[fix] show more descriptive error if data returned from load is a non-POJO

[errmayank]

Closes #7262

• Tested it out with return json

• with new Error

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🦋 Changeset detected

Latest commit: 4cf5e43

The changes in this PR will be included in the next version bump.

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@sveltejs/kit	Patch

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[dummdidumm]

Thanks, looks mostly good, added some small comments

[dummdidumm]

This could be from server_data deeper in the load tree, so I think it should be

Suggested change

	const constructorName = branch[0]?.server_data?.data?.constructor?.name;
	const constructorName = branch.find(({ server_data }) => server_data?.data?.constructor?.name)?.server_data.data.constructor.name;

[dummdidumm]

Suggested change

	`Data returned from \`load\` must be a plain object${
	constructorName ? ` rather than ${constructorName} constructor` : ''
	}`
	`Data returned from \`load\` (while rendering ${event.routeId}) must be a plain object${
	constructorName ? ` rather than an instance of ${constructorName}` : ''
	}`

[errmayank]

Thanks @dummdidumm, updated.

[dummdidumm]

Could you add a changeset by running npx changeset in the root and following the prompts (patch release)? Then we're good to go 👍

[errmayank]

Added changeset

[dummdidumm]

Thank you!

[aypm-dev]

Hey! I was trying to return a response with headers in a load, silly me. Now I understand how load works, but I noticed that I'm getting the old error when returning a response with headers and not the new one. Is this case in the scope of the pull request?

Code: (+page.server.ts)

import type { PageServerLoad } from './$types';

export const load: PageServerLoad = async ({ locals, url, cookies }) => {
  const headers = new Headers()
  headers.append("set-cookie",  " cookies here ")

  return new Response( null, { headers: headers } )
}

Error: Cannot stringify arbitrary non-POJOs (data.headers)