Add Authorization header with firebase user token to every request made with axios across the application

controller.ts

/*
* Backend implementation: express middleware which checks the firebase user token and execute the request only when the token is verified
*/
import * as express from "express";
import * as admin from "firebase-admin";
import * as functions from "firebase-functions";

admin.initializeApp({
  credential: admin.credential.applicationDefault(),
});
  
const app = express();

app.use('/', (req, res, next) => {
  const headerToken = req.header('Authorization');
  if (!headerToken) {
    res.status(401).send({message: 'Auth token must be provided'});
    return;
  }
  const headerParts = headerToken.split(" ");
  if (headerParts.length < 2 || headerParts[0] !== 'Bearer' || !headerParts[1]) {
    res.status(401).send({message: 'Invalid token'});
    return;
  }

  admin
    .auth()
    .verifyIdToken(headerParts[1])
    .then((decodedToken) => {
      next();
    })
    .catch((error) => {
      res.status(403).send({message: error.message}).end();
    });
});

app.get("/", async (req, res) => {
  res.status(200).send('Request executed successfully').end();
});

export const users = functions.region("europe-west6").https.onRequest(app);

fetch.ts

/*
* How to use the fetchClient in the frontend
*/
import fetchClient from "./fetchClient";

fetchClient.get('https://url.api.com/users').then(res => {
  console.log(res);
});

fetchClient.patch('https://url.api.com/users/{uid}', payload).then(res => {
  // ...
});

fetchClient.ts

/*
* Frontend implementation of a custom axios instance
*/
import axios from 'axios';
import firebase from 'firebase/app';
import "firebase/auth";

const fetchClient = (() => {
    const getAuthToken = async () => {
        try {
            return "Bearer " + await firebase.auth().currentUser?.getIdToken();
        } catch(err) {
            console.log("getAuthToken", err);
        };
    };

    const instance = axios.create({
        baseURL: 'https://url.api.com'
    });

    instance.interceptors.request.use(async (config) => {
        config.headers.Authorization = await getAuthToken();
        return config;
    });

    return instance;
})();

export default fetchClient;