Make /client directory configurable

[bbuhler]

Is your feature request related to a problem? Please describe.
Yes. I've sapper running behind a proxy on domain root path. The path "/client" is been proxy passed to a different application. Since the generated client JS and CSS files are served from this path they can not be reached.

Describe the solution you'd like
A CLI argument or an option in config.client.output() rollup config to define an alternative path to serve generated client files from.

Describe alternatives you've considered
I have considered writing a script which renames the output folder and alters build.json or server.js by replacing all occurrences.

How important is this feature to you?
It is currently blocking me, so I consider to writing the script as workaround until there is a better solution.

If there is an alternative solution to my problem that I have not thought of, I am happy to hear about.

[benmccann]

This happens in middleware:

sapper/runtime/src/server/middleware/index.ts

Line 60 in a6fff98

prefix: '/client/',

We'd need to first figure out how to handle configuration in Sapper: #1216

[antony]

I've just stumbled across this and I am totally in favour. The reason being is that it fixes this long-standing issue:

#389

If we were able to specify the asset serving dir we could point an instances assets at it's real url (in the instance of red-green deployment where you deploy to an alternative url and then switch over the DNS) - see the Vercel solution here: https://twitter.com/PierB/status/1294311963363889152

And also would allow people to hash the client directory after builds, meaning that in the instance of a non red-green deployment, open instances of the app would continue to point to their own assets rather than the latest deployment.

I will see if I can come up with a PR for this.