firebase.firestore() is not a function when trying to initialize Cloud Firestore

import { firebase } from '@firebase/app';
import '@firebase/firestore'

If you're using authentication as well

import '@firebase/auth';

First, make sure you have latest version of firebase:

npm install [email protected] --save

Next, add both firebase and firestore:

const firebase = require("firebase");
// Required for side-effects
require("firebase/firestore");

Initialize the firebase app:

firebase.initializeApp({
  apiKey: '### FIREBASE API KEY ###',
  authDomain: '### FIREBASE AUTH DOMAIN ###',
  projectId: '### CLOUD FIRESTORE PROJECT ID ###'
});

// Initialize Cloud Firestore through Firebase
var db = firebase.firestore();

source: https://firebase.google.com/docs/firestore/quickstart?authuser=0

I fixed it by importing multiple libraries: firebase and firebase/firestore. That's because the firebase core library does not include the firestore library.

import firebase from 'firebase/app';
import 'firebase/firestore';