It’s not an array.
var json = {"cool":"34.33","alsocool":"45454"};
json.coolness = 34.33;
or
var json = {"cool":"34.33","alsocool":"45454"};
json['coolness'] = 34.33;
you could do it as an array, but it would be a different syntax (and this is almost certainly not what you want)
var json = [{"cool":"34.33"},{"alsocool":"45454"}];
json.push({"coolness":"34.33"});
Note that this variable name is highly misleading, as there is no JSON here. I would name it something else.