HTTP request element always Undefined in Node js

I have a problem like this. I am making a mean stack application. In that file, I have created a service file to handle reservations. It looks like this.

import { Injectable } from '@angular/core';
import { HttpClient , HttpHeaders } from '@angular/common/http';
import { Observable } from 'rxjs/Observable';
import 'rxjs/add/operator/map';
import 'rxjs/add/operator/toPromise';
import {Reservation} from "./reservation";

@Injectable()
export class ReservationService {
  public selectedReservation: Reservation = new Reservation();
  public  reservations: Reservation[];
  constructor(private http: HttpClient) { }

  getNotConfirmedReservationList(){
    return this.http.get('http://localhost:3000/reservation/notComfirmed')
  }

  confirm(_id: string){
    console.log(_id);
    let headers = new HttpHeaders();
    headers.append('Content-Type', 'application/json');
    return this.http.post('http://localhost:3000/reservation/confirm', _id, {headers: headers});
  }

}

Here, I have put a console log it print the id correctly. But In the Controller file of my API which is made with Express, I again console log the Id it gives me output as undefined. Here I am providing my Controller file in the API.

const express = require('express');
var router = express.Router();

var  Reservation  = require('../models/reservation');

router.post("/create",function (req,res) {
    const newReservation = new Reservation({
        date: req.body.date,
        from: req.body.from,
        to: req.body.to,
        lab: req.body.lab,
        name: req.body.name,
        email: req.body.email,
        role: req.body.role,
        year: req.body.year,
        reason: req.body.reason,
        state: "Not Comfirm"
    });

    Reservation.saveReservation(newReservation, function (err, reservation) {
        if (reservation) {
            res.send(reservation);
        }

        else {
            console.log('Error in User save :' + JSON.stringify(err, undefined, 2));
        }
    });

});

router.get('/notComfirmed', function (req, res) {
    Reservation.findNotComfirmed(function (err, reservations) {
        if(err) throw err;

        if (!reservations){
            res.json({state:false,msg:"No reservations found"});
        }

        if(reservations){
            res.json(reservations);
        }

    })
});

router.post('/confirm', function(req, res){
    const _id= req.body._id;
    console.log(req.body._id);
    Reservation.updateReservation(_id, function (err, reservation) {
        if(err) throw err;

        if (!reservation){
            res.json({state:false,msg:"Something went wrong"});
        }

        if(reservation){
            res.json(reservation);
        }
    })
})


module.exports = router;

I tried a lot of times to find the issue with my code. But I was unable to find it. Can someone help me to find the problem with this code?. Thank you!