fix: preserve path when target includes path

[chimurai]

regression introduced in #137

fixes #138

tested patch in chimurai/http-proxy-middleware#1234

When target is configured with path the outgoing path is not preserved:
(in 0.5.2 a trailing slash is added)

Reproduction:

const app = express()

app.use('/', createProxyMiddleware({
    changeOrigin: true,
    target: `http://localhost:9000/api`,        // <--- configure target with path
}));

app.listen(3000)

0.5.1
curl http://localhost:3000  -> http://localhost:9000/api

0.5.2
curl http://localhost:3000  -> http://localhost:9000/api/

PR
curl http://localhost:3000  -> http://localhost:9000/api

Summary by CodeRabbit

• Bug Fixes
  • Fixed URL path resolution to correctly handle root path scenarios in path joining logic.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 978f9e9e-5482-4548-98da-8d2619119844

📥 Commits

Reviewing files that changed from the base of the PR and between c68013c and 1bd439c.

📒 Files selected for processing (2)

• src/_utils.ts
• test/_utils.test.ts

---

📝 Walkthrough

Walkthrough

This PR fixes a regression where joinURL incorrectly preserved a trailing slash when the incoming path was / and the base had a path component. The implementation now treats path === "/" as an absent path, returning the base unchanged. Test coverage is expanded to validate this behavior and adjacent edge cases.

Changes

Root Path Joining Fix

Layer / File(s)	Summary
joinURL root-path handling fix src/_utils.ts	Updated joinURL early-return condition to treat path === "/" the same as falsy path, returning base instead of continuing with slash-normalization logic.
Test coverage for path-joining edge cases test/_utils.test.ts	Expanded test coverage for setupOutgoing (new assertion for root path with base) and joinURL (comprehensive cases for "/", "", and nested paths with/without trailing slashes on base).

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Possibly related issues

• #138: Regression where trailing slash is added when target is used with a path; this PR fixes that by making joinURL return the base path unchanged when the incoming path is "/".

Possibly related PRs

• unjs/httpxy#137: Prior PR that introduced the regression in joinURL handling of path === "/" that this PR corrects.

Suggested reviewers

• pi0

Poem

🐰 A trailing slash caused quite a mess,
When "/" met a path, it made things less.
Now joinURL knows when to stay,
Base paths unchanged, no extra sway!

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately reflects the main change: fixing behavior when target includes a path, which is the core issue addressed by the regression fix.
Linked Issues check	✅ Passed	The PR directly addresses issue #138 by fixing the trailing slash regression in joinURL when target includes a path, restoring the original behavior from version 0.5.1.
Out of Scope Changes check	✅ Passed	All changes are focused on fixing the joinURL regression reported in issue #138; modifications to both implementation and tests are scoped to the specific path-joining behavior issue.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

---

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[chimurai]

Copilot shouldn't have modified existing tests.

[pi0]

Thanks for PR. Do you prefer to revert last patch? behavior change was kinda known behavior change in #137

[chimurai]

Hi @pi0

Think reverting the last patch makes sense, since some url paths are being changed after #137.

With http-proxy-middleware@4.0.0 released, httpxy is now getting around 10k+ daily new users.
Quite a large ecosystem will be impacted as projects are slowly adapting v4.

Some projects like @rspack/dev-server already migrated to v4
webpack-dev-server is currently upgrading as well: webpack/webpack-dev-server#5663

After reverting the patch. This missing unit test can be added to guard the specific proxy configuration/behaviour.

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");
});

from: https://github.com/unjs/httpxy/pull/139/changes#diff-10beca856b0075affd0be8845cd175f88b977fc852ca7f23b4b96b23a1254e66R380-R391

[pi0]

reverting now (3b5f8b0). Also added #141 to track adding better ecosystem CI testing.