Using mergeMap for parallel asynchronous calls

index.ts

const EMAIL_PARALLELISM = 5;

const users$ = getUsers$();
users$.pipe(
  filter(user => user.isActive),
  mergeMap(user => sendEmail$(user.email), EMAIL_PARALLELISM)
).subscribe(
  (success) => emailsSent++,
  (err) => failedEmails++,
  () = > console.log('Emails sent:'+emailsSent+' Emails failed:'+failedEmails)
)