undefined before the database is connected in nodejs and mongodb

Question

I created an app, what I want to achieve is when the app runs, get the uid from the table user and insert it into a new table msg, code like following.

conn.js

'use strict'
const { MongoClient } = require('mongodb');
const client = new MongoClient(dbUrl, { useNewUrlParser: true });
client.connect(function (err) {
    if (err) {
        console.log(err);
    } else {
        console.log('mongodb connected');
    }
});
const db = client.db(dbName);
module.exports = db;

app.js

'use strict';
const db = require('./conn');
try {
    let to = "nick";
    let from = "susan";
    let content = "test";

    let userinfo = db.collection("user").findOne({name: to});
    let uid = userinfo.uid;
    console.log(uid)
    
    db.collection("msg").insertOne({from: from, to: to})
} catch(e) {
    console.log(e)
}

But it runs, the displayed order of the printed log is

undefined
mongodb connected

uid is undefined, because database isn't connected yet, how to fix this? Thank you.

Answer 1

You need to make sure your db queries are async in nature.

If you are using async/await, you can do

let userinfo = await db.collection("user").findOne({name: to});
let uid = userinfo.uid;
console.log(uid);
    
await db.collection("msg").insertOne({from: from, to: to});

You can also use then/catch as well

db.collection("user").findOne({name: to}).then(userinfo => {
 let uid = userinfo.uid;
}