[breaking] remove createIndexFiles option, derive from trailingSlash instead

[Rich-Harris]

createIndexFiles has been removed — it is now controlled by the trailingSlash option. See https://kit.svelte.dev/docs/configuration#trailingslash for more details

Original PR message:

Fixes #3799.

One thing I've noticed about this option is that people (including myself) get confused talking about it because the default is true, and it feels unusual to have to explicitly set an option to false. Two possible solutions:

1. we come up with a better name for it that means the opposite, allowing a default of false
2. we change the default to false, and make about/index.html instead of about.html opt-in

Might be worth doing a quick survey of popular static file hosts and seeing if about.html will work as expected in the majority of cases.

Also, it might make sense to vary the default based on the value of trailingSlash or the host being used, which we could do on a per-adapter basis, but not globally.

• update docs

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[benmccann]

This seems like a good idea to me. It'd be nice to do it automatically for adapter-netlify (I guess based on the value of trailingSlash?) and some of the others

[Rich-Harris]

Via https://www.zachleat.com/web/trailing-slash/, I found https://github.com/slorber/trailing-slash-guide. My takeaway is that we should indeed vary it based on trailingSlash (which effectively means defaulting to false).

I even wonder if it's worth exposing the option at all. I suspect there aren't really any cases where you'd need to control createIndexFiles independently of trailingSlash, and on the basis that APIs are easy to add and hard to remove, we should make the breaking change now and see if people complain.

[benmccann]

I suspect there aren't really any cases where you'd need to control createIndexFiles independently of trailingSlash,

If you just drop the files in Apache or whatever you will need createIndexFiles: true. Does that mean users will also need trailingSlash: true in that case? That could be reasonable, but I feel like it'd need a screaming warning at the top of adapter-static or something because people wouldn't expect to need to look at the docs for that option most of the time and it could break a good handful of people.

[Rich-Harris]

Is that true? If I create file.html and folder/index.html in the relevant directory and fire up Apache, this is what I get:

/file renders correctly, /file/ 404s. /folder redirects to /folder/ which renders correctly. Every webserver seems to have subtly different semantics, but AFAICT prettifying foo.html and bar/index.html to /foo and /bar/ respectively seems pretty consistent

[benmccann]

Huh. TIL. I didn't think you could leave off the .html extension in the file case. Consider me corrected then

[benmccann]

lgtm after updating the validation message