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Question

I have a code block where I want to concatenate results of two database queries. So I tried implementing Promises.all

 const promise_list = []
let appData = [];
let promise = new Promise((resolve, reject) => {
    let database = new Database();
    database.query(`select * from configuration where  version = (select max(version) from configuration) OR version= ? ORDER BY version `, [req.body.app_version])
        .then(rows => {
            appData=rows[0];
            database.close()
            resolve()
        }, err => {
            return database.close().then(() => { throw err; })
        })
        .catch(err => {
            console.log(err);
            res.status(500).json("Database Error");
            reject()
        })
});
promise_list.push(promise)
let promise2 = new Promise((resolve, reject) => {
    let database = new Database();
    database.query(`select points from users where id=?`, [req.user.id])
        .then(rows => {
            appData.points=rows[0]['points'];
            database.close()
            resolve()
        }, err => {
            return database.close().then(() => { throw err; })
        })
        .catch(err => {
            console.log(err);
            res.status(500).json("Database Error");
            reject()
        })
});
promise_list.push(promise2)
Promise.all(promise_list).then(result => {
    res.status(200).json(appData);
});

The second query works sometimes and sometimes it doesnt. What could be the issue?

Answer 1

appData.points=rows[0]['points']; only works if appData has been initialized by the other promise first. But with Promise.all , either promise can be resolved first. If the first promise resolve second, it will simply override whatever value appData currently has.

It looks like you are using promises incorrectly. Instead of using them with side-effects (assigning to appData ), you should resolve them properly.

The whole code can be cleaned up a lot to something like this:

let database = new Database();
let promise = database.query(`select * from configuration where  version = (select max(version) from configuration) OR version= ? ORDER BY version `, [req.body.app_version])
  .then(rows => rows[0]);
let promise2 = database.query(`select points from users where id=?`, [req.user.id])
  .then(rows => rows[0].points);

Promise.all([promise, promise2])
  .then(
    ([appData, points]) => {
      appData.points = points;
      res.status(200).json(appData);
    },
    err => {
      console.log(err);
      database.close().then(() => {
        res.status(500).json("Database Error");
      });
    }
  );

Don't know what Database does, so it's not clear whether it's OK to only call it once. But it should you a better idea for how to use promises.