Add mailchimp subscriber on Firebase user signup (firebase function)

index.js

const functions = require('firebase-functions');
const Mailchimp = require('mailchimp-api-v3');

// Listens for new firebase signups
exports.addMailchimpSubscriber = functions.auth.user().onCreate((user) => {

  const mailchimp = new Mailchimp('{API-KEY}');

  mailchimp.post('/lists/{listID}/members', { 

    email_address: user.email,
    status: 'subscribed'

  }).then(function(results) {
    console.log('added new firebase user', user.email, 'to mailchimp list');
  })
  .catch(function(err) {
    console.log(err);
  })

  return null
});

package.json

{
  "name": "functions",
  "description": "Cloud Functions for Firebase",
  "scripts": {
    "serve": "firebase serve --only functions",
    "shell": "firebase functions:shell",
    "start": "npm run shell",
    "deploy": "firebase deploy --only functions",
    "logs": "firebase functions:log"
  },
  "dependencies": {
    "firebase-admin": "~6.0.0",
    "firebase-functions": "^2.0.3",
    "mailchimp-api-v3": "^1.12.0"
  },
  "private": true
}

In case the log of the Firebase Function says:

Billing account not configured. External network is not accessible and quotas are severely limited. Configure billing account to remove these restrictions

You need to switch to the Blaze plan (pay-as-go) to make outbound network requests to work. This plan can stay free, if you do not exceed the free usage limits.

https://stackoverflow.com/questions/47622607/is-it-safe-to-use-firebase-blaze-plan

The Firebase Blaze plan is a "pay as you go" plan, as indicated on the Firebase pricing page. It is a post-billing service, meaning that you get billed for your usage at the end of the month.

During the month, you can track your usage (and thus pricing) for the service in the Firebase Console.