在Cakephp 3中，Ajax调用返回undefined

Question

我正在尝试从jquery表单验证进行Ajax调用，但它不断返回undefined。我已经尝试在堆栈溢出中搜索解决方案，但没有任何帮助。这是我的Ajax调用：

jQuery(document).ready(function() {
    var form = $("#ClientAdminAddForm");
    form.validate({
        rules: {   
            tel: {
                number: true,
            },
            mobile: {
                number: true,
            },
            website: {
                url: true
            },
            client_reference_no:{
                remote: {
                    url: "<?php echo $this->Url->build(array('controller'=>'clients','action'=>'check')) ?>",
                    type: "post",
                    cache: "false",
                    dataType: "json",
                    data: {
                      'client_reference_no': function() {
                        return $( "#client_reference_no" ).val();
                    },
                    success: function(response) {
                        console.log(response);
                        // console.log(response);
                        // if (response == '0') 
                        //     return false;
                        // else 
                        //     return true;
                    }  
                }
            }, 
        },
        messages: {
            client_reference_no: { 
                remote: "The value is already taken."            
            }
        }
    }
});

这是我的控制器动作：

public function check()
{
    $this->autoRender = false;

    $client_reference_no = isset($this->request->data['client_reference_no']) && !empty($this->request->data['client_reference_no']) ? $this->request->data['client_reference_no']:'';
    $pid = isset($this->request->data['pid']) && !empty($this->request->data['pid']) ? $this->request->data['pid']:'';
    if (!empty($client_reference_no)) {     
        if($pid == '') {
            if ($this->Clients->exists(['client_reference_no' => $client_reference_no])) {
                $status = 0;
            }
        } 
        else {
            $data = $this->Clients->find()
                ->where(['client_reference_no' => $client_reference_no])
                ->select(['id'])
                ->first()
            ;
            if(!iterator_count($data)) {
                $status = 1;
            }
            else {
                if(!empty($data->id) && $data->id == $pid  ) {
                    $status = 1;
                }
                else {
                    $status = 0; 
                }
            }
        }
    }

    $this->set([
        'output' => $status,
        '_serialize'=>'output'
    ]);
}

我已尝试调试控制器中的$status并显示该值。但是ajax响应中的值未定义。我也在路由中加载了RequestHandler组件和集合$routes->setExtensions(['json', 'xml', 'ajax']);。 console.log()两次输出undefined：

我做错了什么？请帮忙！