[Vuejs]-Trying to import firebase config into my vue file

In below line, you have not declared variable app, so it’s undefined

let db = app.database()

You can change firebase.js to

import firebase from 'firebase'
import config from './firebaseConfig'

const app = firebase.initializeApp(config)
let db = app.database()

export default db

and from your component

import db from './firebase.js'
let booksRef = db.ref('books')
...