You should use a datastore and proxy in ExtJs. There are plenty of examples of this, and the JSON reader automatically parses the JSON message into the model you specified.
There is no need to use basic Javascript when using ExtJs, everything is different, you should use the ExtJs ways to get everything right. Read there documentation carefully, it's good.
By the way, these examples also hold for Sencha Touch (especially v2), which is based on the same core functions as ExtJs.