How to pass array instead of required string NodeJS - node.js

Question

Script logic - script receives data from Binance API > Then I have aggregation $avg to calculate the average of one asset. I will have more than one collection so I need to calculate average of every asset.
I have an array where I store collection names for MongoDB.
const symbols = ["ADABTC", "AEBTC", "AIONBTC"]
And I want to calculate average from MongoDB collection.
const collection = db.collection(symbols);
Here - symbols doesn't work for me, but if I simply add "ADABTC" then it works, but it doesn't fixes my problem since I want to use different collection names one after another one.
How I can pass an array if it's required to be a string? I need to use more than 1 collection names.
FULL CODE
const MongoClient = require('mongodb').MongoClient;
const assert = require('assert');
// Connection URL
const url = 'mongodb://username:password#serveripadress:port/dbname?retryWrites=true&w=majority';
const symbols = ["ADABTC", "AEBTC", "AIONBTC"]
// Database Name
const dbName = 'Crypto';
// Create a new MongoClient
const client = new MongoClient(url, { useUnifiedTopology: true });
// Use connect method to connect to the Server
client.connect(function(err, client) {
assert.equal(null, err);
console.log("Connected correctly to server");
const db = client.db(dbName);
simplePipeline(db, function() {
client.close();
});
});
function simplePipeline(db, callback) {
const collection = db.collection(symbols);
collection.aggregate(
[{
'$group': {
_id: null,
'Volume': {
'$avg': '$Volume'
}
}
}],
function(err, cursor) {
assert.equal(err, null);
cursor.toArray(function(err, documents) {
console.log(documents)
callback(documents);
});
}
);
}

Answer 1

It is not possible to pass an array into a function that is asking for a string. In your case what you need to do is join three collections. If you need to aggregate across multiple collections you can use the $lookup aggregation pipeline operator. You can connect using the first collection:
db.collection(symbols) => db.collection(symbols[0])
Then modify your query to join the three collections:
// Join with AEBTC collection
{
$lookup:{
from: symbols[1],
localField: //name of a value in first collection
foreignField: //name of same value in second collection
as: //what you want to call it in the second table
}
},
{ $unwind: //what you called it },
// Join with AIONBTC collection
{
$lookup:{
from: symbols[2],
localField: //name of value in joined collections 1 and 2
foreignField: //name of that value in collection 3,
as: //whatever you want to call it in the joined collection
}
},
{ $unwind: //what you called it },
// define some conditions here
{
$match {}
},
// define which fields are you want to fetch
{
$group: {
_id: null,
'Volume': {
'$avg': '$Volume'
}
}