From 40fba964cdfd5e0aeee038304b514de183f61ef1 Mon Sep 17 00:00:00 2001
From: Hamid Reza Ghavami <hamidr.ghavami@gmail.com>
Date: Thu, 9 Jul 2026 23:43:04 +0300
Subject: [PATCH 1/5] fs: return Buffer from mkdtemp when prefix is a Buffer

Signed-off-by: Hamid Reza Ghavami <hamidr.ghavami@gmail.com>
---
 lib/fs.js                               | 12 +++++++++---
 lib/internal/fs/promises.js             |  9 ++++++---
 test/parallel/test-fs-mkdtemp-buffer.js | 26 +++++++++++++++++++++++++
 test/parallel/test-fs-mkdtemp.js        | 13 ++-----------
 4 files changed, 43 insertions(+), 17 deletions(-)
 create mode 100644 test/parallel/test-fs-mkdtemp-buffer.js

diff --git a/lib/fs.js b/lib/fs.js
index 1ea70ff192d6dd..fbc985a2678ddb 100644
--- a/lib/fs.js
+++ b/lib/fs.js
@@ -3571,7 +3571,9 @@ function mkdtemp(prefix, options, callback) {
   if (h !== null && vfsResult(h.mkdtemp(prefix, typeof options === 'function' ? undefined : options), callback)) return;
 
   options = getOptions(options);
-
+  if (BufferIsBuffer(prefix)) {
+    options = { ...options, encoding: 'buffer' };
+  }
   prefix = getValidatedPath(prefix, 'prefix');
   warnOnNonPortableTemplate(prefix);
 
@@ -3594,7 +3596,9 @@ function mkdtempSync(prefix, options) {
   }
 
   options = getOptions(options);
-
+  if (BufferIsBuffer(prefix)) {
+    options = { ...options, encoding: 'buffer' };
+  }
   prefix = getValidatedPath(prefix, 'prefix');
   warnOnNonPortableTemplate(prefix);
   return binding.mkdtemp(prefix, options.encoding);
@@ -3610,7 +3614,9 @@ function mkdtempSync(prefix, options) {
  */
 function mkdtempDisposableSync(prefix, options) {
   options = getOptions(options);
-
+  if (BufferIsBuffer(prefix)) {
+    options = { ...options, encoding: 'buffer' };
+  }
   prefix = getValidatedPath(prefix, 'prefix');
   warnOnNonPortableTemplate(prefix);
 
diff --git a/lib/internal/fs/promises.js b/lib/internal/fs/promises.js
index 3e336024a15a3c..2208864556c8aa 100644
--- a/lib/internal/fs/promises.js
+++ b/lib/internal/fs/promises.js
@@ -2024,9 +2024,10 @@ async function mkdtemp(prefix, options) {
     const promise = h.mkdtemp(prefix, options);
     if (promise !== undefined) return await promise;
   }
-
   options = getOptions(options);
-
+  if (BufferIsBuffer(prefix)) {
+    options = { ...options, encoding: 'buffer' };
+  }
   prefix = getValidatedPath(prefix, 'prefix');
   warnOnNonPortableTemplate(prefix);
 
@@ -2039,7 +2040,9 @@ async function mkdtemp(prefix, options) {
 
 async function mkdtempDisposable(prefix, options) {
   options = getOptions(options);
-
+  if (BufferIsBuffer(prefix)) {
+    options = { ...options, encoding: 'buffer' };
+  }
   prefix = getValidatedPath(prefix, 'prefix');
   warnOnNonPortableTemplate(prefix);
 
diff --git a/test/parallel/test-fs-mkdtemp-buffer.js b/test/parallel/test-fs-mkdtemp-buffer.js
new file mode 100644
index 00000000000000..4403db03570a9a
--- /dev/null
+++ b/test/parallel/test-fs-mkdtemp-buffer.js
@@ -0,0 +1,26 @@
+'use strict';
+const common = require('../common');
+const assert = require('assert');
+const fs = require('fs');
+const path = require('path');
+
+const tmpdir = require('../common/tmpdir');
+tmpdir.refresh();
+
+const prefixString = path.join(tmpdir.path, 'buffer-');
+const prefixBuffer = Buffer.from(prefixString);
+
+// 1. Test Sync API
+const resultSync = fs.mkdtempSync(prefixBuffer);
+assert.strictEqual(Buffer.isBuffer(resultSync), true);
+
+// 2. Test Callback API
+fs.mkdtemp(prefixBuffer, common.mustSucceed((resultCb) => {
+  assert.strictEqual(Buffer.isBuffer(resultCb), true);
+}));
+
+// 3. Test Promises API
+fs.promises.mkdtemp(prefixBuffer)
+  .then(common.mustCall((resultPromise) => {
+    assert.strictEqual(Buffer.isBuffer(resultPromise), true);
+  }));
diff --git a/test/parallel/test-fs-mkdtemp.js b/test/parallel/test-fs-mkdtemp.js
index e93809d5b44546..a7d678d21ee02a 100644
--- a/test/parallel/test-fs-mkdtemp.js
+++ b/test/parallel/test-fs-mkdtemp.js
@@ -64,22 +64,13 @@ function handler(err, folder) {
 {
   const tmpFolder = fs.mkdtempSync(Buffer.from(tmpdir.resolve('foo.')));
 
-  assert.strictEqual(path.basename(tmpFolder).length, 'foo.XXXXXX'.length);
+  assert.strictEqual(path.basename(tmpFolder.toString()).length, 'foo.XXXXXX'.length);
   assert(fs.existsSync(tmpFolder));
 
   const utf8 = fs.mkdtempSync(Buffer.from(tmpdir.resolve('\u0222abc.')));
-  assert.strictEqual(Buffer.byteLength(path.basename(utf8)),
+  assert.strictEqual(Buffer.byteLength(path.basename(utf8.toString())),
                      Buffer.byteLength('\u0222abc.XXXXXX'));
   assert(fs.existsSync(utf8));
-
-  fs.mkdtemp(Buffer.from(tmpdir.resolve('bar.')), common.mustCall(handler));
-
-  // Same test as above, but making sure that passing an options object doesn't
-  // affect the way the callback function is handled.
-  fs.mkdtemp(Buffer.from(tmpdir.resolve('bar.')), {}, common.mustCall(handler));
-
-  // Warning fires only once
-  fs.mkdtemp(Buffer.from(tmpdir.resolve('bar.X')), common.mustCall(handler));
 }
 
 // Test with Uint8Array

From f30698f67a6344793b739818b06a7b801ad13f46 Mon Sep 17 00:00:00 2001
From: Hamid Reza Ghavami <hamidr.ghavami@gmail.com>
Date: Sat, 11 Jul 2026 22:10:38 +0300
Subject: [PATCH 2/5] test: restore missing mkdtemp async buffer tests

---
 test/parallel/test-fs-mkdtemp.js | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/test/parallel/test-fs-mkdtemp.js b/test/parallel/test-fs-mkdtemp.js
index a7d678d21ee02a..3c2323440e0803 100644
--- a/test/parallel/test-fs-mkdtemp.js
+++ b/test/parallel/test-fs-mkdtemp.js
@@ -71,6 +71,14 @@ function handler(err, folder) {
   assert.strictEqual(Buffer.byteLength(path.basename(utf8.toString())),
                      Buffer.byteLength('\u0222abc.XXXXXX'));
   assert(fs.existsSync(utf8));
+  fs.mkdtemp(Buffer.from(tmpdir.resolve('bar.')), common.mustCall(handler));
+
+  // Same test as above, but making sure that passing an options object doesn't
+  // affect the way the callback function is handled.
+  fs.mkdtemp(Buffer.from(tmpdir.resolve('bar.')), {}, common.mustCall(handler));
+
+  // Warning fires only once
+  fs.mkdtemp(Buffer.from(tmpdir.resolve('bar.X')), common.mustCall(handler));
 }
 
 // Test with Uint8Array

From c137267bd485caaaed41e061546442c0c94c47c3 Mon Sep 17 00:00:00 2001
From: Hamid Reza Ghavami <hamidr.ghavami@gmail.com>
Date: Sun, 12 Jul 2026 10:27:52 +0300
Subject: [PATCH 3/5] doc: update fs.mkdtemp() return types and history

---
 doc/api/fs.md | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/doc/api/fs.md b/doc/api/fs.md
index 51e1a6f40b985a..990a989a53469a 100644
--- a/doc/api/fs.md
+++ b/doc/api/fs.md
@@ -3882,6 +3882,9 @@ See the POSIX mkdir(2) documentation for more details.
 <!-- YAML
 added: v5.10.0
 changes:
+- version: REPLACEME
+    pr-url: https://github.com/nodejs/node/pull/64397
+    description: The resulting path is now passed as a Buffer if the provided prefix is a Buffer.
   - version:
     - v20.6.0
     - v18.19.0
@@ -3915,7 +3918,7 @@ changes:
   * `encoding` {string} **Default:** `'utf8'`
 * `callback` {Function}
   * `err` {Error}
-  * `directory` {string}
+  * `directory` {string|Buffer}
 
 Creates a unique temporary directory.
 
@@ -3925,8 +3928,7 @@ inconsistencies, avoid trailing `X` characters in `prefix`. Some platforms,
 notably the BSDs, can return more than six random characters, and replace
 trailing `X` characters in `prefix` with random characters.
 
-The created directory path is passed as a string to the callback's second
-parameter.
+The created directory path is passed to the callback's second parameter. If `prefix` is a `Buffer` , then the resulting directory path is passed as a  `Buffer`. Otherwise, the path is passed as a string using the specified encoding.
 
 The optional `options` argument can be a string specifying an encoding, or an
 object with an `encoding` property specifying the character encoding to use.

From b82da4761ad189268d8be9979adf83289398e38f Mon Sep 17 00:00:00 2001
From: Hamid Reza Ghavami <hamidr.ghavami@gmail.com>
Date: Sun, 12 Jul 2026 12:51:04 +0300
Subject: [PATCH 4/5] doc: physically wrap long lines in fs.md to fix markdown
 linter

---
 doc/api/fs.md | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/doc/api/fs.md b/doc/api/fs.md
index 990a989a53469a..ee0c7c2f55e9d8 100644
--- a/doc/api/fs.md
+++ b/doc/api/fs.md
@@ -3928,7 +3928,10 @@ inconsistencies, avoid trailing `X` characters in `prefix`. Some platforms,
 notably the BSDs, can return more than six random characters, and replace
 trailing `X` characters in `prefix` with random characters.
 
-The created directory path is passed to the callback's second parameter. If `prefix` is a `Buffer` , then the resulting directory path is passed as a  `Buffer`. Otherwise, the path is passed as a string using the specified encoding.
+The created directory path is passed to the callback's second parameter. If
+`prefix` is a `Buffer`, then the resulting directory path is passed as a
+`Buffer`. Otherwise, the path is passed as a string using the specified
+encoding.
 
 The optional `options` argument can be a string specifying an encoding, or an
 object with an `encoding` property specifying the character encoding to use.

From a5ef8882da578589e029fa0068ab29d17949d893 Mon Sep 17 00:00:00 2001
From: Hamid Reza Ghavami <hamidr.ghavami@gmail.com>
Date: Sun, 12 Jul 2026 13:17:09 +0300
Subject: [PATCH 5/5] doc: fix markdown and yaml line lengths in fs.md

---
 doc/api/fs.md | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/doc/api/fs.md b/doc/api/fs.md
index ee0c7c2f55e9d8..60ac9614b238e8 100644
--- a/doc/api/fs.md
+++ b/doc/api/fs.md
@@ -3882,9 +3882,10 @@ See the POSIX mkdir(2) documentation for more details.
 <!-- YAML
 added: v5.10.0
 changes:
-- version: REPLACEME
+  - version: REPLACEME
     pr-url: https://github.com/nodejs/node/pull/64397
-    description: The resulting path is now passed as a Buffer if the provided prefix is a Buffer.
+    description: The resulting path is now passed as a Buffer if the
+      provided prefix is a Buffer.
   - version:
     - v20.6.0
     - v18.19.0