下面是我的完整app.js代码。

//jshint esversion:6 

const express = require("express");
const bodyParser = require("body-parser");
const mongoose = require("mongoose");
const _ = require("lodash");

const app = express();

app.set('view engine', 'ejs');

app.use(bodyParser.urlencoded({extended: true}));
app.use(express.static("public"));

mongoose.connect("mongodb://localhost:27017/todolistDB", {useNewUrlParser: true});

const itemsSchema = {
  name: String
};

const Item = mongoose.model("Item", itemsSchema);

const item1 = new Item({
  name: "Welcome to your todolist!"
});

const item2 = new Item({
  name: "Hit the + button to add a new item."
});

const item3 = new Item({
  name: "<-- Hit this to delete an item."
});

const defaultItems = [item1, item2, item3];

const listSchema = {
  name: String,
  items: [itemsSchema]
};

const List = mongoose.model("List", listSchema);

app.get("/", function(req, res) {
  Item.find({}, function(err, foundItems){

      if (foundItems.length === 0) {
        Item.insertMany(defaultItems, function(err){
          if (err) {
            console.log(err);
          } else {
            console.log("Successfully savevd default items to DB.");
          }
        });
        res.redirect("/");
      } else {
        res.render("list", {listTitle: "Today", newListItems: foundItems});
      }
  });
});

app.get("/:customListName", function(req, res){
  const customListName = _.capitalize(req.params.customListName);

  List.findOne({name: customListName}, function(err, foundList){
    if (!err){
      if (!foundList){
        //Create a new list
        const list = new List({
          name: customListName,
          items: defaultItems
        });
      
        list.save();
        res.redirect("/" + customListName);
      } else {
      //Show an existing list
        res.render("list", {listTitle: foundList.name, newListItems: foundList.items});
      }
    }
  });
});

app.post("/", function(req, res){
  const itemName = req.body.newItem;
  const listName = req.body.list;

  const item = new Item({
    name: itemName
  });

  if (listName === "Today"){
    item.save();
    res.redirect("/");
  } else {
    List.findOne({name: listName}, function(err, foundList){
      foundList.items.push(item);
      foundList.save();
      res.redirect("/" + listName);
    });
  }
});

app.post("/delete", function(req, res){
  const checkedItemId = req.body.checkbox;
  const listName = req.body.listName;

  if (listName === "Today") {
    Item.findByIdAndRemove(checkedItemId, function(err){
      if (!err) {
        console.log("Successfully deleted checked item.");
        res.redirect("/");
      }
    });
  } else {
    List.findOneAndUpdate({name: listName}, {$pull: {items: {_id: checkedItemId}}}, function(err, foundList){
      if (!err){
        res.redirect("/" + listName);
      }
    });
  }
});

app.get("/about", function(req, res){
  res.render("about");
});

app.listen(3000, function() {
  console.log("Server started on port 3000");
});

无法理解错误在哪里！
错误为：

Model.find() no longer accept a call back.