fix(node): keep derived content-length when FastResponse materializes

src/adapters/_node/response.ts

@@ -134,10 +134,36 @@ export const NodeResponse: {
         body = stream as unknown as ResponseBody;
       }
 
-      this.#response = new NativeResponse(
-        body,
-        this.#headers ? { ...this.#init, headers: this.#headers } : this.#init,
-      );
+      // Carry the content-type / content-length that `_toNodeResponse()` would
+      // derive, so materializing (reading `.body`, `.clone()`, wrapping the
+      // body in a proxy layer, …) doesn't silently switch to chunked framing.
+      let contentType: string | undefined;
+      let contentLength: number | undefined;
+      if (typeof body === "string") {
+        contentType = "text/plain; charset=UTF-8";
+        contentLength = Buffer.byteLength(body);
+      } else if (
+        body instanceof ArrayBuffer ||
+        body instanceof Uint8Array ||
+        body instanceof DataView
+      ) {
+        contentLength = body.byteLength;
+      } else if (body instanceof Blob) {
+        contentType = body.type;
+        contentLength = body.size;
+      }
+      let headers = this.#headers;
+      if (contentLength !== undefined) {
+        headers ??= new Headers(this.#init?.headers);
+        if (contentType && !headers.has("content-type")) {
+          headers.set("content-type", contentType);
+        }
+        if (!headers.has("content-length")) {
+          headers.set("content-length", String(contentLength));
+        }
+      }
+
+      this.#response = new NativeResponse(body, headers ? { ...this.#init, headers } : this.#init);
       this.#init = undefined;
       this.#headers = undefined;
       this.#body = undefined;

test/node-adapters.test.ts

@@ -1274,6 +1274,78 @@ describe("FastResponse header dedup", () => {
   });
 });
 
+// https://github.com/h3js/srvx/issues/332
+describe("FastResponse keeps derived content-length once materialized", () => {
+  const view = new DataView(new Uint8Array([0, 1, 2, 3, 4, 5]).buffer, 1, 3);
+  const cases: [string, () => BodyInit, string, string | null][] = [
+    ["string", () => "héllo", "6", "text/plain; charset=UTF-8"],
+    ["empty string", () => "", "0", "text/plain; charset=UTF-8"],
+    ["ArrayBuffer", () => new ArrayBuffer(4), "4", null],
+    ["Uint8Array", () => new Uint8Array(7), "7", null],
+    ["DataView", () => view, "3", null],
+    ["Blob", () => new Blob(["abc"], { type: "text/css" }), "3", "text/css"],
+  ];
+
+  test.skipIf(noBunCompat).each(cases)("%s body", (_, body, length, type) => {
+    const res = new FastResponse(body());
+    void res.body;
+    expect(res.headers.get("content-length")).toBe(length);
+    expect(res.headers.get("content-type")).toBe(type);
+  });
+
+  test.skipIf(noBunCompat)("user headers take precedence", () => {
+    const res = new FastResponse("hello", {
+      headers: { "Content-Type": "text/html", "Content-Length": "999" },
+    });
+    void res.body;
+    expect(res.headers.get("content-type")).toBe("text/html");
+    expect(res.headers.get("content-length")).toBe("999");
+  });
+
+  test.skipIf(noBunCompat)("keeps headers mutated before materializing", () => {
+    const res = new FastResponse("hello");
+    res.headers.set("x-custom", "1");
+    void res.body;
+    expect(res.headers.get("x-custom")).toBe("1");
+    expect(res.headers.get("content-length")).toBe("5");
+  });
+
+  test.skipIf(noBunCompat)("does not mutate a shared Headers init", () => {
+    const shared = new Headers({ a: "1" });
+    void new FastResponse("hello", { headers: shared }).body;
+    expect([...shared]).toEqual([["a", "1"]]);
+  });
+
+  test.skipIf(noBunCompat)("streams get no content-length", () => {
+    const res = new FastResponse(new ReadableStream());
+    void res.body;
+    expect(res.headers.get("content-length")).toBeNull();
+  });
+
+  test("is sent content-length framed after reading .body", async () => {
+    const server = serve({
+      port: 0,
+      fetch() {
+        const res = new FastResponse("hello");
+        void res.body;
+        return res;
+      },
+    });
+    await server.ready();
+    try {
+      const res = await fetch(server.url!);
+      expect(res.headers.get("content-length")).toBe("5");
+      expect(res.headers.get("transfer-encoding")).toBeNull();
+      expect(await res.text()).toBe("hello");
+
+      const headRes = await fetch(server.url!, { method: "HEAD" });
+      expect(headRes.headers.get("content-length")).toBe("5");
+    } finally {
+      await server.close(true);
+    }
+  });
+});
+
 // https://github.com/h3js/srvx/issues/250
 describe("FastResponse copies init Headers", () => {
   const headerPairs = (headers: string[]) => {