Javascript How to use Async/Await with a mysql

Question

I'm trying to query my mysql database for a value using Async/Await but my constant 'map' always returns undefined, I am not too sure why

const get_map = () =>{
    db.query('SELECT game,game2 FROM battles WHERE player1=? OR player2=?',[result.char,result.char],(err,results)=>{
        if(err){
            return(err)
        }
        if(!results[0]){
            return('No such battle?')
        }
        console.log(results[0])
        return(results[0])
    })
}
const proccessTurn = async ()=>{
    const map = await get_map()
    console.log(map)
    let game = JSON.parse(map.game)
    let game2 = JSON.parse(map.game2)
    const char = result.char
    const playerTurn = game2.turn[0]
}

The console doesn't even log

console.log(results[0])

This line so why does the await function resolve before it actually returns something? Shouldn't it wait for the returns?

Thank you for the help

Remove {...} from get_map or change it to return db.query(...) and it should work fine. — ulou
– ulou, Commented Aug 1, 2021 at 19:22
@ulou What is the problem and how does your suggestion solve it? — showdev
– showdev, Commented Aug 1, 2021 at 19:23
The problem is that he forgot about return, I think it's good enough explanation. — ulou
– ulou, Commented Aug 1, 2021 at 19:23

Guerric P · Accepted Answer · 2021-08-01 20:21:53Z

3

As the mysql library doesn't support promises, you can use the promisify util function from Node.js like this:

const { promisify } = require('util');

const query = promisify(db.query).bind(db);

const get_map = async () => {
    try {
         const result = await query('SELECT game,game2 FROM battles WHERE player1=? OR player2=?',[result.char,result.char]);
         if(!results[0]){
             return 'No such battle?';
         }
         console.log(results[0])
         return results[0];
    }
    catch(e) {
         return e;
    }
}
const proccessTurn = async ()=>{
    const map = await get_map()
    console.log(map)
    let game = JSON.parse(map.game)
    let game2 = JSON.parse(map.game2)
    const char = result.char
    const playerTurn = game2.turn[0]
}

edited Aug 1, 2021 at 20:21

answered Aug 1, 2021 at 19:26

Guerric P

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6 Comments

rauf543 Over a year ago

Hi, I tried this out but it returns TypeError: Cannot read property 'typeCast' of undefined

Guerric P Over a year ago

It was a mistake in my code I replaced .bind(db.query) with .bind(db). You should use this code rather than reinventing the wheel like the other answer suggests.

DJ Hemath Over a year ago

@GuerricP Returning a new promise is not re-inventing the wheel, I think. And by creating a new promise you can handle your biz specific edge cases in the same function.

Guerric P Over a year ago

Returning a promise is not reinventing the wheel, returning a promise with our own code is. The Node.js callback signature is a standard, Promise is a standard, that's why promisify exists.

Guerric P Over a year ago

@rauf543 historically Node.js handled asynchronism with a callback taking three parameters: error, data and next. The current trend is to use promises which highly helps write cleaner code, especially with async/await so they provided a native tool: promisify in order to facilitate the migration to the new promises pattern. It just takes a function which takes a Node.js callback as last parameter and transforms it in a function returning a Promise. Wrapping into a Promise with custom code is not advised, as this buit-in solution exists.

|

DJ Hemath · Accepted Answer · 2021-08-01 19:30:14Z

The third parameter to db.query() is a callback. If you're returning anything in callback it's actually returned to the function itself, (db.query in this case). You're not returning anything from get_map(). So return a new promise from get_map like this,

const get_map = () =>{
    return new Promise((resolve, reject) => {
        db.query('SELECT game,game2 FROM battles WHERE player1=? OR player2=?',[result.char,result.char],(err,results)=>{
            if(err){
                reject(err)
            }
            if(!results[0]){
                reject('No such battle?')
            }
            console.log(results[0])
            resolve(results[0])
        }
    });
}
const proccessTurn = async ()=>{
    const map = await get_map()
    console.log(map)
    let game = JSON.parse(map.game)
    let game2 = JSON.parse(map.game2)
    const char = result.char
    const playerTurn = game2.turn[0]
    });

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