How can I stop my function from returning "Reference error: (...) is not defined"

I've got this code and it's returning that reference error, but I have no idea why, I've seen lots of Q&As about reference errors, but I still can't find it. Any help is appreciated.

Thanks in advance and may the force be with you.

My code:

 var data = '[{"name":"node1","id":1,"is_open":true,"children":[{"name":"node2","id":6,"children":[{"name":"child3","id":7}]},{"name":"child1","id":2}]}]';

 var dadSon = [];

 (function printDadSon(data, parent) {
   if (!data) return;
   for (var i = 0; i < (data.length); i++) {
     if (parent && parent != 'undefined') {
       console.log('Dad: ' + parent + ' & Son: ' + data[i].id);
       dadSon += ('Dad: ' + parent + ' & Son: ' + data[i].id);
     }

     printDadSon(data[i].children, data[i].id);

   }
 })(JSON.parse(data));

 printDadSon(data);