added comments and updated tests

Author: thisalihassan

lib/buffer.js

@@ -394,6 +394,7 @@ Buffer.copyBytesFrom = function copyBytesFrom(view, offset, length) {
     let end;
     if (length !== undefined) {
       validateInteger(length, 'length', 0);
+      // The old code used TypedArrayPrototypeSlice which clamps internally.
       end = MathMin(offset + length, viewLength);
     } else {
       end = viewLength;
@@ -1096,6 +1097,8 @@ function _fill(buf, value, offset, end, encoding) {
       value = 0;
     } else if (value.length === 1) {
       // Fast path: If `value` fits into a single byte, use that numeric value.
+      // ASCII shares this branch with utf8 since code < 128 covers the full
+      // ASCII range; anything outside falls through to C++ bindingFill.
       if (normalizedEncoding === 'utf8' || normalizedEncoding === 'ascii') {
         const code = StringPrototypeCharCodeAt(value, 0);
         if (code < 128) {

test/parallel/test-buffer-swap-fast.js

@@ -5,18 +5,30 @@ const common = require('../common');
 const assert = require('assert');
 
 function testFastSwap16() {
-  const buf = Buffer.alloc(256);
-  buf.swap16();
+  const buf = Buffer.from([0x01, 0x02, 0x03, 0x04]);
+  const expected = Buffer.from([0x02, 0x01, 0x04, 0x03]);
+  const padded = Buffer.alloc(256);
+  buf.copy(padded);
+  padded.swap16();
+  assert.deepStrictEqual(padded.subarray(0, 4), expected);
 }
 
 function testFastSwap32() {
-  const buf = Buffer.alloc(256);
-  buf.swap32();
+  const buf = Buffer.from([0x01, 0x02, 0x03, 0x04]);
+  const expected = Buffer.from([0x04, 0x03, 0x02, 0x01]);
+  const padded = Buffer.alloc(256);
+  buf.copy(padded);
+  padded.swap32();
+  assert.deepStrictEqual(padded.subarray(0, 4), expected);
 }
 
 function testFastSwap64() {
-  const buf = Buffer.alloc(256);
-  buf.swap64();
+  const buf = Buffer.from([0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08]);
+  const expected = Buffer.from([0x08, 0x07, 0x06, 0x05, 0x04, 0x03, 0x02, 0x01]);
+  const padded = Buffer.alloc(256);
+  buf.copy(padded);
+  padded.swap64();
+  assert.deepStrictEqual(padded.subarray(0, 8), expected);
 }
 
 eval('%PrepareFunctionForOptimization(Buffer.prototype.swap16)');