either-monad-solution.js

const validateLogin = auth => {
  const checkUsername = auth => auth.username
    ? Right(auth)
    : Left('No Username')
  const checkPassword = auth => auth.password
    ? Right(auth)
    : Left('Password Empty')
  const checkMatch = auth => (auth.username === 'jihad' && auth.password === 'jihad123')
    ? Right(auth.username)
    : Left('Username && password mismatch')
  
  const formatError = e => `error: ${e}`
  const successfulMsg = username => `Welcome back, ${username}!`

  return Either.of(auth)
    .bind(checkUsername)
    .bind(checkPassword)
    .bind(checkMatch)
    .cata(formatError, successfulMsg)
}

console.log(validateLogin(auth)())