JSONP is a common technique used to communicate with a JSON-serving Web Service with a Web browser over cross-domains, in place of a XHR request. There is a lot written about JSONP out there, but the tl;dr on it is a Javascript http client requesting JSONP will write a <script> tag to the head of a page, with the src to an API endpoint, with the addition of a callback (or jsonp) query parameter that represents a randomly-named listener function that will parse the request when it comes back from the server.

This middleware will work with anything that supports standard http.Handler. The code is small, so go read it, but it just buffers the response from the rest of the chain, and if its a JSON request with a callback, then it will wrap the response in the callback function before writing it to the actual response writer.

Any feedback is welcome and appreciated!

// JSONP example using Chi http router.. but anything that accepts
// a http.Handler will work
package main
import (
	"log"
	"net/http"
	"github.com/go-chi/chi"
	"github.com/go-chi/chi/middleware"
	"github.com/go-chi/jsonp"
	"github.com/go-chi/render"
)
func main() {
	mux := chi.NewRouter()
	mux.Use(middleware.Logger)
	mux.Use(jsonp.Handler)
	mux.Get("/", func(w http.ResponseWriter, r *http.Request) {
		data := &SomeObj{"superman"}
		render.JSON(w, r, data)
	})
	err := http.ListenAndServe(":4444", mux)
	if err != nil {
		log.Fatal(err)
	}
}
type SomeObj struct {
	Name string `json:"name"`
}

Output:

$ curl -v "https://localhost:4444/"
*   Trying ::1...
* Connected to localhost (::1) port 4444 (#0)
> GET / HTTP/1.1
> Host: localhost:4444
> User-Agent: curl/7.43.0
> Accept: */*
>
< HTTP/1.1 200 OK
< Content-Type: application/json; charset=UTF-8
< Date: Fri, 14 Aug 2015 19:11:44 GMT
< Content-Length: 19
<
* Connection #0 to host localhost left intact
{"name":"superman"}
$ curl -v "https://localhost:4444/?callback=X"
*   Trying ::1...
* Connected to localhost (::1) port 4444 (#0)
> GET /?callback=X HTTP/1.1
> Host: localhost:4444
> User-Agent: curl/7.43.0
> Accept: */*
>
< HTTP/1.1 200 OK
< Content-Length: 122
< Content-Type: application/javascript
< Date: Fri, 14 Aug 2015 19:11:49 GMT
<
* Connection #0 to host localhost left intact
X({"meta":{"content-length":19,"content-type":"application/json; charset=UTF-8","status":200},"data":{"name":"superman"}})

Since JSONP must always respond with a 200, as thats what the browser <script> tag expects, a nice pattern that is also used in the GitHub API is to put the HTTP response headers in a "meta" hash, and the HTTP response body in "data". Like so..

JsonpCallbackFn_abc123etc({
  "meta": {
    "Status": 200,
    "Content-Type": "application/json",
    "Content-Length": "19",
    "etc": "etc"
  },
  "data": { "name": "yummy" }
})

MIT - see LICENSE file