docs: show how to handle redirects from shared helpers in command functions

[Nic-Polumeyv]

Fixes #16275.

The docs promote shared getRequestEvent() helpers, and the canonical requireLogin() example throws redirect(), which explodes with a confusing client error when reused inside a command. This expands the existing Redirects section with the intended pattern, catch with isRedirect and return the location for the client to goto.

For context, allowing redirect() inside command() natively was floated by Conduitry on #14858 and didn't get maintainer buy-in, so this documents the intended pattern rather than a stopgap.

[pkg-svelte-dev]

Install the latest version of @sveltejs/kit from c4ad7d7:

pnpm add https://pkg.svelte.dev/@sveltejs/kit/c/c4ad7d7c2b24b26840bfe21ad390bd5a0de379c0

Open in pkg.svelte.dev: https://pkg.svelte.dev/repos/kit/pr/16328

Note

This PR is from a fork. A maintainer must approve approve each commit before it can be built and installed.

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[Rich-Harris]

You know... I think we've been thinking about this all wrong. My first reaction was that perhaps we should just throw the redirect to the caller...

<!--- file: src/routes/+page.svelte --->
<script>
	import { isRedirect } from '@sveltejs/kit';
	import { goto } from '$app/navigation';
	import { addTodo } from './todos.remote';

	async function add(text) {
		try {
			await addTodo(text);
		} catch (e) {
			if (isRedirect(e)) {
				goto(result.redirect);
			} else {
				throw e;
			}
		}
	}
</script>

...and I do think we should do that, but not because it's a command. The distinction isn't between commands and non-commands, it's between render and non-render.

In render (i.e. in the template, or while otherwise running an effect), a thrown redirect should cause a goto.

Outside render (i.e. in an event handler), it should be like any other exception, and you should have to catch it. The reason for this becomes clear when you consider what currently happens with queries:

const todo = await getTodo(id);
console.log(todo.text);

If getTodo redirects, todo is undefined, directly contradicting both the types and common sense. The second line will throw a TypeError.

This is one of those deeply-obvious-in-retrospect things — it should allow some simplification of the code, and will make commands behave the same as other remote functions with respect to redirects. form might seem like an anomaly here since await submit() in an enhance callback will redirect, but that's because enhanced form submissions are emulating non-enhanced form submissions, which are a form of render.

Working on this locally.

[Nic-Polumeyv]

I can redo this against your change once it's up, since #16275 will still want a docs example either way, or just close it if docs are part of your change.

[Rich-Harris]

Docs will need to be part of the other PR, so I'll go ahead and close this — thanks