dataType: "json"

意味着:给我json，别无其他.一个空字符串不是json，因此接收到一个空字符串意味着它并不成功...

means: give me json, nothing else. an empty string is not json, so recieving an empty string means that it wasn't a success...

request = $.ajax({
    type: "POST",
    url: url,
    data: {
        ....
    },
    error: function() {         
        console.log("error");
    },
    success: function(res) {
        var response = jQuery.parseJSON(res);
        if(typeof response == 'object'){
            if(response.reply == '2') {
                getResult();
            }  
        } else {
              //response is empty 
        }
    }
});