[lprnmns]

inject() returns the body a handler writes for a HEAD request, or for a 204 or 304 response. A real Node.js server never sends it: http.ServerResponse sets _hasBody = false in these cases and ignores the writes.

const dispatch = (req, res) => {
  res.writeHead(200, { 'content-type': 'text/plain', 'content-length': 5 })
  res.end('hello')
}

await inject(dispatch, { method: 'HEAD', url: '/' })   // payload: 'hello'
// the same handler behind http.createServer() + a HEAD request: no body

I found this by sending the same requests to a dispatch function through inject() and through a real http.createServer(). It is the same kind of mismatch that #388 fixed for request headers.

The fix checks this._hasBody in Response.prototype.write after delegating to http.ServerResponse, and drops the chunk when Node would (both the buffered payload and payloadAsStream). Headers, including content-length, are kept as they are, as on a real server.

Tests: three new cases at the end of test/index.test.js (HEAD, 204/304, HEAD with payloadAsStream) fail without the change. "chainable api: http methods should work correctly" echoed req.method in the body and expected it back for head too. It now expects an empty body for head. npm test (182 tests, 100% coverage, tstyche) and npm run lint pass.

Note for fastify: I ran fastify's test suite against this branch (by symlinking it into node_modules). 2 of 2388 tests read a HEAD body through fastify.inject(): "Should allow registering a constrained GET route after a constrained HEAD route" (test/constrained-routes.test.js) and "fastify.all should add all the methods to the same url" (test/internals/all.test.js, which parses the HEAD payload as JSON). A real fastify server sends no body in either case. I'm happy to open the matching fastify PR if you want to take this.

[gurgunday]

The behaviour is correct, but with this branch in place 2 fastify core tests fail that pass without it. They are constrained-routes.test.js:640 ("constrained GET route after a constrained HEAD route") and "fastify.all should add all the methods" in test/internals/all.test.js, both asserting a HEAD body through inject(). Fastify core needs updating before this ships in a minor release.

[lprnmns]

Thanks! I opened fastify/fastify#7148, which updates those two tests to check the HEAD response headers (content-type / content-length) instead of the body. They pass with both the current light-my-request release and this branch, so it can land first.

[gurgunday]

lgtm