Add autocomplete to XFrameOptionsOptions

[louy2]

Changes the XFrameOptionsOptions to let typescript suggest "DENY" and
"SAMEORIGIN" in autocomplete with the trick in
microsoft/TypeScript#29729

[EvanHahn]

Thanks for the pull request. It might be a few weeks before I get to it, but it's on my list.

[EvanHahn]

Thanks for your work here.

I know that some people have these options as configuration files, so they want to pass strings and not more specific types. Does this still work if you pass a variable of type string, or does it now have to be one of those two options?

Also, if we do this, I think it's worth adding some kind of explainer comment.

(I'm away from my computer at the moment, otherwise I'd test this myself!)

[louy2]

Does this still work if you pass a variable of type string

Yes, at type check this is equivalent to just string. The (string & {}) is to prevent the type checker from collapsing the outer union into just string, therefore losing the literals and disabling the autocomplete. But because string is a subtype of {} (which is the type of all non-nullish values), the intersection is equivalent to string, and thus the whole union is equivalent to string.

[louy2]

This is also equivalent to LiteralUnion in sindresorhus/type-fest, but I was not sure about pulling in a whole dependency just for this. If you find other parts of type-fest useful, maybe that's the way to go.

I agree about adding comments, but not sure how detailed it should be.

[louy2]

There is a rule in typescript-eslint that bans {} due to the confusion it may cause when not used within an intersection, so type-fest uses a longer form to work around that.

[EvanHahn]

Thanks for the detailed answer. I'll take another look in the next few weeks.

[EvanHahn]

I haven't forgotten about this but haven't had time to look at it yet. It's still on my list!

[EvanHahn]

I made a couple of small changes and merged this in 2f13523.

As before, let me know if you'd like to be added to the contributors page.