src: replace var for let / const.

Replace var for let or const.

PR-URL: nodejs#18649
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Julian Duque <julianduquej@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/internal/async_hooks.js

@@ -262,7 +262,7 @@ function getOrSetAsyncId(object) {
 // the user to safeguard this call and make sure it's zero'd out when the
 // constructor is complete.
 function getDefaultTriggerAsyncId() {
-  var defaultTriggerAsyncId = async_id_fields[kDefaultTriggerAsyncId];
+  let defaultTriggerAsyncId = async_id_fields[kDefaultTriggerAsyncId];
   // If defaultTriggerAsyncId isn't set, use the executionAsyncId
   if (defaultTriggerAsyncId < 0)
     defaultTriggerAsyncId = async_id_fields[kExecutionAsyncId];
@@ -276,7 +276,7 @@ function defaultTriggerAsyncIdScope(triggerAsyncId, block, ...args) {
   const oldDefaultTriggerAsyncId = async_id_fields[kDefaultTriggerAsyncId];
   async_id_fields[kDefaultTriggerAsyncId] = triggerAsyncId;
 
-  var ret;
+  let ret;
   try {
     ret = Reflect.apply(block, null, args);
   } finally {

lib/internal/bootstrap_node.js

@@ -176,7 +176,7 @@
           const fs = NativeModule.require('fs');
           // read the source
           const filename = Module._resolveFilename(process.argv[1]);
-          var source = fs.readFileSync(filename, 'utf-8');
+          const source = fs.readFileSync(filename, 'utf-8');
           checkScriptSyntax(source, filename);
           process.exit(0);
         }
@@ -222,7 +222,7 @@
           // Read all of stdin - execute it.
           process.stdin.setEncoding('utf8');
 
-          var code = '';
+          let code = '';
           process.stdin.on('data', function(d) {
             code += d;
           });
@@ -495,7 +495,7 @@
     const versionTypes = icu.getVersion().split(',');
 
     for (var n = 0; n < versionTypes.length; n++) {
-      var name = versionTypes[n];
+      const name = versionTypes[n];
       const version = icu.getVersion(name);
       Object.defineProperty(process.versions, name, {
         writable: false,
@@ -670,7 +670,7 @@
   ];
 
   NativeModule.prototype.compile = function() {
-    var source = NativeModule.getSource(this.id);
+    let source = NativeModule.getSource(this.id);
     source = NativeModule.wrap(source);
 
     this.loading = true;

lib/internal/readline.js

@@ -9,8 +9,8 @@ const ansi =
 
 const kEscape = '\x1b';
 
-var getStringWidth;
-var isFullWidthCodePoint;
+let getStringWidth;
+let isFullWidthCodePoint;
 
 function CSI(strings, ...args) {
   let ret = `${kEscape}[`;