Passing key names to json object in jquery

I'm trying to write a generalised function that validates my field through Jquery Ajax. My problem is, I want to give key names via function input. However in the field_value variable, my field_label parameter stays as "field_label". I want to change it like "email", "username" so i can generate different jsons. So any idea ?

  function validate_field(span_id,field_id,field_label,url){
           var message_field = $("#" + span_id)

           $("#" + field_id).keyup(function(){
                if(this.value != this.lastValue){
                      if(this.timer){
                          clearTimeout(this.Timer);
           开发者_运维问答           }
                message_field.html('<img src="/media/img/loader.gif">');
                var field_value = {field_label : $("#"+field_id).val()};
                var json_response = JSON.stringify(field_value);
                this.timer = setTimeout(function(){
                      $.ajax({
                            type:'POST',
                            url:url,
                            contentType:'application/json; charset=utf-8',
                            data:json_response,
                            success:function(msg){
                                 message_field.html(msg);
                            }
                      });
                });
             }
          });
    }

change this line:

var field_value = {field_label : $("#"+field_id).val()};

to this:

var field_value = {};
field_value[field_label] = $("#"+field_id).val();