Push inside forEach with query not working properly

Question

I\'m working with mongodb stitch/realm and I\'m trying to modify objects inside an array with a foreach and also pushing ids into a new array.

For each object that i\'m m

Answer 1

It's an asynchronous issue. You're populating the value of the array inside a callback. But because of the nature of the event loop, it's impossible that any of the callbacks will have been called by the time the console.log is executed.

You mentioned a solution involving promises, and that's probably the right tack. For example something like the following:

exports = function(orgLoc_id, data) {
  // ...
  let stream_ids = [];
  const promises = data.map(function(stream) {
    return streamsCollection.findOne({ _id: stream.stream_id }, { type: 1, sizes: 1 })
      .then(res => { //if I comment this query it will push without any problem
        if (res) {
          let newId = new BSON.ObjectId();
          // ...
          stream_ids.push(newId);
        }
      })
  })

  Promise.all(promises).then(function() {
    console.log('stream ids: ' + stream_ids);

    //TODO
    // any code that needs access to stream_ids should be in here...
  });
};

Note the change of forEach to map...that way you're getting an array of all the Promises (I'm assuming your findOne is returning a promise because of the .then).

Then you use a Promise.all to wait for all the promises to resolve, and then you should have your array.

Side note: A more elegant solution would involve returning newId inside your .then. In that case Promise.all will actually resolve with an array of the results of all the promises, which would be the values of newId.