auth.js

const handler = <T>(
  users: Array<T>,
  metaData: Array<MockResponse>,
  authFn: (Auth, T) => bool,
  respondFn: (T) => Object,
  req: $Request,
  res: $Response,
) => {
  const eitherAuth = validateAuth(req.body)
  if (eitherAuth instanceof Error) {
    res.send(400)
  }
  else {
    const maybeUser = users.find(authFn.bind(null, eitherAuth))
    if (!maybeUser) {
     res.send(401) // or whatever code is the right one
    } else {
      res.send(200)
    }
  }
}

// usage
app.post('/auth/v1/students', handler.bind(null, students, (auth: Auth, s: StudentAuthenticationRequest) => {
  return s.username == auth.username && s.password == auth.password
}, (s: StudentAuthenticationRequest) => ({ studentBid: s.studentBid }))
app.post('/auth/v1/test-sessions', handler.bind(null, testSessions, (auth: Auth, t: TestSessionAuthenticationRequest) => {
  return t.sessionName == auth.username && t.pin == auth.password
}, (t: TestSessionAuthenticationRequest) => ({ testSessionBid: t.testSessionBid })))