Why is this async function returning empty on the first try, but then it returns everything?-CodePudding

I'm trying to insert data to my db, and the query is inside a map loop, so i'm using an async function to wait for the loop to end, and store all the results in my variable "practicasAgregadas".

This is how i call the function:

insertarPracticas(turno_id, req.body.lista_codigos_practicas, queryInsertarPracticas)
  .then(result => {
    res.status(200).json({
      "Practicas Agregadas": result
    })
  })

This is the function:

async function insertarPracticas(turno_id, req, queryInsertarPracticas) {
  const res = await Promise.all(req.map(r => {
    connection.query(
      queryInsertarPracticas, [turno_id, r], (error2, row2) => {
        if (error2) {
          console.log("Error al insertar turno detalle (prácticas): "   r   " "   error2);
          practicasNoAgregadas  = r   "-";
        } else {
          console.log("Turnos detalle agregados "   r);
          practicasAgregadas  = r   "-";
          console.log("practicas "   practicasAgregadas);
        }
      });
    return practicasAgregadas;
  })
  )

  console.log("en async "   res[0]);
  return res;
}

On the first try, it returns empty:

This is the console:

And on the second try, it does return, but it repeats 3 times:

And the console:

CodePudding user response：

Most modern JS libs that use async functions will return a promise, but will often not return a promise if the callback is supplied.

So assuming connection.query does return a promise, then the code below should do what your after..

async function insertarPracticas(turno_id, req, queryInsertarPracticas) {
    const res = await Promise.all(
        req.map(async (r) => {
            try {
                const row2 = await connection.query(queryInsertarPracticas, [
                    turno_id, r
                ]);
                console.log("Turnos detalle agregados "   r);
                practicasAgregadas  = r   "-";
                console.log("practicas "   practicasAgregadas);
            } catch (e) {
                console.log(
                    "Error al insertar turno detalle (prácticas): "  
                        r   " "   error2
                );
                practicasNoAgregadas  = r   "-";
            }
            return practicasAgregadas;
        })
    );

    console.log("en async "   res[0]);
    return res;
}

Remember, don't use a callback, or that could indicate to the lib that your not wanting a promise returning.

If for some reason the lib has not been updated for a long while, you will need to promisify the query..

In node, there is a nice utility called promisify https://nodejs.org/api/util.html#utilpromisifyoriginal that will make this bit easier.

eg.

const conQuery = util.promisify(connection.query);
const row2 = await conQuery(queryInsertarPracticas....

If the callback is not in the form (error, result) you would need to use a Promise constructor.. https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise/Promise , but looking at your code (error2, row2) => that shouldn't be required.