What is wrong with my AJAX code (sending Firebase token to server)?

I am a beginner. I am trying to send via AJAX from the logged in user's state the token that was generated. However, when I run the below code, I get internal server error, so something is off. What am I missing?

app.post('/auth', function(req,res,next){
  var idToken =
  admin.auth().verifyIdToken(idToken)
  .then(function(decodedToken) {
    var uid = decodedToken.uid
    console.log(uid)
    res.send(uid)
  }).catch(function(error) {
  console.log(error.message)
})
})

$("#sign-in").on('click', function(event) {
  event.preventDefault()
  var email = $("#email").val()
  var password = $("#password").val()
  firebaseAUTH.signInWithEmailAndPassword(email, password).then(function (user) {
    console.log('user has signed in with e-mail address: '+user.email+' and user ID: '+user.uid)
    //window.location = '/members-area'
    firebaseAUTH.currentUser.getToken(true).then(function(idToken) {
    $.ajax(
    {
      url: '/auth',
      type: 'POST',
      data: idToken,
      success: function (response){
      console.log('sent successfully')
      console.log(uid)}
    }
)
console.log(idToken)
// Send token to your backend via HTTPS (JWT)
}).catch(function(error) {
// Handle error
console.log(error.message)
})
  }).catch(function(error) {
    $('#errorbox').html('Error: '+error.message).css('color', 'red')
  })
 })

via huzal12