with-firebase-user

A higher order function that decodes a Firebase Auth JWT and decorates the NextJS API request object with a Firebase user.

Introduction

I wanted to use Firebase Auth to authenticate and secure routes in my NextJS API.

The Problem

Unable to add middleware to express middleware chain for NextJS API routes.
Don't want to add a large library for a lightweight operation to be run on a serverless function.

My Solution

Follow the Verify ID Tokens using a third party JWT library instructions from the firebase auth docs to securely verify and decode the JWT.
Write a higher order function to decorate the NextJS request object with authenticated Firebase user data.

Installation

npm i --save with-firebase-user

yarn add with-firebase-user

Usage

Pass your NextJS handler to withFirebaseUser and it will add the authenticated user to the request object user key.

import { withFirebaseUser } from 'with-firebase-user';

const handler = async (req, res) => {
  res.send(req.user); // FirebaseUser
};

export default withFirebaseUser(handler);

Make sure to add your JWT to the request headers on the client.

fetch('/api/users', {
  headers: {
    Authorization: 'Bearer <JWT>',
  },
});

Decoded user data

interface FirebaseUser {
  name?: string;
  user_id: string;
  email?: string;
  email_verified?: boolean;
}

API

const withFirebaseUser: (
  handler: (
    req: NextApiRequestWithFirebaseUser,
    res: NextApiResponse
  ) => Promise<void>,
  options?: WithFirebaseUserOptions | undefined
) => (req: NextApiRequest, res: NextApiResponse) => Promise<void>;

WithFirebaseUserOptions (passed as a second argument to withFirebaseUser function)

interface WithFirebaseUserOptions {
  clientCertUrl?: string; // defaults to url provided in Firebase auth docs
  projectId?: string; // verifies the audience and issuer of the JWT when provided
}