Check Whether or not email is already registered (ignoring dots & underscore)

function isRegistered(email) {
  const registeredEmail = 'test.123@gmail.com'
  let [registeredUsername, registeredDomain] = registeredEmail.split('@')
  let [username, domain] = email.split('@')
  if (domain == registeredDomain) {
    registeredUsername = registeredUsername.replace(/\.|_/gi, '')
    username = username.replace(/\.|_/gi, '')
    return username == registeredUsername
  }
  return false
}

const email = 'tes.t123@gmail.com'
const cek = isRegistered(email)
console.log(email + (cek ? ' is' : ' isn\'t') + ' already registered')