There is a restriction to access response headers when you are using Fetch API over CORS. Due to this restriction, you can access only following standard headers:
Cache-Control
Content-Language
Content-Type
Expires
Last-Modified
Pragma
When you are writing code for Google Chrome extension, you are using CORS, hence you can’t access all headers. If you control the server, you can return custom information in the response body
instead of headers
More info on this restriction – https://developers.google.com/web/updates/2015/03/introduction-to-fetch#response_types