How to implement a JSONP call - jsonp

Question

Let me preface this saying I'm still very new to ClojureScript.
I'm trying to parse an API that has CORS turned off. My only choice is to use JSONP requests. But I have no idea how this is done. As far as I understand JSONP, it's the server that returns a callback script.
How do I implement this in ClojureScript? I have this snippet that I try to use:
(defn get-json [url callback]
(xhr/send url callback "GET"))
(defn logcallback [event]
(let [response (.-target event)]
(.log js/console (response))))
(get-json ("http://www.apiurl.com/method")
logcallback)
But I still receive a Origin "localhost:3449" is not allowed (CORS) error. I guess this is not really a JSONP request. How would I actually do this?

Answer 1

You can use goog.net.Jsonp. Here's a simple example:
(ns jsonp.core
(:import [goog.net Jsonp]
[goog Uri]))
(defn success-handler [res]
(.log js/console res))
(defn error-handler [res]
(js/alert (str "Found an error: " res)))
(let [url "http://en.wikipedia.org/w/api.php?action=opensearch&format=json&search=clojure"
jsonp (goog.net.Jsonp. (Uri. url))]
(.send jsonp nil success-handler error-handler))
If your endpoint returns a "wrapped payload" and the param for setting the callback name is not "callback", you can pass the callback param name as the second parameter to the Jsonp constructor. The following example gets a flickr feed which expects the "jsoncallback" param:
(let [url "http://api.flickr.com/services/feeds/photos_public.gne?format=json"
jsonp (goog.net.Jsonp. (Uri. url) "jsoncallback")]
(.send jsonp nil success-handler error-handler))
goog.net.Jsonp will take care of passing the extra param (jsoncallback in the example) and unwrap the payload by calling the function. In either case it will end calling the success or error handler.
You can take a look into clojurescript 101 by David Nolen. The post is from 2 years ago, but it should still work. There's also another clojurescript tutorial which has a similar example.