如何将角形数据发送到nodejs后端

Question

我对棱角非常陌生。我正在将发布请求从angular7发送到nodejs后端服务器，但没有任何反应，并在控制台中收到此错误：

  

net :: ERR_CONNECTION_REFUSED和消息：“ http失败响应为   http://localhost:4000/business/add：0未知错误”名称：   “ HttpErrorResponse”

服务文件

export class BusinessService {

 uri = 'http://localhost:4000/business';

  constructor(private http: HttpClient) { }

  addBusiness(person_name, business_name, business_gst_number) {
    const obj = {
      person_name: person_name,
      business_name: business_name,
      business_gst_number: business_gst_number
    };
    this.http.post(`${this.uri}/add`,obj)
        .subscribe(res => console.log('Done'));
  }
}

Node.js后端：

app.use('/business', businessRoute);

businessRoutes.route('/add').post( function(req, res){

    console.log('hello');
    let business = new Business(req.body);
    business.save()
    .then(business => {
        res.status(200).json({'business': 'business in added sucessfully'});
    }).catch(err => {
        res.status(400).send('Unable to save to database');
        console.log(err);
    });
});