I use next.js on the front end, and there is an "api" folder that provides us with a node.js environment on the next.js server side.

We can have some proxy "api" between frontend and backend (cloud functions).

1. From the frontend, I send a request to next.js api
2. Send request from next/api to cloud function
3. Get the response on the front end and return it

Result: Get CORS

Next.js api proxy code:

import { getFunctions, httpsCallable } from 'firebase/functions';

import { firebase } from '@/lib';

export async function deleteUserAccount(userId: string) {
  if (!userId) {
    console.error('deleteUserAccount: no userId provided');
    return;
  }

  try {
    const functions = getFunctions(firebase, 'us-central1');
    const deleteUserData = httpsCallable(functions, 'deleteUserData');

    const { data }: any = await deleteUserData({
      userId,
    });

    console.log('deleteUserAccount', data);
  } catch (error) {
    console.error('deleteUserAccount error', error);
  }
}

This is the cloud function implementation:

const admin = require('firebase-admin');
const functions = require('firebase-functions');

exports.deleteUserData = functions.https.onRequest(async (req, res) => {
    const uid = request.auth.uid;
    
    try {
        await admin.auth().deleteUser(uid);
        return { message: 'Data deleted successfully!' };
    } catch (error) {
        console.error('Error deleting user data', error);
        return Promise.reject(error);
    }
});

How to resolve CORS securely?