dynamically assign key in json using javasctipt

I am trying to find a way to dynamically assign keynames when creating my json object using java script.
This issue I have is that the stringfy option ignores the value for the key name but it does interpet the key value
..
..

  var xxKeyName = "MyTestKey1"
  var xxKeyValue = "12345"

   var oTableDataJson =
        JSON.stringify(
            {xxKeyName : xxKeyValue});
..
this result in a json looking like:
{"xxKeyName":"12345"}

but what I want is
{"MyTestKey1":"12345"}