$.getJSon expects a callback functions either you pass it to the callback function or in callback function assign it to global variale.
var globalJsonVar;
$.getJSON("http://127.0.0.1:8080/horizon-update", function(json){
//do some thing with json or assign global variable to incoming json.
globalJsonVar=json;
});
IMO best is to call the callback function. which is nicer to eyes, readability aspects.
$.getJSON("http://127.0.0.1:8080/horizon-update", callbackFuncWithData);
function callbackFuncWithData(data)
{
// do some thing with data
}