Missing headers in Fetch response

The Headers class instance that fetch returns is an iterable, as opposed to a plain object like axios returns. Some iterable's data, such as Headers or URLSearchParams, aren't viewable from the console, you have to iterate it and console.log each element, like:

fetch('http://localhost:9876/test/sample-download', {
    method: 'post',
    headers: {},
    body: {}
})
.then(response => {
  // Inspect the headers in the response
  response.headers.forEach(console.log);
  // OR you can do this
  for(let entry of response.headers.entries()) {
    console.log(entry);
  }
})
.catch(err => console.error(err));

Headers are limited for CORS requests. See https://stackoverflow.com/a/44816592/2047472

(Use access-control-expose-headers to allow exposing headers to requests from a different origin.)

To get a specific header property you can use the following:

response.headers.get(yourProperty)

Although the correct answer is the one from @AndyTheEntity for me is a little bit incomplete.

CORS requests have limitations and show only a subset of headers:

• Cache-Control
• Content-Language
• Content-Type
• Expires
• Last-Modified
• Pragma

In order to show more headers on the response, the server has to add a header to allow more extra headers.

For example, after a POST request if a new resource is created you should return a 201 CREATED response and add a Location header. If you need to accept CORS also you need to add the next headers (on the server response):

Location: $url-of-created-resource
Access-Control-Expose-Headers: Location

With this you will see on the client the header Location.

If you need to send an specific header (like a SESSION_ID), then you need to add the next header in the request:

Access-Control-Request-Headers: SESSION_ID
SESSION_ID: $current_session_id

I hope this cover all needs to handle request/response using CORS.