buffer: change var to let

PR-URL: nodejs#30292
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Jiawen Geng <technicalcute@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>

lib/buffer.js

@@ -317,7 +317,7 @@ Buffer.from = function from(value, encodingOrOffset, length) {
 // Refs: https://esdiscuss.org/topic/isconstructor#content-11
 const of = (...items) => {
   const newObj = createUnsafeBuffer(items.length);
-  for (var k = 0; k < items.length; k++)
+  for (let k = 0; k < items.length; k++)
     newObj[k] = items[k];
   return newObj;
 };
@@ -433,7 +433,7 @@ function fromString(string, encoding) {
 function fromArrayLike(obj) {
   const length = obj.length;
   const b = allocate(length);
-  for (var i = 0; i < length; i++)
+  for (let i = 0; i < length; i++)
     b[i] = obj[i];
   return b;
 }
@@ -1044,7 +1044,7 @@ Buffer.prototype.write = function write(string, offset, length, encoding) {
 Buffer.prototype.toJSON = function toJSON() {
   if (this.length > 0) {
     const data = new Array(this.length);
-    for (var i = 0; i < this.length; ++i)
+    for (let i = 0; i < this.length; ++i)
       data[i] = this[i];
     return { type: 'Buffer', data };
   }
@@ -1090,7 +1090,7 @@ Buffer.prototype.swap16 = function swap16() {
   if (len % 2 !== 0)
     throw new ERR_INVALID_BUFFER_SIZE('16-bits');
   if (len < 128) {
-    for (var i = 0; i < len; i += 2)
+    for (let i = 0; i < len; i += 2)
       swap(this, i, i + 1);
     return this;
   }
@@ -1105,7 +1105,7 @@ Buffer.prototype.swap32 = function swap32() {
   if (len % 4 !== 0)
     throw new ERR_INVALID_BUFFER_SIZE('32-bits');
   if (len < 192) {
-    for (var i = 0; i < len; i += 4) {
+    for (let i = 0; i < len; i += 4) {
       swap(this, i, i + 3);
       swap(this, i + 1, i + 2);
     }
@@ -1122,7 +1122,7 @@ Buffer.prototype.swap64 = function swap64() {
   if (len % 8 !== 0)
     throw new ERR_INVALID_BUFFER_SIZE('64-bits');
   if (len < 192) {
-    for (var i = 0; i < len; i += 8) {
+    for (let i = 0; i < len; i += 8) {
       swap(this, i, i + 7);
       swap(this, i + 1, i + 6);
       swap(this, i + 2, i + 5);