JSON to data structure in Javascript
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]
    //....
}