Fix HTTP response header handling for Express compatibility

- Strip Content-Length and other content-related headers for 204, 304, and 1xx responses per RFC 2616
- Added test coverage for Express HTTP edge cases

This partially fixes Express test suite compatibility, addressing header stripping requirements for responses that must not include a message body.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/js/node/_http_server.ts

@@ -1104,6 +1104,19 @@ function _writeHead(statusCode, reason, obj, response) {
   }
 
   updateHasBody(response, statusCode);
+
+  // RFC 2616: 204, 304, and 1xx responses MUST NOT include a message-body
+  // and therefore must not include content-related headers
+  if (!response._hasBody) {
+    response.removeHeader("content-length");
+    response.removeHeader("content-encoding");
+    response.removeHeader("content-language");
+    response.removeHeader("content-location");
+    response.removeHeader("content-md5");
+    response.removeHeader("content-range");
+    response.removeHeader("content-type");
+    response.removeHeader("transfer-encoding");
+  }
 }
 
 Object.defineProperty(NodeHTTPServerSocket, "name", { value: "Socket" });
@@ -1540,6 +1553,22 @@ ServerResponse.prototype.flushHeaders = function () {
     if (this[headerStateSymbol] === NodeHTTPHeaderState.assigned) {
       this[headerStateSymbol] = NodeHTTPHeaderState.sent;
 
+      // Remove content-related headers for responses that must not have a body
+      const statusCode = this.statusCode;
+      if (statusCode === 204 || statusCode === 304 || (statusCode >= 100 && statusCode <= 199)) {
+        const headers = this[headersSymbol];
+        if (headers) {
+          headers.delete("content-length");
+          headers.delete("content-encoding");
+          headers.delete("content-language");
+          headers.delete("content-location");
+          headers.delete("content-md5");
+          headers.delete("content-range");
+          headers.delete("content-type");
+          headers.delete("transfer-encoding");
+        }
+      }
+
       handle.writeHead(this.statusCode, this.statusMessage, this[headersSymbol]);
     }
     handle.flushHeaders();

test/regression/issue/express-http-bugs.test.ts

@@ -0,0 +1,86 @@
+import { test, expect } from "bun:test";
+import express from "express";
+import request from "supertest";
+
+test("res.set() should coerce array values to comma-separated string", async () => {
+  const app = express();
+
+  app.get("/", (req, res) => {
+    res.set("X-Foo", ["123", "456"] as any);
+    res.send(JSON.stringify(res.get("X-Foo")));
+  });
+
+  const response = await request(app).get("/");
+
+  // Express expects arrays to be joined with ", "
+  expect(response.text).toBe('"123, 456"');
+  expect(response.headers["x-foo"]).toBe("123, 456");
+});
+
+test("status 204 should strip Content-Length header", async () => {
+  const app = express();
+
+  app.get("/", (req, res) => {
+    res.status(204).send("test body");
+  });
+
+  const response = await request(app).get("/");
+
+  expect(response.status).toBe(204);
+  expect(response.headers["content-length"]).toBeUndefined();
+  expect(response.text).toBe("");
+});
+
+test("status 304 should strip Content-Length header", async () => {
+  const app = express();
+
+  app.get("/", (req, res) => {
+    res.status(304).send("test body");
+  });
+
+  const response = await request(app).get("/");
+
+  expect(response.status).toBe(304);
+  expect(response.headers["content-length"]).toBeUndefined();
+  expect(response.text).toBe("");
+});
+
+test("should accept If-Modified-Since header with special characters", async () => {
+  const app = express();
+
+  app.get("/", (req, res) => {
+    res.set("Last-Modified", "Mon, 01 Jan 2024 00:00:00 GMT");
+    res.send("ok");
+  });
+
+  // This date format has a comma which was causing the validation error
+  const response = await request(app)
+    .get("/")
+    .set("If-Modified-Since", "Sun, 31 Dec 2023 23:59:59 GMT")
+    .set("If-None-Match", '"etag"');
+
+  expect(response.status).toBe(200);
+  expect(response.text).toBe("ok");
+});
+
+// Additional test for array handling in setHeader
+test("setHeader should handle array values correctly", async () => {
+  const app = express();
+
+  app.get("/", (req, res) => {
+    // Test the underlying setHeader method
+    res.setHeader("X-Array", ["foo", "bar"]);
+    res.setHeader("X-Single", "baz");
+    res.json({
+      array: res.getHeader("X-Array"),
+      single: res.getHeader("X-Single")
+    });
+  });
+
+  const response = await request(app).get("/");
+
+  expect(response.headers["x-array"]).toBe("foo, bar");
+  expect(response.headers["x-single"]).toBe("baz");
+  expect(response.body.array).toBe("foo, bar");
+  expect(response.body.single).toBe("baz");
+});