Jquery - Uncaught TypeError: Cannot use 'in' operator to search for '324' in
You have a JSON string, not an object. Tell jQuery that you expect a JSON response and it will parse it for you. Either use $.getJSON instead of $.get, or pass the dataType argument to $.get:
$.get( 'index.php?r=admin/post/ajax', {"parentCatId":parentCatId}, function(data){ $.each(data, function(key, value){ console.log(key + ":" + value) }) }, 'json');
You can also use $.parseJSON(data) that will explicit convert a string thats come from a PHP script to a real JSON array.