Is it possible to add dynamically named properties to JavaScript object?

In JavaScript, I’ve created an object like so:

var data = {
    'PropertyA': 1,
    'PropertyB': 2,
    'PropertyC': 3
};

Is it possible to add further properties to this object after its initial creation if the properties name is not determined until run time? i.e.

var propName="Property" + someUserInput
//imagine someUserInput was 'Z', how can I now add a 'PropertyZ' property to 
//my object?

Yes.

var data = {
    'PropertyA': 1,
    'PropertyB': 2,
    'PropertyC': 3
};

data["PropertyD"] = 4;

// dialog box with 4 in it
alert(data.PropertyD);
alert(data["PropertyD"]);