From 91cac31f795409b3a729ac67f53f287aa37bda8c Mon Sep 17 00:00:00 2001
From: tsctx <91457664+tsctx@users.noreply.github.com>
Date: Mon, 13 Nov 2023 18:35:10 +0900
Subject: [PATCH 1/4] fix: implement `Headers#set` correctly

---
 lib/fetch/headers.js | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lib/fetch/headers.js b/lib/fetch/headers.js
index f37c5c99e03..3df839f71f6 100644
--- a/lib/fetch/headers.js
+++ b/lib/fetch/headers.js
@@ -401,7 +401,7 @@ class Headers {
     // 7. Set (name, value) in this’s header list.
     // 8. If this’s guard is "request-no-cors", then remove
     //    privileged no-CORS request headers from this
-    return this[kHeadersList].set(name, value)
+    this[kHeadersList].set(name, value)
   }
 
   // https://fetch.spec.whatwg.org/#dom-headers-getsetcookie

From ea135c8f1d9022531a26ee26764f1374448bdb0e Mon Sep 17 00:00:00 2001
From: tsctx <91457664+tsctx@users.noreply.github.com>
Date: Mon, 13 Nov 2023 18:37:42 +0900
Subject: [PATCH 2/4] fix: HeadersList

---
 lib/fetch/headers.js | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lib/fetch/headers.js b/lib/fetch/headers.js
index 3df839f71f6..035560428bc 100644
--- a/lib/fetch/headers.js
+++ b/lib/fetch/headers.js
@@ -190,7 +190,7 @@ class HeadersList {
     //    the first such header to value and remove the
     //    others.
     // 2. Otherwise, append header (name, value) to list.
-    return this[kHeadersMap].set(lowercaseName, { name, value })
+    this[kHeadersMap].set(lowercaseName, { name, value })
   }
 
   // https://fetch.spec.whatwg.org/#concept-header-list-delete

From 013d1e94a906b191dd0a0ffb469dde42f69db225 Mon Sep 17 00:00:00 2001
From: tsctx <91457664+tsctx@users.noreply.github.com>
Date: Tue, 14 Nov 2023 03:45:33 +0900
Subject: [PATCH 3/4] test: add

---
 test/fetch/headers.js | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/test/fetch/headers.js b/test/fetch/headers.js
index c4b4e03d297..1ee66445d98 100644
--- a/test/fetch/headers.js
+++ b/test/fetch/headers.js
@@ -250,7 +250,7 @@ tap.test('Headers has', t => {
 })
 
 tap.test('Headers set', t => {
-  t.plan(4)
+  t.plan(5)
 
   t.test('sets valid header entry to instance', t => {
     t.plan(2)
@@ -294,6 +294,16 @@ tap.test('Headers set', t => {
     t.throws(() => headers.set('undici'), 'throws on missing value')
     t.throws(() => headers.set('invalid @ header ? name', 'valid value'), 'throws on invalid name')
   })
+
+  // https://github.com/nodejs/undici/issues/2431
+  t.test('`Headers#set` returns undefined', t => {
+    t.plan(2)
+    const headers = new Headers()
+
+    t.same(headers.set('a', 'b'), undefined)
+
+    t.notOk(headers.set('c') instanceof Map)
+  })
 })
 
 tap.test('Headers forEach', t => {

From 2faa1297a66de0448e23558a299a94dac19a6a94 Mon Sep 17 00:00:00 2001
From: tsctx <91457664+tsctx@users.noreply.github.com>
Date: Tue, 14 Nov 2023 04:00:38 +0900
Subject: [PATCH 4/4] test: fix

---
 test/fetch/headers.js | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/test/fetch/headers.js b/test/fetch/headers.js
index 1ee66445d98..5da1a1ded1f 100644
--- a/test/fetch/headers.js
+++ b/test/fetch/headers.js
@@ -302,7 +302,7 @@ tap.test('Headers set', t => {
 
     t.same(headers.set('a', 'b'), undefined)
 
-    t.notOk(headers.set('c') instanceof Map)
+    t.notOk(headers.set('c', 'd') instanceof Map)
   })
 })