From ee4332244028f884b2b97a672d33791b09dfc993 Mon Sep 17 00:00:00 2001
From: linhongkuan <linhk8@mail2.sysu.edu.cn>
Date: Wed, 24 Jun 2026 21:56:58 +0800
Subject: [PATCH] fix(res.append): preserve empty header values

---
 lib/response.js    |  2 +-
 test/res.append.js | 15 +++++++++++++++
 2 files changed, 16 insertions(+), 1 deletion(-)

diff --git a/lib/response.js b/lib/response.js
index b4755a5c060..268f8f7980c 100644
--- a/lib/response.js
+++ b/lib/response.js
@@ -633,7 +633,7 @@ res.append = function append(field, val) {
   var prev = this.get(field);
   var value = val;
 
-  if (prev) {
+  if (prev !== undefined) {
     // concat the new and prev vals
     value = Array.isArray(prev) ? prev.concat(val)
       : Array.isArray(val) ? [prev].concat(val)
diff --git a/test/res.append.js b/test/res.append.js
index 2dd17a3a1fb..93a52d97ba2 100644
--- a/test/res.append.js
+++ b/test/res.append.js
@@ -82,6 +82,21 @@ describe('res', function () {
         .end(done)
     })
 
+    it('should preserve an existing empty header value', function (done) {
+      var app = express()
+
+      app.use(function (req, res) {
+        res.set('Warning', '')
+        res.append('Warning', '199 Miscellaneous warning')
+        res.end(JSON.stringify(res.get('Warning')))
+      })
+
+      request(app)
+        .get('/')
+        .expect(200, '["","199 Miscellaneous warning"]')
+        .end(done)
+    })
+
     it('should work together with res.cookie', function (done) {
       var app = express()