Chained async calls returning undefined response - node.js

Question

I seem to be getting undefined from my async code, and none of the answers I have found seem to work.
So my setup is router calls a controller, which calls a service, which calls a database. The controller is like so (removed some validation etc to reduce code)
const { userService } = require('../services/index.services');
const postUser = async (req, res) => {
try {
const user = {
user_id: req.body.userId,
user_status: req.body.userStatus,
created_at: new Date(),
updated_at: new Date(),
};
const result = await userService.createUser(user);
console.log(result);
} catch (err) {
return apiResponse.ErrorResponse(res, err);
}
};
module.exports = {
postUser,
};
You can see I have a console log of the result. Then my service class is like so
const { addUserToDb } = require('../database/users.db');
const createUser = async (user) => {
try {
const createdUser = await addUserToDb(user);
console.log(createdUser);
} catch (err) {
throw new Error(err);
}
};
module.exports = {
createUser,
};
That one also has a console log. Then finally users.db
const pool = require('./database');
const addUserToDb = (user) => {
pool.getConnection((err, connection) => {
if (err) {
throw new Error(err);
}
connection.query('INSERT INTO `users` SET ?', user, (err) => {
if (err) { throw new Error(err); }
pool.releaseConnection(connection);
return user;
});
});
};
module.exports = {
addUserToDb,
};
So you can see this adds the user to the database and returns the user. So my user is successfully being inserted into the database.
My question is why am I getting undefined in both my console logs? I am returning from all modules which is where I noticed a lot of other people had issues because of no returns.
So can someone explain why these may be undefined?
Thanks

Answer 1

It's because in your service class you have,
const createdUser = await addUserToDb(user);
But addUserToDb in users.db doesn't return a promise.
addUserToDb is a regular function that executes the query using callbacks.
Hence, even though you have return user; nested within the 2nd callback, it never returns the user value back to createUser in the service class as callbacks are asynchronous.
In fact, addUserToDb function in users.db doesn't wait for the callback to complete and thus returns undefined back to the service class.
And hence you see undefined being logged in your service class.
async/await only works with promises, not callbacks.
You need to refactor ./database module to use promises instead of callbacks for pool.getConnection and connection.query.
I use mysql2 npm module as it has a promise wrapper,
https://www.npmjs.com/package/mysql2#using-promise-wrapper
And the reason why console.log(result); in your controller is undefined is because you don't return anything from your service class.
You need to add return createdUser; in your service class's createUser method.