lib: replace var with let/const

PR-URL: #30390
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>

lib/internal/encoding.js

@@ -36,7 +36,7 @@ const {
   encodeUtf8String
 } = internalBinding('buffer');
 
-var Buffer;
+let Buffer;
 function lazyBuffer() {
   if (Buffer === undefined)
     Buffer = require('buffer').Buffer;
@@ -281,8 +281,8 @@ const encodings = new Map([
 // Unfortunately, String.prototype.trim also removes non-ascii whitespace,
 // so we have to do this manually
 function trimAsciiWhitespace(label) {
-  var s = 0;
-  var e = label.length;
+  let s = 0;
+  let e = label.length;
   while (s < e && (
     label[s] === '\u0009' ||
     label[s] === '\u000a' ||
@@ -378,7 +378,7 @@ function makeTextDecoderICU() {
       if (enc === undefined)
         throw new ERR_ENCODING_NOT_SUPPORTED(encoding);
 
-      var flags = 0;
+      let flags = 0;
       if (options !== null) {
         flags |= options.fatal ? CONVERTER_FLAGS_FATAL : 0;
         flags |= options.ignoreBOM ? CONVERTER_FLAGS_IGNORE_BOM : 0;
@@ -406,7 +406,7 @@ function makeTextDecoderICU() {
       }
       validateArgument(options, 'object', 'options', 'Object');
 
-      var flags = 0;
+      let flags = 0;
       if (options !== null)
         flags |= options.stream ? 0 : CONVERTER_FLAGS_FLUSH;
 
@@ -422,7 +422,7 @@ function makeTextDecoderICU() {
 }
 
 function makeTextDecoderJS() {
-  var StringDecoder;
+  let StringDecoder;
   function lazyStringDecoder() {
     if (StringDecoder === undefined)
       ({ StringDecoder } = require('string_decoder'));
@@ -444,7 +444,7 @@ function makeTextDecoderJS() {
       if (enc === undefined || !hasConverter(enc))
         throw new ERR_ENCODING_NOT_SUPPORTED(encoding);
 
-      var flags = 0;
+      let flags = 0;
       if (options !== null) {
         if (options.fatal) {
           throw new ERR_NO_ICU('"fatal" option');