From 1bd439cffdfc91044d088575d393062fe5746222 Mon Sep 17 00:00:00 2001
From: Steven Chim <655241+chimurai@users.noreply.github.com>
Date: Tue, 19 May 2026 22:34:39 +0000
Subject: [PATCH] fix: preserve path when target includes path

---
 src/_utils.ts       |  2 +-
 test/_utils.test.ts | 31 ++++++++++++++++++++++++++++---
 2 files changed, 29 insertions(+), 4 deletions(-)

diff --git a/src/_utils.ts b/src/_utils.ts
index 97597a2..42b263e 100644
--- a/src/_utils.ts
+++ b/src/_utils.ts
@@ -194,7 +194,7 @@ export function joinURL(base: string | undefined, path: string | undefined): str
   if (!base || base === "/") {
     return path || "/";
   }
-  if (!path) {
+  if (!path || path === "/") {
     return base;
   }
   // eslint-disable-next-line unicorn/prefer-at
diff --git a/test/_utils.test.ts b/test/_utils.test.ts
index ac5fe6d..0ceee0c 100644
--- a/test/_utils.test.ts
+++ b/test/_utils.test.ts
@@ -377,6 +377,19 @@ describe("lib/http-proxy/common.js", () => {
       expect(outgoing.path).to.eql("/?project=example&path=");
     });
 
+    it("should not add trailing slash when target has base path and request path is root", () => {
+      const outgoing = createOutgoing();
+      common.setupOutgoing(
+        outgoing,
+        {
+          target: URL.parse("http://localhost/api")!,
+        },
+        stubIncomingMessage({ url: "/" }),
+      );
+
+      expect(outgoing.path).to.eql("/api");
+    });
+
     it("should not modify the query string", () => {
       const outgoing = createOutgoing();
       common.setupOutgoing(
@@ -694,13 +707,25 @@ describe("lib/http-proxy/common.js", () => {
       expect(common.joinURL("/base/", "path")).to.eql("/base/path");
     });
 
-    it("should preserve trailing slash when path is exactly /", () => {
-      expect(common.joinURL("/maildev", "/")).to.eql("/maildev/");
+    it("should return base when path is exactly /", () => {
+      expect(common.joinURL("/maildev", "/")).to.eql("/maildev");
     });
 
-    it("should keep a single trailing slash when both base ends and path is /", () => {
+    it("should return base when base ends with slash and path is /", () => {
       expect(common.joinURL("/maildev/", "/")).to.eql("/maildev/");
     });
+
+    it("should return base when path is empty", () => {
+      expect(common.joinURL("/api", "")).to.eql("/api");
+    });
+
+    it("should join base and nested path", () => {
+      expect(common.joinURL("/api", "/users")).to.eql("/api/users");
+    });
+
+    it("should join base with trailing slash and nested path", () => {
+      expect(common.joinURL("/api/", "/users")).to.eql("/api/users");
+    });
   });
 
   describe("#rewriteCookieProperty", () => {