test: improve child_process tests

Replaced var keyword with const and let in the tests for child process
stdin and stdio.

PR-URL: https://github.com/nodejs/node/pull/8617
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>

test/parallel/test-child-process-stdin.js

@@ -1,10 +1,10 @@
 'use strict';
-var common = require('../common');
-var assert = require('assert');
+const common = require('../common');
+const assert = require('assert');
 
-var spawn = require('child_process').spawn;
+const spawn = require('child_process').spawn;
 
-var cat = spawn(common.isWindows ? 'more' : 'cat');
+const cat = spawn(common.isWindows ? 'more' : 'cat');
 cat.stdin.write('hello');
 cat.stdin.write(' ');
 cat.stdin.write('world');
@@ -14,7 +14,7 @@ assert.ok(!cat.stdin.readable);
 
 cat.stdin.end();
 
-var response = '';
+let response = '';
 
 cat.stdout.setEncoding('utf8');
 cat.stdout.on('data', function(chunk) {

test/parallel/test-child-process-stdio-big-write-end.js

@@ -1,7 +1,7 @@
 'use strict';
 require('../common');
-var assert = require('assert');
-var BUFSIZE = 1024;
+const assert = require('assert');
+const BUFSIZE = 1024;
 
 switch (process.argv[2]) {
   case undefined:
@@ -13,11 +13,11 @@ switch (process.argv[2]) {
 }
 
 function parent() {
-  var spawn = require('child_process').spawn;
-  var child = spawn(process.execPath, [__filename, 'child']);
-  var sent = 0;
+  const spawn = require('child_process').spawn;
+  const child = spawn(process.execPath, [__filename, 'child']);
+  let sent = 0;
 
-  var n = '';
+  let n = '';
   child.stdout.setEncoding('ascii');
   child.stdout.on('data', function(c) {
     n += c;
@@ -34,7 +34,7 @@ function parent() {
   } while (child.stdin.write(buf));
 
   // then write a bunch more times.
-  for (var i = 0; i < 100; i++) {
+  for (let i = 0; i < 100; i++) {
     const buf = Buffer.alloc(BUFSIZE, '.');
     sent += BUFSIZE;
     child.stdin.write(buf);
@@ -47,7 +47,7 @@ function parent() {
 }
 
 function child() {
-  var received = 0;
+  let received = 0;
   process.stdin.on('data', function(c) {
     received += c.length;
   });

test/parallel/test-child-process-stdio.js

@@ -1,9 +1,9 @@
 'use strict';
-var common = require('../common');
-var assert = require('assert');
+const common = require('../common');
+const assert = require('assert');
 
-var options = {stdio: ['pipe']};
-var child = common.spawnPwd(options);
+let options = {stdio: ['pipe']};
+let child = common.spawnPwd(options);
 
 assert.notEqual(child.stdout, null);
 assert.notEqual(child.stderr, null);