What does this handler send to the client, and why?

func h(w http.ResponseWriter, r *http.Request) {
	w.Write([]byte("hello"))
	w.Header().Set("Content-Type", "application/json")
	w.WriteHeader(http.StatusCreated)
}

The client gets status 200, Content-Type: text/plain; charset=utf-8, body hello. The server logs http: superfluous response.WriteHeader call.

Why: the first Write implicitly calls WriteHeader(200), and since no Content-Type was set, the server sniffs the first 512 bytes (http.DetectContentType). Headers are flushed at that point; later Header().Set modifies a map nobody reads, and the second WriteHeader is ignored.

Correct order: set headers → WriteHeader → Write.

func h(w http.ResponseWriter, r *http.Request) {
	w.Header().Set("Content-Type", "application/json")
	w.WriteHeader(http.StatusCreated)
	json.NewEncoder(w).Encode(map[string]string{"status": "created"})
}

Related gotcha: http.Error after partially writing a body produces a garbled response; decide on errors before writing. Trailers must be declared in the Trailer header before writing, or use the http.TrailerPrefix.