Express JS: Send Databse as Variable / Parameter while requiring a module - node.js

Question

Say I have the following code
const functions = require("firebase-functions");
const admin = require("firebase-admin");
const serviceAccount = require("./permissions.json");
admin.initializeApp({
credential: admin.credential.cert(serviceAccount),
databaseURL: "https://blah-blah-blah.firebaseio.com"
});
const db = admin.firestore();
app.use("/auth", require("./auth"));
Now the problem is that I have to use the cloud firestore databse functionality, which is curently stored in the variable db, in my auth.js file as well, without using admin.initializeApp a second time. Is there any way to acomplish that, like importing/exporting the db variable etc.
This is where I am using it in auth.js:
const express = require("express");
const auth = express.Router();
db.collection("data")
.doc("sample")
.get()
.then(
document => {
if (document.exists) {
return done(null, {
id: user.id,
name: user.name,
email: user.email
});
} else {
//create document
}
},
err => {
//handle error
}
);
Of course, right now db will be undefined, which is the problem I need to tackle

Answer 1

Since version 1.0.0 of the Firebase SDK for Cloud Functions you have to initialize with admin.initializeApp();, see the doc here.
Then, to interact with Firestore, you just need to use the Admin SDK, for example as follows:
admin.firestore().collection('....').get();
So, if I understand correctly your question, the following changes should do the trick:
index.js
const functions = require("firebase-functions");
const admin = require("firebase-admin");
admin.initializeApp();
//const db = admin.firestore(); not sure you need that here, it depends if you interact (or not) with Firestore in index.js
app.use("/auth", require("./auth"));
auth.js
const admin = require("firebase-admin");
const db = admin.firestore();
//....
db.collection("data")
.doc("sample")
.get()
.then(
document => {
if (document.exists) {
return done(null, {
id: user.id,
name: user.name,
email: user.email
});
} else {
//create document
}
},
err => {
//handle error
}
);
//....