Javascript to access nested keys by category

Ok, I think there are some misconceptions that lead you to some errors in handlig the dictionaries in SJ.
You main proble is in this code:

var subdict = new Dict(keys[0]);
if(typeof(subdict) == "string")
subdict = [subdict];

.
the var subdict = new Dict(keys[0]) will create a new dict that will be named as the first key of the first Dict. That doesn't mean that it is the a sub Dict. The correct way to get the sub dict is by the get("keyName") method of dict like so:
var subdict = dict.get(keys[0]);. This works because thekeys[0]` will return (in this case) the name of the 1st key, that happens to be another dictionary. To make sure that the dictionary element you get is another dict you should check that is it object but not array.
Secondly, here: if(typeof(subdict) == "string") subdict = [subdict]; you are comparing the dict element to see if it is a string, not its keys.
So it will return an array with on object inside, that is in this the sub dictionary.

So, to get the keys you do var mykeys = mydict.getkeys(). This will return either an array of string, or a string (not ver cool but it is the way it is...). Only here you should do this check if(typeof(mykeys ) == "string") subdict = [mykeys ];, and that is so you can easier write that handles the keys i.e. be sure that it is an array, and not have to check every time if its not.
To get a subdictionary you should do var thesubdict = mydict.get(keys[N]);. Here you should make sure that it is indeed a dictionary - unless you are 100% sure that the Nth key is a subdictionary, bu still...
the var thesubdict (or whatever name) you should treat as a dict (because it is a dict!), meaning the way you got the first dicts keys etc, the same way you will for the subdict, like var sudictkeys = thesubdict .getkeys() etc...

I made the above corrections to your code and it works:

var dict = new Dict;
dict.import_json("fruits and peaches 3.json");

var keys = dict.getkeys();
if(typeof(keys) == "string") // Force the ‘keys’ to be an array, even with single element
keys = [keys];

var name = dict.name;
post("dict name: ", name);
post("\nkey: ", keys);
post("\ndict type: ", typeof(dict), "array? ", Array.isArray(dict)); // object, not an array
post("\nkey type: ", typeof(keys), "array? ", Array.isArray(keys)); // object, is an array
post("\nkeys[0] type: ", typeof(keys[0]), "array? ", Array.isArray(keys[0])); // says it’s a string
post();
post();

var subdict = dict.get(keys[0]);
post(subdict.stringify());
post("subdict type: ", typeof(subdict), "array? ", Array.isArray(subdict));
post();

var subkeys = subdict.getkeys();
post("subkeys type: ", typeof(subkeys), "array? ", Array.isArray(subkeys));
post(subkeys);
post();
post(Object.getOwnPropertyNames(subkeys));
post();
post();

if(typeof(subkeys) == "string")
subkeys = [subkeys];
post(subkeys);
post();

also the json you posted wasn't formatted correctly, so here it is
(also make sure that the json you use is correctly formated. To test it you can load it in an object [dict], if it is OK it will be read, else it will not) :

{
    "fruitspeaches" :     {
        "business" : "Francis Motuzick Jr",
        "category" : "Fruit",
        "farmer_id" : "3402",
        "item" : "Peaches",
        "l" : "0",
        "location_1" :         {
            "type" : "Point",
            "coordinates" : [ -73.096275, 41.779892 ]
        }
    }
}

Using these two, it works.
-N