mongodb method without function not working in nodejs - node.js

Question

I don't know much Javascript and was making a nodejs app. My mongodb query in nodejs is working only when the query has a function method like .toArray
Here's the database.js file
const {MongoClient} = require('mongodb');
const uri = "mongodb+srv://name:pass#clusterurl/metro4?retryWrites=true&w=majority";
// all fields are correctly filled
const client = new MongoClient(uri);
try {
// Connect to the MongoDB cluster
client.connect(err =>{
if(err) throw err;
let db = client.db('metro4');
db.collection('Station').find().toArray(function(err, result){
if(err) throw err;
console.log(result);
});
let a = db.collection('Station').findOne({'_id':4});
if(a) {
console.log(a);
}
else{
console.log("No a\n");
}
module.exports = db;
});
} catch (e) {
console.error(e);
} finally {
client.close();
}
when I run the app, the db.collection('Station').find().toArray runs fine and output the result but the second query of findOne doesn't work.
Any help is appreciated.

Answer 1

The findOne method returns a Promise. You should handle its result in a callback function:
db.collection('Station').findOne({ _id: 4 }, function (err, a) {
if (err) {
console.log(err);
} else if (a) {
console.log(a);
} else {
console.log('No a\n');
}
});
Or using async - await:
client.connect(async (err) => {
...
let a = await db.collection('Station').findOne({ _id: 4 })
...
});
EDIT
To handle the import - export problem you should handle the datase connection operations separate async functions.
You may use the connection function to return the database instance:
const {MongoClient} = require('mongodb');
const uri = "mongodb+srv://name:pass#clusterurl/metro4?retryWrites=true&w=majority";
// all fields are correctly filled
const client = new MongoClient(uri);
const connectDB = async () => {
try {
// Connect to the MongoDB cluster
await client.connect();
return client.db('metro4');
} catch (e) {
throw e;
}
}
const disconnectDB = () => {
client.close();
}
module.exports = { connectDB, disconnectDB };
Then use these functions to handle your database related operations:
const { connectDB, disconnectDB } = require('../database');
const getStations = async () => {
const db = connectDB();
if (!db) return;
try {
const data = await db.collection('Station').find().toArray();
return data;
} catch (err) {
throw err;
} finally {
disconnectDB();
}
}
const getStation = async (id) => {
const db = connectDB();
if (!db) return;
try {
const data = await db.collection('Station').findOne({ _id: id});
return data;
} catch (err) {
throw err;
} finally {
disconnectDB();
}
}