require('dotenv').config();
const mongoose = require('mongoose');
function connectDB() {
    // Database connection 🥳
    mongoose.connect(process.env.MONGO_CONNECTION_URL, 
    { useNewUrlParser: true, useCreateIndex:true, useUnifiedTopology: true, useFindAndModify : true });
    const connection = mongoose.connection;
    connection.once('open', () => {
        console.log('Database connected ');
    }).catch(err => {
        console.log('Connection failed ');
    });`

}每次我都得到catch不是connectDB错误中的函数的错误：

C:\Users\user\Desktop\mern\config\db.js:15
    }).catch(err => {
            ^

TypeError: connection.once(...).catch is not a function
    at connectDB (C:\Users\user\Desktop\mern\config\db.js:15:13)