无法读取“ ID参数”的属性

这是我导入了cart.js模型的Router的代码

var router = require('express').Router();
var Product = require('../models/products');
var Cart = require('../models/cart'); 

router.get('/add-to-cart/:_id',(req,res,next)=>{
  const productID = req.params._id;
  var cart = new Cart(req.session.cart ? req.session.cart : {});

  Product.findById(productID,(err,product)=>{
    if(err){
      res.redirect('/');
    }
    cart.add(product, product.id);
    req.session.cart = cart;
    res.redirect('/');
  })  
})

这是我的Cart.js代码

module.exports = function cart(oldCart){
   this.items = oldCart.items;
   this.totalQty = oldCart.totalQty;
   this.totalPrice = oldCart.totalPrice;

   this.add = function(item,id){
      var storedItem = this.items[id]; // Here is Problem
      if(!storedItem){
        storedItem = this.items[id] = {item: item, qty: 0, price: 0};
      }
      storedItem.qty++;
      storedItem.price = storedItem.item.price * storedItem.qty;
      this.totalQty++;
      this.totalPrice = storedItem.price;
   };

   this.generateArray = function(){
      var array = [];
      for (var id in this.items){
        array.push(this.items[id]);
      }
      return array;
   };
}

错误：

events.js：174投掷者//未处理的“错误”事件^

TypeError：无法读取未定义的属性'5d9182475f539435e81a7bb9'在cart.add（C：\ Users \ Manav \ Documents \ Github \ E_com_App \ models \ cart.js：7：36）在Product.findById（C：\ Users \ Manav \ Documents \ Github \ E_com_App \ routes \ routes.js：85：10）在C：\ Users \ Manav \ Documents \ Github \ E_com_App \ node_modules \ mongoose \ lib \ model.js：4589：16在C：\ Users \ Manav \ Documents \ Github \ E_com_App \ node_modules \ mongoose \ lib \ query.js：4323：12在process.nextTick（C：\ Users \ Manav \ Documents \ Github \ E_com_App \ node_modules \ mongoose \ lib \ query.js：2805：28）在process._tickCallback（内部/进程/next_tick.js:61:11）在以下位置发出了“错误”事件：在C：\ Users \ Manav \ Documents \ Github \ E_com_App \ node_modules \ mongoose \ lib \ model.js：4591：13在C：\ Users \ Manav \ Documents \ Github \ E_com_App \ node_modules \ mongoose \ lib \ query.js：4323：12在process.nextTick（C：\ Users \ Manav \ Documents \ Github \ E_com_App \ node_modules \ mongoose \ lib \ query.js：2805：28）在process._tickCallback（内部/进程/next_tick.js:61:11）[nodemon]应用程序崩溃-等待文件更改，然后再开始...

请参见我的完整代码：.git

我看到您的代码，在此行Product.findById(productID,(err,product)=>{中，您从findById模块调用了product函数。但我只在此文件中看到

const mongoose = require('mongoose');
const schema = mongoose.Schema;

const productSchema = new schema({
    imagePath: {type: mongoose.Schema.Types.String, required:true},
    // imagePath2: {type: mongoose.Schema.Types.String, required:true},
    // imagePath3: {type: mongoose.Schema.Types.String, required:true},
    // imagePath4: {type: mongoose.Schema.Types.String, required:true},
    // imagePath5: {type: mongoose.Schema.Types.String, required:true},
    name: {type: mongoose.Schema.Types.String, required:true},
    desc: {type: mongoose.Schema.Types.String, required:true},
    price: {type: mongoose.Schema.Types.Number, required:true},
});

module.exports = mongoose.model("DND - Products",productSchema);

您必须在本节中定义所需的功能。例如：findById