The string looks like `"jsonCallback( {"/duck/index.vwf/732f17b6e704e3d1":{"clients":{"d6570d1f55f5d4c1":null}}} )"`. It's intended to be used with a script element as in `<script src="/duck/instances/jsonp?..."></script>`, which evaluates to `<script>jsonCallback( {"...":{"clients":{...}}} )</script>`.

A `script` element is used because it doesn't have the cross-site restrictions that a regular XHR call does. But the result has to _do_ something; an unwrapped value would just be evaluated and dropped. Here it's presuming that the client has a `jsonCallback()` function that can receive the result.

This implementation hard codes the client function name. Typically, clients will create the function under a randomized name and pass a `?callback=uniquename12345` parameter to tell the server to wrap the result with the requested name. It would be better to support the `callback` param instead of requiring a fixed name. We wouldn't need a separate JSONP resource in that case. Just look for the `callback` param and if it's there, return JSONP; otherwise return JSON. `vwf/application/component.rb` does this for component requests.