Setting Custom User Claims in Firebase

firestore.rules

rules_version = '2';
service cloud.firestore {
  match /databases/{database}/documents {
   match /AdminStuff/{StuffId} {
     allow read, write: if isAdmin();
   }
    
    function isAdmin() {
      return request.auth.token['isAdmin'] == true;
    } 
  }
}

index.ts

// Firebase Functions

import * as admin from 'firebase-admin';
import * as functions from 'firebase-functions';

export const UserUpdated = functions.firestore.document('Users/{UserId}').onUpdate(async (update, context) => {
  const userAfter = update.after.data();

  // Let's assume the user document has a field called isAdmin that is a boolean
  if(userAfter.isAdmin === true) {
    await admin.auth().setCustomUserClaims(context.params.UserId, {admin: true});
  } else {
    await admin.auth().setCustomUserClaims(context.params.UserId, {admin: false});
  }

 // do whatever else

  return Promise.resolve(null);
});