If you've ever wanted to handle JSONP callbacks within a Go powered API, here's a simple wrapper that you can use to intercept the callback query variable and properly wrap the response. It's designed to take an entire http.Handler, so works particularly well when using a router such as mux from gorilla, however can work with any handler.

jsonphandler.go

import (
  "io"
  "net/http"
)

//With HandlerFunc
func main () {
  http.Handle("/", JsonpHandler(http.HandlerFunc(func (w http.ResponseWriter, r *http.Request) {
    w.Write("Hello World")
  })))
}

func JsonpHandler(h http.Handler) http.Handler {
  return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
    callback := r.FormValue("callback")
    if callback != "" {
      w.Header().Add("Content-Type", "text/javascript")
      io.WriteString(w, callback+"(")
      defer func() {
        io.WriteString(w, ");")
      }()
    }
    h.ServeHTTP(w, r)
  })
}