golang http response headers being removed

Question

I'm not sure if this is a bug or how the http response package is supposed to work.

In this example the Content-Type response header will not be set

// Return the response
w.WriteHeader(http.StatusCreated)
w.Header().Set("Content-Type", "application/json")
w.Write(js)

How ever if I flip the order of how the headers are set it does work:

// Return the response
w.Header().Set("Content-Type", "application/json")
w.WriteHeader(http.StatusCreated)
w.Write(js)

Now this will actual set the header to application/json. Is this behavior intended?

Answer 1

Headers can only be written once to the response so you must set all the headers before writting them. Once the headers are written they are sent to the client.

You should only call w.WriteHeader(http.StatusCreated) once you have set all your headers.

Read in the GOLANG spec how WriteHeader works

This rule is the same for the body once the body is written (writting to the response is literally sending it to the client) it can not be resent or changed.