Uncaught (in promise) SyntaxError: Unexpected end of JSON input

Question

I am trying to send a new push subscription to my server but am encountering an error "Uncaught (in promise) SyntaxError: Unexpected end of JSON input" and the console says it's in my index page at line 1, which obviously is not the case.

The function where I suspect the problem occurring (because error is not thrown when I comment it out) is sendSubscriptionToBackEnd(subscription) which is called in the following:

function updateSubscriptionOnServer(subscription) {
  const subscriptionJson = document.querySelector('.js-subscription-json');
  const subscriptionDetails = document.querySelector('.js-subscription-details');

  if (subscription) {
    subscriptionJson.textContent = JSON.stringify(subscription);
    sendSubscriptionToBackEnd(subscription);
    subscriptionDetails.classList.remove('is-invisible');
  } else {
    subscriptionDetails.classList.add('is-invisible');
  }
}

The function itself (which precedes the above function):

 function sendSubscriptionToBackEnd(subscription) {
  return fetch('/path/to/app/savesub.php', {
    method: 'POST',
    headers: {
      'Content-Type': 'application/json'
    },
    body: JSON.stringify(subscription)
  })
  .then(function(response) {
    if (!response.ok) {
      throw new Error('Bad status code from server.');
    }

    return response.json();
  })
  .then(function(responseData) {
    if (!(responseData.data && responseData.data.success)) {
      throw new Error('Bad response from server.');
    }
  });
}

I have tried replacing single quotes with double quotes in the fetch call but that yields the same results.

I know that the JSON should be populated because it prints to the screen in the updateSubscriptionOnServer() function with subscriptionJson.textContent = JSON.stringify(subscription);, and I used that output in the google codelab's example server to receive a push successfully.

EDIT: Here is the JSON as a string, but I don't see a mistake in syntax:

{"endpoint":"https://fcm.googleapis.com/fcm/send/dLmthm1wZuc:APA91bGULRezL7SzZKywF2wiS50hXNaLqjJxJ869y8wiWLA3Y_1pHqTI458VIhJZkyOsRMO2xBS77erpmKUp-Tg0sMkYHkuUJCI8wEid1jMESeO2ExjNhNC9OS1DQT2j05BaRgckFbCN","keys":{"p256dh":"BBz2c7S5uiKR-SE2fYJrjPaxuAiFiLogxsJbl8S1A_fQrOEH4_LQjp8qocIxOFEicpcf4PHZksAtA8zKJG9pMzs=","auth":"VOHh5P-1ZTupRXTMs4VhlQ=="}}

Any ideas??

Answer 1

This might be a problem with the endpoint not passing the appropriate parameters in the response's header.

In Chrome's console, inside the Network tab, check the headers sent by the endpoint and it should contain this: Example of proper response to allow requests from localhost and cross domains requests

Ask the API developer to include this in the headers:

"Access-Control-Allow-Origin" : "*", 
"Access-Control-Allow-Credentials" : true