Generate Google access token for service-account in Cloudflare workers environment

index.js

/**
You can use the code below in Cloudfalre worker environment to generate a Google access token 
for a service-account. 
Run this code in a worker cron so you always have a fresh token in your KV that can be used by other workers. 
The generated token here will be valid for 3600 seconds, but you can change that in your code.
The code assumes that you have a KV mapped to 'PROPERTIES' in you wrangler.toml file

You must create a service-user and assign to it the roles that you need in google console. 
Then take the JSON that you get and paste it below

This code also assumes that you have your private key mapped to a variable `PRIVATE_KEY`
Note that currently `wrangler secret put PRIVATE_KEY` will not be able to save private keys because they are too long.
Instead, use Cloudflare dashboard UI to save this as an environemnt variable for your worker
**/

import jwt from 'jsonwebtoken';
/* global PRIVATE_KEY, PROPERTIES */
const ServiceAccountJson = {
  "type": "service_account",
  "project_id": "project-id",
  "private_key_id": "92c482736826384a88",
  "private_key": PRIVATE_KEY,
  "client_email": "service-account-user@project-id.iam.gserviceaccount.com",
  "client_id": "87263847628736478",
  "auth_uri": "https://accounts.google.com/o/oauth2/auth",
  "token_uri": "https://oauth2.googleapis.com/token",
  "auth_provider_x509_cert_url": "https://www.googleapis.com/oauth2/v1/certs",
  "client_x509_cert_url": "https://www.googleapis.com/robot/v1/metadata/x509/service-account-user%40project-id.iam.gserviceaccount.com"
}

addEventListener('scheduled', event => {
  event.waitUntil(handleScheduled())
})

//store it under whatever key that you want.
//Use the permission scope that you need for your access token
async function handleScheduled() {
  await generateKey(ServiceAccountJson, 'GOOGLE_FIRESTORE_ACCESS_TOKEN', 'https://firestore.googleapis.com/')
}



async function generateKey(serviceAccount, property, aud) {
  const now = new Date()
  const NOW = Math.floor( now.getTime() / 1000);
  const token = jwt.sign({
      iss: serviceAccount.client_email,
      sub: serviceAccount.client_email,
      aud,
      iat: NOW,
      exp: NOW + 3600
    },
    serviceAccount.private_key,
    {
      algorithm: 'RS256',
      keyid: serviceAccount.private_key_id
    });
  //store token in KV
  await PROPERTIES.put(property, token)
  console.log('generated token', property)
}