Iam trying to implement a progress bar for my charts because it takes some time to get the large json file and display it in the charts.

I found some examples at sencha docs but I dont understand it completely...

Can someone explain me how it works?
I know javascript is not multi-threading and I read some posts that people put something like that in their json results: {"success":true, "progress":0.2} and then update the progress bar on success with the progress value.

Is this the only and correct way?