AJAX Dynamic table using JSON template

I am trying to make a dynamic table in the success part of an AJAX call. Depending on the user’s selections, the table can have totally different field names & data (different queries) from one call to the next. I am attempting to do this using JSON data to supply the field names & data for the different scenarios.

JSON

var fields = { 
                "Customer Name": "cust_nm",
                "Circuit ID": "ckt_id",
                "Speed": "speed",
            };

AJAX

success: function (data) {                      
            for (key in fields) {
                if (fields.hasOwnProperty(key)) {
                    var x = 'data.' + fields[key];
                    var row = '<tr><td>' + key + '</td><td> ' + x + '</td></tr>';
                    $('#circuitDetails').append(row);
                }
            }
        }

As can be seen in the result screenshot below, it is recognizing 'data.' + fields[key] as a string rather legitimate data definition. In this case the data returned would have had 3 fields named cust_nm, ckt_id & speed.

Suggestions on how to accomplish this would be appreciated.