feat: support function-based dynamic target for ServerOptions

[tbontb-iaq]

Allow target option to accept a function (req) => ProxyTarget in addition to a static ProxyTarget, enabling per-request dynamic routing.

Changes

• lib/http-proxy/index.ts: Added ProxyTargetFunction type, changed ServerOptions.target to accept ProxyTarget | ProxyTargetFunction. Function target is resolved in createRightProxy before any passes run, so all downstream code sees a resolved static target.
• lib/index.ts: Exported ProxyTargetFunction type.
• lib/test/http/dynamic-target.test.ts: 6 new tests covering path-based routing, {port, host} object return, and per-request override.

Example

const proxy = createProxyServer({
  target: (req) => {
    if (req.url?.startsWith("/api/v2")) return "http://localhost:3001";
    return "http://localhost:3000";
  },
});

---

🤖 This code was written by AI (OpenCode / DeepSeek v4 Pro).

[williamstein]

Wow, that's a good idea, I think. Code looks good.

[williamstein]

Further feedback: Tests include a placeholder-ish override test, there is no websocket coverage even though the change affects ws too, and a throwing target function currently looks like it would escape instead of going through proxy error handling. Can you clean this up a bit?

[tbontb-iaq]

Thanks for the review! I've addressed all three points in b0063bf:

1. Removed the placeholder test — the first per-request "override" test was messy with dead code and unused target2, didn't actually test override. Kept the real one that tests proxy.web(req, res, { target: ... }) override.

2. Added WebSocket test coverage — lib/test/websocket/dynamic-target-ws.test.ts creates two WS echo servers and uses a dynamic target function to route /ws1 to one and /ws2 to the other, verifying correct upgrade and routing.

3. Wrapped function target call in try-catch — if the target function throws, the error is now emitted via this.emit("error", err, req, res) and the request is aborted (same pattern as the existing "Must set target or forward" error handling).

All 310 tests pass, no regressions.

[williamstein]

Thanks!

[tbontb-iaq]

Is there any progress?