insertBackend.js

var express = require('express');
var User = require('../model/user');
var mongoose = require('mongoose');

var router = express.Router();

router.post('/insertUser', function(req, res, next) {
  var newUser = new User(req.body);
  newUser._id = mongoose.Types.ObjectId();

  newUser.save(function(err) {
    if (err) {
      console.log("not saved!");
      res.status(400);
      res.send();
    }

    console.log("saved!");
    res.send({ id : newUser._id });
  });
});