Node.js+MongoDB开发CRUD程序遇未知编译错误，无具体错误信息求助

Let's break down the issues in your code and fix them one by one. The error stack points directly to line 13 of your Node.js file, and there are a couple of other potential pitfalls we'll address to prevent future problems.

1. Direct Cause of the Runtime Error: Wrong View Engine Setup #

You used app.use() to set the view engine, but Express requires app.set() for configuration settings like view engines. app.use() is meant for mounting middleware, not setting app-wide configurations. This mismatch is the immediate reason for your runtime error.

Fix: #

Replace this line:

app.use("view engine","ejs");

With:

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

2. Mongoose Model Definition Issue #

You're defining the Mongoose model inside the POST route handler (/addbook), which means every time a request hits this route, you're trying to re-register the same model with Mongoose. This will cause unexpected errors after the first request, as Mongoose doesn't allow duplicate model registrations.

Fix: #

Move the model definition to the global scope, right after you define the schema:

var booksSchema = new mongoose.Schema({
    title: String,
    author: String
});

// Define model once, outside routes
var Book = mongoose.model("Book", booksSchema);

Then update your POST route to use this pre-defined model:

app.post("/addbook",function(req,res){
    var title2 = req.body.title1;
    var author2 = req.body.author1;
    Book.create({title:title2,author:author2},function(err,newBook){
        if(err){
            console.log(err);
            res.status(500).send("Failed to create book");
        } else{
            res.send(newBook);
        }
    });
});

3. Add Mongoose Connection Error Handling #

Your current MongoDB connection code doesn't include error handling, so if MongoDB isn't running or the connection fails, you won't get clear feedback. This makes it hard to debug connection-related issues.

Fix: #

Add a promise-based error handler to your connection code:

mongoose.connect("mongodb://localhost:27017/practiceofmn")
.then(() => console.log("Connected to MongoDB successfully"))
.catch(err => console.error("MongoDB connection error:", err));

Corrected Full Node.js Code #

Here's the complete, fixed version of your server code:

var express = require("express"),
    app = express(),
    bodyParser = require("body-parser"),
    mongoose = require("mongoose");

// MongoDB connection with error handling
mongoose.connect("mongodb://localhost:27017/practiceofmn")
.then(() => console.log("Connected to MongoDB successfully"))
.catch(err => console.error("MongoDB connection error:", err));

app.use(bodyParser.urlencoded({extended:true}));
// Correct view engine setup
app.set("view engine", "ejs");

// Schema and Model definition (global scope)
var booksSchema = new mongoose.Schema({
    title: String,
    author: String
});
var Book = mongoose.model("Book", booksSchema);

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

app.post("/addbook",function(req,res){
    var title2 = req.body.title1;
    var author2 = req.body.author1;
    Book.create({title:title2,author:author2},function(err,newBook){
        if(err){
            console.log(err);
            res.status(500).send("Failed to create book entry");
        } else{
            res.send(newBook);
        }
    });
});

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

Notes on Your EJS View #

Your book.ejs code is actually correct - no changes needed there. Just make sure you have the ejs package installed (run npm install ejs if you haven't already).

Steps to Test the Fix #

1. Make sure MongoDB is running locally on port 27017.
2. Install any missing dependencies: npm install express body-parser mongoose ejs.
3. Replace your server code with the corrected version above.
4. Run the server: node practiceofmongonode.js.
5. Visit http://localhost:3000 in your browser, submit the form, and you should see the created book object returned.

内容的提问来源于stack exchange，提问作者user12142401