docs: handle immutable headers error

[hyunbinseo]

Fixes https://github.com/sveltejs/kit/issues/11883. Stumbled on it again after 5 months.

---

Please don't delete this checklist! Before submitting the PR, please make sure you do the following:

• [x] It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
• [x] This message body should clearly illustrate what problems it solves.
• [ ] Ideally, include a test that fails without this PR but passes with it.

Tests

• [ ] Run the tests with pnpm test and lint the project with pnpm lint and pnpm check

Changesets

• [ ] If your PR makes a change that should be noted in one or more packages' changelogs, generate a changeset by running pnpm changeset and following the prompts. Changesets that add features should be minor and those that fix bugs should be patch. Please prefix changeset messages with feat:, fix:, or chore:.

Edits

• [x] Please ensure that 'Allow edits from maintainers' is checked. PRs without this option may be closed.

[changeset-bot[bot]]

⚠️ No Changeset found

Latest commit: a1825cff0c12fbc04865ef3641c88a990c5d0480

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes no changesets

When changesets are added to this PR, you'll see the packages that this PR includes changesets for and the associated semver types

Click here to learn what changesets are, and how to add one.

Click here if you're a maintainer who wants to add a changeset to this PR

[Conduitry]

I don't think this is the proper way to suggest handling this. If it's important that this header be set, you don't want to just be silently failing and ignoring that. See e.g. the way this is handled in #10030.

[hyunbinseo]

It definitely is not a proper way to handle this problem.

However, the current documentation is prone to runtime errors.

And I didn't want to mention cloning responses just for this exception.

// the returned Response might have immutable Headers
// so we should clone them before trying to mutate them
response = new Response(response.body, {
  status: response.status,
  statusText: response.statusText,
  headers: new Headers(response.headers)
});

@Conduitry Any suggestions on how to improve the documentation?

Maybe provide an example other than response.headers.set()?

After-all, it is just a server handle function example code.

[Rich-Harris]

preview: https://svelte-dev-git-preview-kit-12543-svelte.vercel.app/

this is an automated message