AJAX没有收到错误

Question

I'm completely new to JSON, JS and AJAX. I'm used some example codes to progress through my targets. What I don't understand is when the JSON is an error object the success section is still firing. The console shows error where object d is null or a non-object when error is true. What have I done incorrectly? TIA

Result type good:

JSON: {"data":["1","breast","2","wing","3","thigh","4","leg","5","half","6","quarter white","7","quarter dark","9","whole"],"error":false}

Result type error:

JSON: {"error":true}

$.ajax({
            type     : 'GET',
            url      : 'getsubtypes2.php',
            data     : dataString,
            dataType : 'JSON',
            cache: false,
            success  : function(data) {          
                var output;            
                var d = data.data;
                    var output = "";
                    for (var i = 0 ; i< d.length; i=i+2) {//error gets to this line
                      var count = d[i];
                      var newOption = d[i+1];
                      output += "<option value='"+count+"'>"+newOption+"</option>";
                    }
                    $('#select3').empty().append(output); 
            },
            error: function(){
            $('#select3').empty();
                    console.log("Ajax failed");
            }
        });

Answer 1

The server must return an error status code such as 4xx or 5xx for the error callback to execute. If you server is returning a 2xx, the success callback will execute.

For testing you can try explicitly sending an error code to see the error callback executing. See this question.