$app/forms enhance/ApplyAction does not handle endpoints that redirect

[jerrythomas]

Describe the bug

Assuming that POST Endpoints (+server.js) or actions (+page.server.js) can both be used for the actions on forms.

api/+server.js

import { redirect } from '@sveltejs/kit'

/** @type {import('./$types').RequestHandler} */
export async function POST(event) {
	console.log('endpoint', event.url.pathname, event.request.method)
	throw redirect(303, '/')
}

form/+page.server.js

import { redirect } from '@sveltejs/kit'

/** @type {import (./$types).Actions} */
export const actions = {
	async default(event) {
		console.log('action', event.url.pathname, event.request.method)
		throw redirect(303, '/')
	}
}

Both of the above two seem to be identical, however the result of using the enhance function on the action path results in

{
    "type": "redirect",
    "status": 303,
    "location": "/"
}

and using it on the endpoint path results in

{
    "type": "error",
    "error": "SyntaxError: Unexpected token '<', \"<!DOCTYPE \"... is not valid JSON
    at JSON.parse (<anonymous>)
    at deserialize (http://localhost:5173/node_modules/.pnpm/@sveltejs+kit@1.0.0-next.571_svelte@3.53.1+vite@3.2.4/node_modules/@sveltejs/kit/src/runtime/app/forms.js?v=5b196e18:21:22)
    at HTMLFormElement.handle_submit (http://localhost:5173/node_modules/.pnpm/@sveltejs+kit@1.0.0-next.571_svelte@3.53.1+vite@3.2.4/node_modules/@sveltejs/kit/src/runtime/app/forms.js?v=5b196e18:106:13)"
}

Shouldn't either of them result in the response to be redirect.

The following error occurs when a ApplyAction is used to make a post request to an endpoint that redirects to some other page.

500
Unexpected token '<', "<!DOCTYPE "... is not valid JSON
SyntaxError: Unexpected token '<', "<!DOCTYPE "... is not valid JSON
    at JSON.parse (<anonymous>)
    at deserialize (http://localhost:5173/node_modules/.pnpm/@sveltejs+kit@1.0.0-next.571_svelte@3.53.1+vite@3.2.4/node_modules/@sveltejs/kit/src/runtime/app/forms.js?v=5b196e18:21:22)
    at HTMLFormElement.handle_submit (http://localhost:5173/node_modules/.pnpm/@sveltejs+kit@1.0.0-next.571_svelte@3.53.1+vite@3.2.4/node_modules/@sveltejs/kit/src/runtime/app/forms.js?v=5b196e18:106:13)

This error does not occur with vanilla post method on the form.

Reproduction

https://github.com/jerrythomas/repro-enhance-apply

Logs

No response

System Info

System:
    OS: macOS 13.0.1
    CPU: (16) x64 Intel(R) Core(TM) i9-9880H CPU @ 2.30GHz
    Memory: 282.48 MB / 16.00 GB
    Shell: 5.8.1 - /bin/zsh
  Binaries:
    Node: 16.15.0 - ~/.nvm/versions/node/v16.15.0/bin/node
    npm: 8.5.5 - ~/.nvm/versions/node/v16.15.0/bin/npm
  Browsers:
    Brave Browser: 107.1.45.133
    Chrome: 108.0.5359.94
    Firefox: 106.0.5
    Safari: 16.1
  npmPackages:
    @sveltejs/adapter-auto: next => 1.0.0-next.90 
    @sveltejs/kit: next => 1.0.0-next.571 
    svelte: ^3.53.1 => 3.53.1 
    vite: ^3.2.4 => 3.2.4

Severity

serious, but I can work around it

Additional Information

I have been trying out auth-helpers from supabase for svelte. It works great but I feel that it might be easier to prepackage some of the endpoints (like sign in, sign out & handle protected routes) using the handle functions and the sequence helper. I hit this snag while working with this experiment.

[dummdidumm]

applyAction is only meant to be used with form actions (+page.server.js)