Date changes datatype in SSR und CSR

Describe the bug

When returning a JSDate from an endpoint. The datatype is preserved during SSR, but then on clientside load the type is the expected string.

Reproduction

The reproduction repo can be found here: https://github.com/tylkomat/sveltekit-date-ssr

The setup is very simple:

index.svelte

<script lang="ts">
  export let now: string;

  console.log(now.toString(), (now as any) instanceof Date,typeof now === 'string');
</script>

index.ts

export async function GET() {
  return {
    body: {
      now: new Date()
    }
  }
}

Logs

Server Log: `Wed Aug 10 2022 16:15:55 GMT+0200 (Mitteleuropäische Sommerzeit) true false`
Browser Log: `2022-08-10T14:15:55.959Z false true`

System Info

System:
    OS: Windows 10 10.0.19044
    CPU: (16) x64 AMD Ryzen 7 3700X 8-Core Processor
    Memory: 2.64 GB / 15.93 GB
  Binaries:
    Node: 18.7.0 - ~\AppData\Local\Volta\tools\image\node\18.7.0\node.EXE
    npm: 8.15.0 - ~\AppData\Local\Volta\tools\image\node\18.7.0\npm.CMD
  Browsers:
    Edge: Spartan (44.19041.1266.0), Chromium (104.0.1293.47)
    Internet Explorer: 11.0.19041.1566
  npmPackages:
    @sveltejs/adapter-auto: next => 1.0.0-next.64
    @sveltejs/kit: next => 1.0.0-next.405
    svelte: ^3.44.0 => 3.49.0
    vite: ^3.0.0 => 3.0.5

Severity

annoyance

Additional Information

No response

[CaptainCodeman]

Providing some way to hook into the deserialization on the client would make it easier to convert ISO date strings back to JS Objects (i.e. as a reviver option to the JSON.parse method).

I hit this all the time (pretty much ever chunk of data always has a date in it) and it’s a new-user foot-gun because the error you end up with isn’t obvious as to the cause.

[Conduitry]

I’d be in favor of testing for serializability in dev mode only, and just passing along the object as-is in prod mode for SSR.