmitter();
+  this._flushSendQueue = this._flushSendQueue.bind(this)
 }
 
 FluentSender.prototype.emit = function(/*[label] <data>, [timestamp], [callback] */){
@@ -46,39 +47,35 @@ FluentSender.prototype.emit = function(/*[label] <data>, [timestamp], [callback]
   // Last argument is an optional callback
   if (typeof args[0] === 'function') callback = args.shift();
 
-
-  var self = this;
-  var item = self._makePacketItem(label, data, timestamp);
+  var item = this._makePacketItem(label, data, timestamp);
 
   var error;
   var options;
   if (item.tag === null) {
     options = {
-      tag_prefix: self.tag_prefix,
+      tag_prefix: this.tag_prefix,
       label: label
     };
     error = new FluentLoggerError.MissingTag('tag is missing', options);
-    self._handleEvent('error', error, callback);
+    this._handleEvent('error', error, callback);
     return;
   }
   if (typeof item.data !== 'object') {
     options = {
-      tag_prefix: self.tag_prefix,
+      tag_prefix: this.tag_prefix,
       label: label,
       record: item.data
     };
     error = new FluentLoggerError.DataTypeError('data must be an object', options);
-    self._handleEvent('error', error, callback);
+    this._handleEvent('error', error, callback);
     return;
   }
 
   item.callback = callback;
 
-  self._sendQueue.push(item);
-  self._sendQueueTail++;
-  self._connect(function(){
-    self._flushSendQueue();
-  });
+  this._sendQueue.push(item);
+  this._sendQueueTail++;
+  this._connect(this._flushSendQueue);
 };
 
 ['addListener', 'on', 'once', 'removeListener', 'removeAllListeners', 'setMaxListeners', 'getMaxListeners'].forEach(function(attr, i){
@@ -115,12 +112,11 @@ FluentSender.prototype._close = function() {
 
 
 FluentSender.prototype._makePacketItem = function(label, data, time){
-  var self = this;
   var tag = null;
-  if (self.tag_prefix && label) {
-    tag = self.tag_prefix + '.' + label;
-  } else if (self.tag_prefix) {
-    tag = self.tag_prefix;
+  if (this.tag_prefix && label) {
+    tag = this.tag_prefix + '.' + label;
+  } else if (this.tag_prefix) {
+    tag = this.tag_prefix;
   } else if (label) {
     tag = label;
   }
@@ -131,7 +127,7 @@ FluentSender.prototype._makePacketItem = function(label, data, time){
 
   var packet = [tag, time, data];
   var options = {};
-  if (self.requireAckResponse) {
+  if (this.requireAckResponse) {
     options = {
       chunk: crypto.randomBytes(16).toString('base64')
     };
@@ -225,10 +221,9 @@ FluentSender.prototype._flushSendQueue = function() {
 };
 
 FluentSender.prototype._handleEvent = function _handleEvent(signal, data, callback) {
-  var self = this;
   callback && callback(data);
-  if (self._eventEmitter.listenerCount(signal) > 0) {
-    self._eventEmitter.emit(signal, data);
+  if (this._eventEmitter.listenerCount(signal) > 0) {
+    this._eventEmitter.emit(signal, data);
   }
 };