From c2609ec2e5ed0c90e2fb77b3ac592af99f98a58a Mon Sep 17 00:00:00 2001
From: Sebastian Beltran <bjohansebas@gmail.com>
Date: Wed, 24 Jun 2026 20:29:51 -0500
Subject: [PATCH] feat: implement support for 2d duplicate headers in writeHead

---
 index.js     | 28 ++++++++++++++++++++++-
 test/test.js | 64 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 91 insertions(+), 1 deletion(-)

diff --git a/index.js b/index.js
index d691cc7..8c2b7d4 100644
--- a/index.js
+++ b/index.js
@@ -17,7 +17,16 @@ var http = require('http')
 
 // older node versions don't have appendHeader
 var isAppendHeaderSupported = typeof http.ServerResponse.prototype.appendHeader === 'function'
+
+// node v22 (https://github.com/nodejs/node/pull/50394) made writeHead preserve
+// duplicate headers passed as a raw array even when setHeader was called before;
+// earlier versions collapse them to the last value. since this monkey-patch
+// always applies headers via setHeader/appendHeader, only preserve duplicates on
+// versions where node itself does, so behavior stays consistent with the runtime
+var preservesDuplicateHeaders = isAppendHeaderSupported && parseInt(process.versions.node, 10) >= 22
+
 var set1dArray = isAppendHeaderSupported ? set1dArrayWithAppend : set1dArrayWithSet
+var set2dArray = preservesDuplicateHeaders ? set2dArrayWithAppend : set2dArrayWithSet
 
 /**
  * Create a replacement writeHead method.
@@ -145,7 +154,24 @@ function setWriteHeadHeaders (statusCode) {
   return args
 }
 
-function set2dArray (res, headers) {
+function set2dArrayWithAppend (res, headers) {
+  var key
+  for (var i = 0; i < headers.length; i++) {
+    key = headers[i][0]
+    if (key) {
+      res.removeHeader(key)
+    }
+  }
+
+  for (var j = 0; j < headers.length; j++) {
+    key = headers[j][0]
+    if (key) {
+      res.appendHeader(key, headers[j][1])
+    }
+  }
+}
+
+function set2dArrayWithSet (res, headers) {
   var key
   for (var i = 0; i < headers.length; i++) {
     key = headers[i][0]
diff --git a/test/test.js b/test/test.js
index 2efcf30..23d343e 100644
--- a/test/test.js
+++ b/test/test.js
@@ -7,6 +7,10 @@ var request = require('supertest')
 // older node versions don't have appendHeader
 var isAppendHeaderSupported = typeof http.ServerResponse.prototype.appendHeader === 'function'
 
+// node v22 (https://github.com/nodejs/node/pull/50394) preserves duplicate
+// headers from a raw array passed to writeHead; earlier versions collapse them
+var preservesDuplicateHeaders = isAppendHeaderSupported && parseInt(process.versions.node, 10) >= 22
+
 describe('onHeaders(res, listener)', function () {
   it('should fire after setHeader', function (done) {
     var server = createServer(echoListener)
@@ -429,6 +433,66 @@ describe('onHeaders(res, listener)', function () {
         })
     })
   })
+
+  describe('writeHead(status, 2d duplicate headers)', function () {
+    it('should be respected', function (done) {
+      var server = createServer(listener, handler)
+
+      function handler (req, res) {
+        res.writeHead(201, [['express', 'is good'], ['express', 'is great']])
+      }
+
+      function listener (req, res) {
+        // no need to duplicate existing listener tests further... right?
+      }
+
+      var response = request(server).get('/')
+
+      if (preservesDuplicateHeaders) {
+        response
+          .expect('express', 'is good, is great')
+      } else {
+        response
+          .expect('express', 'is great')
+      }
+
+      response
+        .expect(201)
+        .end(function (err, res) {
+          if (err) throw err
+
+          var expressIsGood = false
+          var expressIsGreat = false
+
+          // very old node versions do not have the `rawHeaders` prop
+          var headers = res.res.rawHeaders || res.res.headers
+
+          if (headers.length) {
+            for (var i = 0; i < headers.length; i++) {
+              const header = headers[i]
+
+              if (header === 'express') {
+                if (headers[i + 1] === 'is good') {
+                  expressIsGood = true
+                } else if (headers[i + 1] === 'is great') {
+                  expressIsGreat = true
+                }
+              }
+            }
+          } else {
+            expressIsGreat = headers.express === 'is great'
+          }
+
+          if (preservesDuplicateHeaders) {
+            assert.ok(expressIsGood)
+          }
+
+          assert.ok(expressIsGreat)
+
+          done()
+        })
+    })
+  })
 })
 
 function createServer (listener, handler) {