result_type.js

type Result<E, V> = [null, V] | [E, null];

const emailList = ['[email protected]'];
function searchForEmail(email: string): Result<Error, bool> {
  if (email.indexOf('@') === -1) {
    return [new Error('this is not an email'), null];
  }

  // here we would do an actual search for the email ...
  return [null, emailList.indexOf(email) !== -1];
}

const [err, hasEmail] = searchForEmail('[email protected]');
if (err) {
  console.log(`invalid email: ${err.stack}`);
} else {
  console.log('found email address: ', hasEmail);
}