Mask an email address

maskEmailAddress.js

function maskEmailAddress (emailAddress = '') {
  const [
    username = '',
    domainName = ''
  ] = emailAddress.split('@')

  return username.substring(0, 2) +
    '***@' +
    domainName
      .split('.')
      .map(domainNamePart => domainNamePart.substring(0, 1) + '**')
      .join('.')
}

maskEmailAddress.test.js

console.assert(maskEmailAddress('julian.casablancas@merch.strokes.com') === 'ju***@m**.s**.c**')
console.assert(maskEmailAddress('julian.casablancas@strokes.com') === 'ju***@s**.c**')
console.assert(maskEmailAddress('julian.casablancas@localhost') === 'ju***@l**')
console.assert(maskEmailAddress('') === '***@**')
console.assert(maskEmailAddress() === '***@**')

I went with the regex approach, might still be able to be simplified.

function replacer(match, username, domain) {
  username = username.replace(/(\w{2}).*/,'$1**@');
  let domainParts = domain.replace(/(.)\w*(\.?)/g, '$1**$2');
  return username + domainParts;
}

function maskEmailAddress(email) {
  if (!email) {
    return "***@***";
  }
  return email.replace(/(.*)@(.*)/, replacer);
}