Progressive Web Application Development by Example

Protected Headers

Headers have a guard property. This flag indicates if headers can be manipulated. If you try to manipulate a header whose guard is set to immutable, an exception is thrown.

These are the possible guard states:

• none: Default
• request: Guard for a Headers object obtained from a Request (Request.headers)
• request-no-cors: Guard for a Headers object obtained from a Request that's been created with mode no-cors
• response: Naturally, for Headers obtained from Response (Response.headers)
• immutable: Mostly used for ServiceWorkers; renders a Headers object

The actual rules controlling how different headers can be manipulated is very detailed. If you want to know more about these details, I recommend reading the Fetch specification ...