JSON stands for JavaScript Object Notation. A JSON object is really a string that has yet to be turned into the object it represents.

To add a property to an existing object in JS you could do the following.

object["property"] = value;

or

object.property = value;

If you provide some extra info like exactly what you need to do in context you might get a more tailored answer.

Using your example above, something like:

data.car[0] = { name: "civic" };
data.car[1] = { name: "s2000" };

This will assign a new object to the array element, with one property 'name'. Alertnatively, for a bit more code re-use:

function car( name) {
    this.name = name
}
data.car[0] = new car("civic");
data.car[1] = new car("s2000");

I'll give you an example from your add function:

function add(time, user, text){
    // this line is all I changed
    var message = {'time' : time, 'user' : user, 'text' : text};

    if (msg.length >= 50)
        msg.shift();

    msg.push(message);        
}

As you can see the message variable is no longer an array but it's the Object you want it to be.

From this you should be able to work out how to create a new array and add the values you want to it.

Could you do the following:

obj = {
    "1":"aa",
    "2":"bb"
};


var newNum = "3";
var newVal = "cc";


obj[newNum] = newVal;



alert(obj["3"]); // this would alert 'cc'