how to run multiple firebase promises and then once completed, execute function - node.js

Question

I need to execute 2 firebase calls to retrieve specific data from the database. Once these promises resolve, I want to call another function with the data retrieved. How can I do this? ..something with Promise.All?
Code below:
app.post('/testtwilio', function(req, res) {
//save request variables
var to_UID = req.body.to;
var from_UID = req.body.from;
var experience_id = req.body.exp_id;
//Query firebase and save 'zone_id' which we need later
firebase.database().ref('experiences').child(experience_id).once('value').then((snap) => {
zone_id = snap.val().ZoneID;
});
//Query firebase and save 'from_name' which we need later
firebase.database().ref('users').child(from_UID).once('value').then((snap) => {
from_name = snap.val().Name;
});
//Once we have the two variables returned and saved
//Call a final firebase query and a twilio function with all the recieved data
firebase.database().ref('users').child(to_UID).once('value').then((snap) => {
//Do something with this aggregated data now
client.messages.create({
//blah blah do something with the saved data that we retrieved
var phone = snap.val().Phone;
var msg = from_name + zone_id + from_UID + experience_id
});
});
});

Answer 1

Yes, you can use Promise.all since once('value') returns one.
Quick n dirty example:
var promises = [];
promises.push(firebase.database().ref('experiences').child(experience_id).once('value'));
promises.push(firebase.database().ref('users').child(from_UID).once('value'));
// Wait for all promises to resolve
Promise.all(promises).then(function(res) {
// res[0] is your experience_id snapshot
// res[1] is your from_UID snapshot
// Do something...
});

Answer 2

If you are using NodeJS of version 7.6 and higher you can also write this code with async function, which much simpler to read and maintain
// ...
const wrap = require('express-async-wrap')
// ...
// need to wrap async function
// to make it compatible with express
app.post('/testtwilio', wrap(async (req, res) => {
const to_UID = req.body.to
const from_UID = req.body.from
const experience_id = req.body.exp_id
const [
snap1,
snap2,
snap3
// waiting for all 3 promises
] = await Promise.all([
firebase.database().ref('experiences').child(experience_id).once('value'),
firebase.database().ref('users').child(from_UID).once('value'),
firebase.database().ref('users').child(to_UID).once('value')
])
const zone_id = snap1.val().ZoneID
const from_name = snap2.val().Name
const phone = snap3.val().Phone
const msg = from_name + zone_id + from_UID + experience_id
// ...
client.messages.create(...)
}))