// call unsubBook() to stop observing
const unsubBook = onSnapshot(bookRef, 
	// snapshot callback re-run with every change
	(snapshot) => {
		// this is one doc
		console.log(snapshot)

		// this is doc data and id
		console.log(snapshot.data(), snapshot.id)

		// this use the metadata to detect from where doc came
		// { fromCach: true, hasPendingWrite: true }
		console.log(snapshot.metadata)
	},
	// optional error handler
	(errr) => console.error(errr.message)
)

const unsubBooks = onSnapshot(booksRef,
	(snapshot) => {
		// this is an array of docs
		console.log(snapshot.docs)

		// you can itrate throgh and map waht you need
		console.log(snapshot.docs.map(d => d.data()))
    
		// document change types
		console.log(snapshot.docChanges[0])
		// { type: 'added', doc: {}, oldIndex: -1, newIndex: 1 }

		// create object indexed by changes
		const change = snapshot.docChanges.reduce((acc, curr) => { 
			acc[curr.type] = { ...curr.doc.data(), curr.doc.id }
		}, { added: [], modified: [], removed: [] })
		// { added: [{}, {}, ...], modified: [], removed: [] }
	},
	(err) => console.error(err.message)
)