emailAddress.js

const emailAddresses = [
  '[email protected]',
  '[email protected]',
  '[email protected]',
  '[email protected]',
]

const isNullOrUndefined = (data) => data === null || data === undefined;

const getEmailServiceProvider = (email) => {
  if (isNullOrUndefined(email) || email.length === 0) {
    return email;
  }
  const beginningOfEmailProvider = email.indexOf('@') + 1;
  const endOfEmailProvider = email.lastIndexOf('.');
  return email.slice(beginningOfEmailProvider, endOfEmailProvider);
}

const getEmailServiceProviders = (emailAddresses = []) => {
  if (isNullOrUndefined(emailAddresses)) {
    return null;
  }
  return emailAddresses.map(emailAddress => getEmailServiceProvider(emailAddress));
}


console.log(getEmailServiceProviders(emailAddresses))