在使用nodejs的文件共享系统中得到错误，表达mongoose

Question

这是我的nodejs代码

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.');
})
}
module.exports = connectDB;

当我试图连接服务器时显示此错误

}).catch(err => {
^
TypeError: connection.once(...).catch is not a function

Answer 1

是否返回一个promise?

db.once('open', function(err, resp){
console.log(resp);
});

或

mongoose.connection
.once('open', function () {
console.log('MongoDB running');
})
.on('error', function (err) {
console.log(err);
});

Answer 2

这意味着once函数不返回一个对象首先承诺。

你可以使用catch和mongoose.connect来处理错误，如果有的话，检查下面的例子:

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

}