How to get return values from Async/await function when fetching the data from mySQL in Nodejs

Question

I am fetching some exchange data from DB, then extracting the name of distinct exchanges and passing again into MYSQL query to fetch data from a different table.

The problem I am facing is that async await does not return the value rather just return Promise { }.

Below is the code that I am trying, wondering where I am going wrong.

//Function that fetches the exchanges from DB

const getExchange = () => {
        return new Promise((resolve, reject) => {
            db.connection.query(`
            SELECT  *
            FROM,
            (
            SELECT
                exchange,
                COUNT(pair) as noOfMarkets
                FROM ticker_data

        ) as t
            `, (err, resp) => {
                if (!err) {
                    resolve(resp)
                } else {
                    reject(err)
                }
            })
        })
    }


// push unique exchanges to an array.
const getExchangesData = async () => {
    const allExchanges = await getExchanges();

    let exchanges = []
    allExchanges.forEach(item => {
        let exchange = {
            exchange: item.exchange
        }
        exchanges.push(exchange)
    })
    return await exchanges
}


// mapping through an array of exchanges and passing to DB query to get data from the DB.

const getSingleExchange = async () => {
    const exchanges = await getExchangesData()
    await Promise.all(exchanges.map(async (item) => {
        db.connection.query(`
        SELECT
       exchange_rank,
       name
       volume24hUSD
       (
            SELECT
            volume24hUSD as tradingVolYesterday
            FROM exchanges
            WHERE name = '${item.exchange}'
            AND createdAt >= now() -interval 1 day
            AND createdAt <  now() -interval 1 day + interval 120 second
            LIMIT 1
       ) volumeDay1
FROM exchanges
WHERE name = '${item.exchange}'
        `, (err, resp) => {
            if (!err) {
                console.log(resp) // getting all the values

                let volData = {
                     name: resp[0].name,
                     exchange_rank: resp[0].exchange_rank,
                     icon: resp[0].icon
                 }
                 return volData 
            }
        })
    }))
}


const data = getSingleExchange()
console.log(data) // returning Promise { <pending> }

Edit

After making changes suggested in an answer, I still have an issue:

//Function that fetches the exchanges from DB

const getExchange = () => {
        return new Promise((resolve, reject) => {
            db.connection.query(`
            SELECT  *
            FROM,
            (
            SELECT
                exchange,
                COUNT(pair) as noOfMarkets
                FROM ticker_data

        ) as t
            `, (err, resp) => {
                if (!err) {
                    resolve(resp)
                } else {
                    reject(err)
                }
            })
        })
    }


// push unique exchanges to an array.
const getExchangesData = async () => {
    const allExchanges = await getExchanges();

    let exchanges = []
    allExchanges.forEach(item => {
        let exchange = {
            exchange: item.exchange
        }
        exchanges.push(exchange)
    })
    return await exchanges
}


    // mapping through an array of exchanges and passing to DB query to get data from the DB.

const getSingleExchange = async () => {
    const exchanges = await getExchangesData()
    await Promise.all(exchanges.map((item) => {
        return new Promise((resolve, reject) => {
            db.connection.query(`...`, (err, resp) => {
                if (!err) {
                    resolve(resp)
                } else {
                    reject(err)
                }
            }).then(resp => {
                console.log(resp)
                let volData = {
                    name: resp[0].name,
                    exchange_rank: resp[0].exchange_rank,
                    icon: resp[0].icon
                }
                return volData
            })
        })
    }))

}

getSingleExchange().then(data => {
    console.log(data)
});

I now get this error:

(node:30583) UnhandledPromiseRejectionWarning: TypeError: db.connection.query(...).then is not a function
at Promise (/getExchanges.js:217:16)
at new Promise ()
at Promise.all.exchanges.map (/getExchanges.js:145:16)
at Array.map ()
at getSingleExchange (/getExchanges.js:144:33)

trincot · Accepted Answer · 2019-06-01 12:15:33Z

3

The main issue is in this part:

await Promise.all(exchanges.map(async (item) => {

That map callback is not returning anything, and it has no await, so using async makes no sense.

Instead remove async:

await Promise.all(exchanges.map((item) => {

... and return a promise in the callback function, much like you had done in the first function:

    return new Promise((resolve, reject) => {
        db.connection.query(`...`), (err, resp) => {
            if (!err) {
                resolve(resp)
            } else {
                reject(err)
            }
        })
    }).then(resp => {
        console.log(resp)
        let volData = {
             name: resp[0].name,
             exchange_rank: resp[0].exchange_rank,
             icon: resp[0].icon
         }
         return volData 
    });

You would benefit from writing one generic function that promisifies query, so that you don't have to do that new Promise-thing for every single query you need.

Finally, you cannot expect to get an asynchronous result synchronously: async functions do not return the asynchronous result synchronously, but return a promise for it. So your last lines (main code) should still await. So either do this:

(async () => {
    const data = await getSingleExchange()
    console.log(data)
})(); // immediately executing (async) function expression

Or:

getSingleExchange().then(data => {
    console.log(data)
});

NB: doing return await exchanges in the second function makes no sense (exchanges is not a promise), so you can just do return exchanges.

edited Jun 1, 2019 at 12:15

answered Jun 1, 2019 at 11:47

trincot

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8 Comments

Tayyab Rahman Over a year ago

Thanks for replying, getting the same response Promise { <pending> }

trincot Over a year ago

That is because you do not await the final promise in the main code (last two lines). See addition to my answer.

Tayyab Rahman Over a year ago

I follow the step and changed the code. still getting the error. I have edited the code showing the error I am getting.

trincot Over a year ago

Where did you do .query(....).then as it says in the error message? That is not in my answer. Please also don't adapt your question like that. It makes my answer look irrelevant. If you want to add information to your question, that is OK, but please leave the original question in place. NB: the new code you added in your question is not like I have it in my answer. The then is at the wrong place.

Tayyab Rahman Over a year ago

I fixed the error in this line db.connection.query(...), (err, resp) => { (wrapped my db query here). now console.log() is printing all values but when use return volData. show undefined. I tried getSingleExchange().then(data => { console.log(data) }); and (async () => { const data = await getSingleExchange() console.log(data) })();

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