ClientResponse<T>.json() returns never when T is a union type of structurally different objects

[moreirathomas]

What version of Hono are you using?

3.11.2

What runtime/platform is your app running on?

Found on Bun (v1.0.15) initially, reproduced on Deno (v1.38.4)

What steps can reproduce the bug?

Hi, thanks for building Hono!

I ran into an unexpected behavior when trying to use ClientResponse.json(). In the following minimal reproduction, the type information is lost, we get never.

import { hc, Hono } from "https://deno.land/x/hono@v3.11.2/mod.ts";

type ErrorResponse = {
  error: Error;
};

type OkResponse = {
  data: string;
};

// Simulate business logic.
declare function condition(): Promise<boolean>;

const app = new Hono();

const api = app.get("/", async (c) => {
  const ok = await condition();
  // Expect response type to be TypedResponse<OkResponse | ErrorResponse>
  if (ok) {
    return c.json<OkResponse>({ data: "foo" });
  }
  return c.json<ErrorResponse>({ error: new Error("error") });
});

const client = hc<typeof api>("http://localhost:8000");

const res = await client.index.$get();

const json = await res.json();

What is the expected behavior?

I expected the type to be OkResponse | ErrorResponse.

Explicitly typing the handler 's return value does not help.

const api = app.get("/", async (c): Promise<TypedResponse<OkResponse | ErrorResponse>> => {
  // ...
});

What do you see instead?

Additional information

It seems that the type breaks only on union of object (record) types:

declare function condition(): boolean;

const route = app
  .get("/primitive|primitve", (c) => {
    return condition() ? c.json(1) : c.json("a");
  })
  .get("/primitive|primitive[]", (c) => {
    return condition() ? c.json([1, 2, 3]) : c.json("a");
  })
  .get("/primitive[]|primitive[]", (c) => {
    return condition() ? c.json([1, 2, 3]) : c.json(["a", "b", "c"]);
  })
  .get("/object[]|primitive[]", (c) => {
    return condition() ? c.json([{ k: true }]) : c.json(["a", "b", "c"]);
  })
  .get("/object[]|object[]", (c) => {
    return condition() ? c.json([{ k: true }]) : c.json([{ l: 0 }]);
  })
  .get("/object|object", (c) => {
    return condition() ? c.json({ k: true }) : c.json({ l: true });
  });

const client = hc<typeof route>("http://localhost:8000");

const a = await client["primitive|primitve"].$get().then((res) => res.json());
//    ^ string | number
const b = await client["primitive|primitive[]"].$get().then((res) => res.json());
//    ^ string | number[]
const c = await client["primitive[]|primitive[]"].$get().then((res) => res.json());
//    ^ string[] | number[]
const d = await client["object[]|primitive[]"].$get().then((res) => res.json());
//    ^ { k: boolean }[] | string[]
const e = await client["object[]|object[]"].$get().then((res) => res.json());
//    ^ { k: boolean }[] | { l: number }[]
const f = await client["object|object"].$get().then((res) => res.json());
//    ^ never

I suspect it has something to do with the signature of ClientResponse.json(): Promise<BlankRecordToNever<T>>.

type BlankRecordToNever<T> = keyof T extends never ? never : T

type X = BlankRecordToNever<{ a: string } | { b: string }>;
//   ^ never (as expected)

Is this an expected behavior?

[yusukebe]

Hi @moreirathomas

Thanks for raising the issue. it might be fixed in latest release v3.11.3. Check git it a try!