Tru-er SPA mode

[ebeloded]

SvelteKit allows building Single Page Apps with the following config:

const adapter = require('@sveltejs/adapter-static');

module.exports = {
  kit: {
    adapter: adapter({
      fallback: '200.html'
    },
    prerender: {
      enabled: false
    },
    ssr: false
  }
};

Is your feature request related to a problem? Please describe.
Even with the ssr disabled in config file, it is impossible to reference window or document without wrapping it in an environment check. This is because each component needs to be evaluated to determine whether it has SSR export, as @Rich-Harris explained here:

The fundamental issue is that the component must be imported in order to determine whether it has an ssr export. That would continue to be true if we checked each page for ssr at build time and added the information to a manifest.

There's a couple of options that spring to mind - we find some other way of establishing whether a given page should be SSR'd, or we determine it with static analysis (which has the usual caveats but should be pretty robust in this case).

Describe the solution you'd like

I'd like to have a way of configuring SvelteKit as a true SPA, in which I need not worry about referencing browser globals.

As a possible way forward, @benmccann suggested to expand the ssr option, allowing not just a boolean value:

The other option would be to make the setting more than just a boolean. E.g. we could have true/false/per-page or something like that or always/never/true-by-default/false-by-default or whatever makes sense

I think this would be a fine solution.

Describe alternatives you've considered

The only alternative I currently see is to preface browser globals with environment check and load naughty dependencies dynamically.

How important is this feature to you?
Slightly annoying. Gets in the way. Forces to add meaningless code.

Additional context
The original issue regarding SPA in SvelteKit #754

[umanghome]

Since load runs on the browser in SPA mode, it makes sense to be able to use localStorage from within load.

[einarpersson]

+1

I’d expect it to be possible to import pixi.js when I run SvelteKit in SPA-mode, but right now I have to do a dynamic import await import(“pixi.js”) because Pixi references the window object.

[Rich-Harris]

Reopening since this isn't actually completely fixed by #2008 — the page's module is still imported, and in a simple test app the server blows up when trying to render a placeholder for a page that imports a client-only dependency

[lemmon]

Since load runs on the browser in SPA mode, it makes sense to be able to use localStorage from within load.

I think there's no need for checking valid variables/libraries. You should be able to reference any browser objects, without warnings, when ssr is set to false.

[brgrz]

Agree, this is an issue that still results in compile error with ssr: false in svelte-kit config, for example

Error when evaluating SSR module /src/routes/...
ReferenceError: localStorage is not defined

The app still builds and runs locally but my Render builds are blocked by this.