The whatwg spec section on RequestInit dictionary has a comment that states that this value "can only be set to null".
The whatwg spec section on RequestInit dictionary has a comment that states that this value "can only be set to null". In the detailed steps section for the Request(input,init) constructor it says even more clearly: "If init's window member is present and it is not null, throw a TypeError."
See RequestInit in Fetch API The underlying object is a dictionary. This trait is here to help encode the types.