javascript – working with multiple promises inside loop – how to return data outside of promise?

I’m struggling to understand how I can return data from multiple promises to build up an array of data.

Is there anyway I can return the data outside of the promise to push to the data variable?

I have the following:

db_sh.find({
    selector: {sh: req.params.sh_id},
    fields: ['_id', 'sh_id', 'time'],
    sort: ['_id']
}).then(function (result) {
    let data = {};
    console.log('Found: ' + result.docs.length);
    if (result.docs.length > 0) {
        for (var i = 0; i < result.docs.length; i++) {
            let student = result.docs[i];

            Promise
                .all([getMealBooking(student._id), getStudentData(student._id)])
                .then(function(response) {
                    var meal_booking_data = response[0];
                    var student_data = response[1];
                    console.log(meal_booking_data);

                    console.log(student_data);
                })
                .catch(function (err) {
                    return res.send(false);
                });

            data[student.time] = [
                meal_booking_data,
                student_data
            ]
        }
    }
    /** Sort Data Oldest First*/
    data = Object.keys(data).sort().reduce((a, c) => (a[c] = data[c], a), {});
    console.log(data);
    res.send(data);
});

I have two promises (getMealBooking() & getStudentData()): and I am using Promise.all() to return me the results of both of these promises. I have tried to return the data but I cannot get the results to build up the data array.

Any help to be able to build up a list of all my data would be great.

>Solution :

Another Promise.all() is needed for the loop that contains the Promise.all() you’ve already figured out. It’s better to factor a little so you can see what’s happening.

function getStudentMealAndData(student) {
  return Promise
    .all([getMealBooking(student._id), getStudentData(student._id)])
    .then(function(response) {
      var meal_booking_data = response[0];
      var student_data = response[1];
      console.log(meal_booking_data);
      console.log(student_data);
      return { student, meal_booking_data, student_data };
    })
    .catch(function (err) {
      return res.send(false);
    });
}

This simplifies the then block a bit…

}).then(function (result) {
    console.log('Found: ' + result.docs.length);
    let promises = []
    for (var i = 0; i < result.docs.length; i++) {
      let student = result.docs[i];
      promises.push(getStudentMealAndData(student));
    }
    return Promise.all(promises);
}).then(results => {
  // results are an array of [{ student, meal_booking_data, student_data }, ...]
  let data = results.reduce((acc, s) => {
    acc[s.student.time] = [ s.meal_booking_data, s.student_data ];
    return acc;
  }, {});
  data = Object.keys(data).sort().reduce((a, c) => (a[c] = data[c], a), {});
    console.log(data);
    res.send(data);
});