Allow server-only load functions to return more than JSON

[Rich-Harris]

closes #6008
fixes #6357

This allows load functions to return things like Date objects, regexes, Map and Set, BigInt and so on. It also allows repeated and cyclical references, for the times when they're useful.

TODO:

• docs
• finish updating tests

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Latest commit: 5eafe2c

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@sveltejs/adapter-netlify	Patch
@sveltejs/adapter-vercel	Patch
@sveltejs/kit	Patch
@sveltejs/adapter-auto	Patch

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[Conduitry]

Did you have an opinion about a statically analyzable config for making a given page use JSON instead of Devalue? Do you think the performance impact is not big enough to warrant that?

[Rich-Harris]

I think it's probably fine — for all but the most complex objects, we're talking fractions of milliseconds. One thing devalue could do to reduce the overhead is, after it completes its initial walk of the object, return JSON.stringify(data) if it didn't encounter any non-JSON-able values (or repeated references of objects or arrays). Though the resulting data will be larger as a result, so it's probably a wash.

[Rich-Harris]

No, I'm afraid that's probably a non-starter. The code generated by devalue needs to know how to reference that class in the client. It would involve jumping through some absurd hoops. Just take care of it on the server

[dummdidumm]

This looks very nice! Looking at the code I think it may even be possible to make this configurable to some extend. The window method stays but what kind of format is written to it could be configured by the user (JSON, superjson, whatever). Not for now but thought for later.

[0gust1]

Oh. Yeah. :)