API does not wait for await - node.js

Question

Hello I am having the following problem with my API.
I am calling a function in my models from my service like so,
async getAccounts(/*offset = 0, limit = 500,*/ res) {
try {
let offset = 0;
let limit = 500;
let result = await this.model.getAllAccounts(offset, limit);
console.log(result, "Debugger 3");
return res.status(200).json(result);
} catch (error) {
logger.error('AccountsService::getAccounts', 39);
logger.error(error);
return errorHandler(error, res);
}
}
And the function in my models class looks something like this.
async getAllAccounts(offset, limit) {
// Need to add offset and limit parameters //done
console.log("URL",url);
MongoClient.connect(url, function(err, db) {
if (err) throw err;
logger.log('Query Accounts');
const dbo = db.db("DB_NAME");
dbo.collection('accounts').find({}).skip(offset).limit(limit).toArray(function(err, result) {
if (err) throw err;
db.close();
logger.log("Debugger 2xxx: ",result);
return result;
});
});
}
The problem I am having is that before Debugger 3's output comes before Debugger 2's output.
I think I need to await the result being returned from MongoDB but not sure how to do it. Could anyone please guide me?

Answer 1

await needs a promise for async to work. Currently, since your function returns undefined (as all functions without an explicit return), await is getting a promise of undefined, which is immediately resolved, and no waiting is necessary. On the other hand, the dbo uses a callback, not a promise; you cannot return a result from an asynchronous callback.
You should do something like this (not tested though):
async getAllAccounts(offset, limit) {
// Need to add offset and limit parameters //done
console.log("URL",url);
return new Promise(function(resolve, reject) {
MongoClient.connect(url, function(err, db) {
if (err) throw err;
logger.log('Query Accounts');
const dbo = db.db("DB_NAME");
dbo.collection('accounts').find({}).skip(offset).limit(limit).toArray(function(err, result) {
if (err) throw err;
db.close();
logger.log("Debugger 2xxx: ",result);
resolve(result);
});
});
});
}

Answer 2

If you're using the mongo db driver following versions 2.x, ie all version 2.x, 3.x, MongoClient.connect return a promise if no callback is passed as argument.
So you can do the next thing :
async getAllAccounts(offset, limit) {
// Need to add offset and limit parameters //done
try {
console.log("URL", url);
let db = await MongoClient.connect(url);
logger.log("Query Accounts");
const dbo = db.db("DB_NAME");
const result = await dbo
.collection("accounts")
.find({})
.skip(offset)
.limit(limit)
.toArray();
db.close();
logger.log("Debugger 2xxx: ", result);
return result;
} catch (e) {
throw e;
}
}

Answer 3

The reason why the getAllAccounts function call didn't await. Await will only work for a function that returns a promise. Here getAllAccounts doesn't return a promise, so wrap the method with a promise.
modul.exports.getAllAccounts = (offset, limit)=> {
// Need to add offset and limit parameters //done
return new Promise((resolve,reject)=>{
console.log("URL",url);
MongoClient.connect(url, function(err, db) {
if (err){
reject(err);
return;
}
logger.log('Query Accounts');
const dbo = db.db("DB_NAME");
dbo.collection('accounts').find({}).skip(offset).limit(limit).toArray(function(err, result) {
if (err){
reject(err);
return;
}
db.close();
logger.log("Debugger 2xxx: ",result);
resolve(result);
});
});
});
}