child_process: refactor self=this in socket_list

The socket list module (used by child_process) currently uses the
`var self = this;` pattern for context in several places, this PR
replaces this with arrow functions or passing a parameter in where
appropriate.
Note that the `var self = this` in the _request is intentioanlly
left in place since it is not trivial to refactor it and the current
pattern isn't bad given the use case.
PR-URL: #5860
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>

Author: benjamingr

lib/internal/socket_list.js

@@ -58,13 +58,11 @@ SocketListSend.prototype.getConnections = function getConnections(callback) {
 functionSocketListReceive(slave,key){
 EventEmitter.call(this);
 
-varself=this;
-
 this.connections=0;
 this.key=key;
 this.slave=slave;
 
-functiononempty(){
+functiononempty(self){
 if(!self.slave.connected)return;
 
 self.slave.send({
@@ -73,36 +71,34 @@ function SocketListReceive(slave, key) {
 });
 }
 
-this.slave.on('internalMessage',function(msg){
-if(msg.key!==self.key)return;
+this.slave.on('internalMessage',(msg)=>{
+if(msg.key!==this.key)return;
 
 if(msg.cmd==='NODE_SOCKET_NOTIFY_CLOSE'){
 // Already empty
-if(self.connections===0)returnonempty();
+if(this.connections===0)returnonempty(this);
 
 // Wait for sockets to get closed
-self.once('empty',onempty);
+this.once('empty',onempty);
 }elseif(msg.cmd==='NODE_SOCKET_GET_COUNT'){
-if(!self.slave.connected)return;
-self.slave.send({
+if(!this.slave.connected)return;
+this.slave.send({
 cmd: 'NODE_SOCKET_COUNT',
-key: self.key,
-count: self.connections
+key: this.key,
+count: this.connections
 });
 }
 });
 }
 util.inherits(SocketListReceive,EventEmitter);
 
 SocketListReceive.prototype.add=function(obj){
-varself=this;
-
 this.connections++;
 
 // Notify previous owner of socket about its state change
-obj.socket.once('close',function(){
-self.connections--;
+obj.socket.once('close',()=>{
+this.connections--;
 
-if(self.connections===0)self.emit('empty');
+if(this.connections===0)this.emit('empty',this);
 });
 };