how do i make this work with async?

app.js

app.get('/savedTracks', function (req, res) {

  var savedTracks = [];

  // get users
  User.find().limit(10).exec(function(err, users) {

    var numberOfAsyncRequests = users.length;

    // for each user, get their saved tracks and add to the array of savedTracks
    users.forEach(function(user, callback) {

      // refresh access tokens for the user
      spotifyApi.setRefreshToken(user['refresh_token']);
      spotifyApi.refreshAccessToken().then(function(data) {

        // set the access token for this user
        spotifyApi.setAccessToken(data['access_token']);
        console.log('The access token has been refreshed!');

        // now you can get the user's saved tracks
        spotifyApi.getMySavedTracks({ limit: 10 }).then(function(data) {

          // add tracks to list of savedTracks
          savedTracks = savedTracks.concat(data['items']);
          --numberOfAsyncRequests;
          if(numberOfAsyncRequests === 0){
            //savedTracks = savedTracks.map(function(track) { return track.track });
            console.log('tracks', { items: savedTracks });
            res.render('tracks', { items: savedTracks });
          }
        }, function(err) {
          console.log('Could not get saved tracks', err);
        });

      }, function(err) {
        console.log('Could not refresh access token', err);
      });

    });

    // how do i wait until the requests for mySavedTracks have returned?
    //console.log('tracks', savedTracks);
    //res.render('tracks', savedTracks);

  });

});