如何在React Native中显示来自Node JS服务器的错误消息作为警报？

Question

当服务器向客户端返回错误消息时，如何显示警报框。如果响应无效，但警报没有来临，我已经做了一些显示警报框的操作。服务器中的错误消息也在控制台内显示为unexpected end of json input。但是，成功情况也在显示。成功情况为{error:0 , token:zxrggsdhsgdjcbuewy656}

时控制台的o / p

nodjs

users.post（'/ login'，function（req，res）{

   var appData ={};
   var phone_no = req.body.contact;
   var pwd      = req.body.password;

   database.connection.getConnection(function(err,connection){
    if(err){
      appData["error"] =1;
      appData["data"]  ="Internal Server Error";
      res.status(500).json(appData);
    }else{
      connection.query("SELECT * FROM user_registration WHERE phone_no = ?",[phone_no],function(err,result,fields){
        if(err){
          appData.error = 1;
          appData["data"] = "Error Occured";
          res.status(400).json(appData);
          console.log(result);

        } else {
         if(result.length >0) {
           if(result[0].pwd == pwd){
             let token = jwt.sign(result[0], process.env.SECRET_KEY,{
               expiresIn :5000
             });
             appData.error =0;
             appData["token"] =token;
             res.status(200).json(appData);
             //console.log(appData);
           } else {
             appData.error= 1;
             appData["data"] = "Phone number and Password does not match";
             res.status(204).json(appData);
             //console.log(appData);
           }
         }else{
           appData.error=1;
           appData["data"] ="Phone number does not exist";
           res.status(204).json(appData);
           //console.log(appData);
         }
        }
      });
      connection.release();
    }

  });

还有反应部分如下

onPressLogin(){
    fetch('http://192.168.1.7:3000/users/login',{
        method: 'POST',
        headers:{
                'Content-Type' : 'application/json',
            },
            body: JSON.stringify({
                contact:this.state.contact,
                password: this.state.password,
            })
})
  .then(response => response.json())
  .then((responseData) =>
        {
    this.setState({
        userdetail: responseData,
        })
console.log(responseData);
if(responseData.error == '0'){
  setTimeout(() => {
    Actions.firstScreen();
  }, 2300);
  AsyncStorage.setItem('userdetail', this.state.userdetail.token);
    AsyncStorage.getItem('userdetail').then((value) => console.log(value));
} else {
  Alert.alert(
  'Alert',
  'Invalid Login',
  [
    {text: 'Cancel', onPress: () => console.log('Cancel Pressed'), style: 'cancel'},
    {text: 'OK', onPress: () => console.log('OK Pressed')},
  ],
  { cancelable: false }
)
}
});

    }

请帮助