JSON to data structure in Javascript I've never formally learned javascript, and I feel very much like I'm unaware of 'good' practice.

I'm working on this open source project, and in particular the code that converts the incoming JSON into a usable data structure in this file 

The relevent function looks like this: 

   function start() {
       key = "toppage";
       utterances = {};
       links = {};
       colours = {};
       icons = {};
       labels = {};
       slide_number = {};
       var req = new XMLHttpRequest();
       req.open("GET", "pageset.json");
       req.overrideMimeType("application/json");
       req.send(null);
       req.onreadystatechange = function() {
           if (req.readyState == 4 &amp;&amp; req.status == 200) {
               var obj = JSON.parse(req.responseText);
               for (grid in obj.Grid) {
           console.log(obj.Grid[grid][0])
                   labels[obj.Grid[grid][0]] = obj.Grid[grid][1];
                   utterances[obj.Grid[grid][0]] = obj.Grid[grid][2];
                   links[obj.Grid[grid][0]] = obj.Grid[grid][3];
                   icons[obj.Grid[grid][0]] = obj.Grid[grid][4];
                   colours[obj.Grid[grid][0]] = obj.Grid[grid][5];
                   slide_number[obj.Grid[grid][0]] = obj.Grid[grid][6];
           if(obj.Grid[grid][6]==0){
            key=obj.Grid[grid][0]
            }
               }
               grid_size_rows = obj.Settings[0];
               grid_size_columns = obj.Settings[0];
               setup_messagewindow();
               setup_table();
               load_page(key);
           }
       };
       //TODO - needs an error message if the JSON doesn't load
   }


I have NO idea if this is a reasonable way to populate structures in javascript.  It works, but I feel like it could be a hell of a lot more elegant.  Any comments people have would be welcome.
First of all,you declare them without anything,so it's probably declared globally,this is not good,with this amount of variables in global.

Second of all,ES6 Destructuring assignment

var arr=[1,2,3,4,5];
var [a,b,c,d,e]=arr;
console.log(a,b,c,d,e); // 1,2,3,4,5


Using the same way,

for (grid in obj.Grid) {
    var i=obj.Grid[grid][0];
    var [unused,labels[i], utterances[i], links[i], icons[i], colours[i], slide_number[i]]
    =obj.Grid[grid]
    //....
}