Fixed the way to provide default empty function.

redboltz · redboltz · commit a981d028970c · 2018-08-29T23:00:53.000+09:00
Default paramter is introduced since ES6.
Replaced the way that supports older ES,
diff --git a/lib/client.js b/lib/client.js
@@ -953,7 +953,8 @@ default:
 
 for now i just suppressed the warnings
 */
-MqttClient.prototype._handlePublish = function (packet, done = function () {}) {
+MqttClient.prototype._handlePublish = function (packet, done) {
+  done = typeof done !== 'undefined' ? done : function () {}
   var topic = packet.topic.toString()
   var message = packet.payload
   var qos = packet.qos
@@ -1076,7 +1077,8 @@ MqttClient.prototype._handleAck = function (packet) {
  * @param {Object} packet
  * @api private
  */
-MqttClient.prototype._handlePubrel = function (packet, callback = function () {}) {
+MqttClient.prototype._handlePubrel = function (packet, callback) {
+  callback = typeof callback !== 'undefined' ? callback : function () {}
   var mid = packet.messageId
   var that = this
 
