Build fails on latest Sveltekit 2 and @auth/sveltekit

[subpublic]

Environment

System:
OS: macOS 14.3.1
CPU: (8) arm64 Apple M1
Memory: 45.52 MB / 8.00 GB
Shell: 5.9 - /bin/zsh
Binaries:
Node: 18.18.2 - ~/.nvm/versions/node/v18.18.2/bin/node
npm: 10.5.0 - ~/.nvm/versions/node/v18.18.2/bin/npm
pnpm: 8.8.0 - ~/Library/pnpm/pnpm
Browsers:
Chrome: 122.0.6261.129
Safari: 17.3.1
npmPackages:
@auth/sveltekit: ^0.14.0 => 0.14.0

Reproduction URL

https://stackblitz.com/edit/sveltejs-kit-template-default-furqfa?file=README.md
or
https://github.com/subpublic/sveltekit2-auth-failing

Describe the issue

Build fails on latest sveltekit and @auth/sveltekit

TypeError: Cannot read properties of undefined (reading 'length')
    at Object.handle (/.svelte-kit/output/server/chunks/hooks.server.js:136:26)
    at respond (/.svelte-kit/output/server/index.js:2603:43)

node:internal/event_target:1033
  process.nextTick(() => { throw err; });
                           ^

How to reproduce

Installing the latest sveltekit 2 demo repo
Installing @auth/sveltekit
Follow usage instructions on
https://authjs.dev/reference/sveltekit
(adding auth.ts and hooks.server.ts)

Expected behavior

Build should succeed

[a0a7]

I have this exact issue as well with Kit version 1.30.4, Svelte version 4.2.12, and TS version 5.3.3.

This is what's responsible:

next-auth/packages/frameworks-sveltekit/src/lib/index.ts

Lines 359 to 364 in e9a3e8e

	const action = url.pathname
	.slice(
	// @ts-expect-error - basePath is defined in setEnvDefaults
	_config.basePath.length + 1
	)
	.split("/")[0]

_config.basePath is not defined here.

[subpublic]

Thanks! Good pointer.

Solved it now by adding basePath inside the config

export const { handle, signIn, signOut } = SvelteKitAuth({
	basePath: ''
});

The documentation says it's optional, so should probably have a default setting
https://authjs.dev/reference/core#authconfig

basePath?: string;

[ThangHuuVu]

thanks for reporting this, the issue happens when you have a prerender page export const prerender = true
The default basePath for SvelteKit is /auth BTW. We are working on the issue. I will close this as duplicated of #9809 for now, please continue the discussion there, thank you.