I am posting a data to mongoosedb, I check my code seems no bug or typo, but when I send data through app.post() , it return 400, I check the data can connect to post app.post(), because I change the return error 400 to 401, the browser say the error is 401.

return error set to 400 POST http://localhost:3000/api/products 400 (Bad Request)

return error set to 401 POST http://localhost:3000/api/products 401 (Unauthorized)

I sure that the db has been connected, because I set something like this

mongoose.connect('<MY key for mongodb>')
  .then(() => {
    console.log('Successfully connected to MongoDB Atlas!');
  })
  .catch((error) => {
    console.log('Unable to connect to MongoDB Atlas!');
    console.error(error);
});

It return Successfully connected to MongoDB Atlas! when I connect.

what will be the bug?

app.js

const mongoose = require('mongoose');

const Product = require('./models/product');

const app = express();

app.use((req, res, next) => {
    res.setHeader('Access-Control-Allow-Origin', '*');
    res.setHeader('Access-Control-Allow-Headers', 'Origin, X-Requested-With, Content, Accept, Content-Type, Authorization');
    res.setHeader('Access-Control-Allow-Methods', 'GET, POST, PUT, DELETE, PATCH, OPTIONS');
    next();
});

app.use(express.json());

app.post('/api/products', (req, res, next) => {
    const product = new Product({
        name: req.body.name,
        description: req.body.description,
        price: req.body.price,
        inStock: req.body.inStock,
        userId: req.body.userId
    });
    product.save().then(
        () => {
          res.status(201).json({
            message: 'Post saved successfully!'
          });
        }
      ).catch(
        (error) => {
          res.status(400).json({
            error: error
          });
        }
      );
});

product.js

const mongoose = require('mongoose');

const productSchema = mongoose.Schema({
  name: { type: String, required: true },
  description: { type: String, required: true },
  price: { type: Number, required: true },
  userId: { type: String, required: true },
  inStock: { type: Boolean, required: true }
});

module.exports = mongoose.model('Product', productSchema);

that the code for requesting data from db

app.use('/api/stuff', (req, res, next) => {
  Thing.find().then(
    (things) => {
      res.status(200).json(things);
    }
  ).catch(
    (error) => {
      res.status(400).json({
        error: error
      });
    }
  );
});