Asynchronously populate Sammy.Storage cache - ajax

Question

I'm having difficulty accessing requestJSON on a jQuery $.ajax object outside of the success callback. If I do:
var ajax_request = $.ajax({
dataType: 'json',
contentType: 'application/json'
});
console.log(ajax_request.responseJSON);
// this results in `undefined`
How can I access the responseJSON without adding a .success() callback? If I inspect ajax_request in Firebug, I can see the responseJSON property, and the data I expect, but I can't access it via:
ajax_request.responseJSON
More specifically, I'm building an SPA using Sammy and Knockout. In some routes, I need to be able to get JSON from cache, and if it doesn't exist, get the value from a service call and then set it into cache:
var cached_json = storage.fetch('cached_json', function() {
// make service call
return $.getJSON(url);
});
event_context.render('template.tpl', {'json': cached_json}).appendTo('#my-target');
But, of course, calling storage.fetch doesn't cause the rest of the code to pause until $.getJSON is complete. This is the part I can't quite figure out how to structure.

Answer 1

here's how i would implement it
responseJSON = "";
$.get("myurl.php",function(jdata){
responseJSON = jdata;
},"json");
i like to see the ajax method at a glace, but in your case you can do the same by
....
success : function(jdata){ responseJSON = jdata; }
....
PS: i believe that initializing the blank responseJSON is not required since any variable without var is in global scope, but it would help for clarity

Answer 2

I ended up solving this by creating a deferred object that gets or creates the value I need:
function get_or_create_cache(storage, key, service_endpoint) {
return $.Deferred(function(deferred) {
var c = storage.get(key);
if (c === null || c === undefined) {
$.when(jsonp_service_request(service_endpoint)).done(function(json) {
storage.set(key, json);
deferred.resolve(json);
});
}
else {
deferred.resolve(c);
}
}).promise();
}
In this function, storage refers to a Sammy.Storage instance. jsonp_service_request is a local function that returns a jsonp response, taking into account the location.hostname for local development, where I'm pointing to local.json files, or a remote environment, where I'm calling into an actual API. jsonp_service_request returns an $.ajax function.
Then in my Sammy route, I can do:
this.get('#/', function(event_context) {
$.when(get_or_create_cache(storage, 'my-cache-key', 'service-endpoint'))
.then(function(json) {
event_context.render('my-template.template', {'value-name': json})
.appendTo('#my-target');
});
});