doc: modernize code examples in the cluster.md

- Fixes https://github.com/nodejs/node/issues/10255
- It also will be consistent with a previous code example.
- `cluster.workers` iteration: `Object.keys().forEach` -> `for`...`in`

PR-URL: https://github.com/nodejs/node/pull/10270
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>

doc/api/cluster.md

@@ -15,8 +15,10 @@ const http = require('http');
 const numCPUs = require('os').cpus().length;
 
 if (cluster.isMaster) {
+  console.log(`Master ${process.pid} is running`);
+
   // Fork workers.
-  for (var i = 0; i < numCPUs; i++) {
+  for (let i = 0; i < numCPUs; i++) {
     cluster.fork();
   }
 
@@ -30,17 +32,20 @@ if (cluster.isMaster) {
     res.writeHead(200);
     res.end('hello world\n');
   }).listen(8000);
+
+  console.log(`Worker ${process.pid} started`);
 }
 ```
 
 Running Node.js will now share port 8000 between the workers:
 
 ```txt
-$ NODE_DEBUG=cluster node server.js
-23521,Master Worker 23524 online
-23521,Master Worker 23526 online
-23521,Master Worker 23523 online
-23521,Master Worker 23528 online
+$ node server.js
+Master 3596 is running
+Worker 4324 started
+Worker 4520 started
+Worker 6056 started
+Worker 5644 started
 ```
 
 Please note that on Windows, it is not yet possible to set up a named pipe
@@ -202,27 +207,27 @@ const http = require('http');
 if (cluster.isMaster) {
 
   // Keep track of http requests
-  var numReqs = 0;
+  let numReqs = 0;
   setInterval(() => {
-    console.log('numReqs =', numReqs);
+    console.log(`numReqs = ${numReqs}`);
   }, 1000);
 
   // Count requests
   function messageHandler(msg) {
-    if (msg.cmd && msg.cmd == 'notifyRequest') {
+    if (msg.cmd && msg.cmd === 'notifyRequest') {
       numReqs += 1;
     }
   }
 
   // Start workers and listen for messages containing notifyRequest
   const numCPUs = require('os').cpus().length;
-  for (var i = 0; i < numCPUs; i++) {
+  for (let i = 0; i < numCPUs; i++) {
     cluster.fork();
   }
 
-  Object.keys(cluster.workers).forEach((id) => {
+  for (const id in cluster.workers) {
     cluster.workers[id].on('message', messageHandler);
-  });
+  }
 
 } else {
 
@@ -285,8 +290,8 @@ the `'disconnect'` event has not been emitted after some time.
 
 ```js
 if (cluster.isMaster) {
-  var worker = cluster.fork();
-  var timeout;
+  const worker = cluster.fork();
+  let timeout;
 
   worker.on('listening', (address) => {
     worker.send('shutdown');
@@ -302,7 +307,7 @@ if (cluster.isMaster) {
 
 } else if (cluster.isWorker) {
   const net = require('net');
-  var server = net.createServer((socket) => {
+  const server = net.createServer((socket) => {
     // connections never end
   });
 
@@ -428,7 +433,7 @@ This example will echo back all messages from the master:
 
 ```js
 if (cluster.isMaster) {
-  var worker = cluster.fork();
+  const worker = cluster.fork();
   worker.send('hi there');
 
 } else if (cluster.isWorker) {
@@ -524,7 +529,7 @@ When a new worker is forked the cluster module will emit a `'fork'` event.
 This can be used to log worker activity, and create your own timeout.
 
 ```js
-var timeouts = [];
+const timeouts = [];
 function errorMsg() {
   console.error('Something must be wrong with the connection ...');
 }
@@ -588,7 +593,7 @@ If you need to support older versions and don't need the worker object,
 you can work around the discrepancy by checking the number of arguments:
 
 ```js
-cluster.on('message', function(worker, message, handle) {
+cluster.on('message', (worker, message, handle) => {
   if (arguments.length === 2) {
     handle = message;
     message = worker;
@@ -807,7 +812,7 @@ before last `'disconnect'` or `'exit'` event is emitted.
 ```js
 // Go through all workers
 function eachWorker(callback) {
-  for (var id in cluster.workers) {
+  for (const id in cluster.workers) {
     callback(cluster.workers[id]);
   }
 }
@@ -821,7 +826,7 @@ the worker's unique id is the easiest way to find the worker.
 
 ```js
 socket.on('data', (id) => {
-  var worker = cluster.workers[id];
+  const worker = cluster.workers[id];
 });
 ```