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App Router and File Conventions

In the App Router, folders are URL segments and special files decide what renders. Know how the file hierarchy nests, why layouts persist while templates remount, and how parallel and intercepting routes behave on soft and hard navigation.

Key points

1
A segment is routable only when it has page or route. Other colocated files are never served, and _folder opts a whole subtree out of routing.
2
Each segment renders layout > template > error > loading > not-found > page. error.tsx can't catch its own segment's layout; the parent boundary or global-error does.
3
params and searchParams are promises, and Next.js 16 removed synchronous access. Await them, or use React use in Client Components. Layouts never receive searchParams.
4
Route groups (name) organise files and can create multiple root layouts. Crossing root layouts is a full page load, and two groups must not resolve to the same URL.
5
Parallel route slots (@slot) keep their active page on soft navigation and render default.js after a reload. In Next.js 16 every named slot needs a default.js.
6
Intercepting routes (.), (..) and (...) count route segments, not folders, so @slot folders don't count. Interception happens only on client navigation; a refresh shows the real page.
7
generateStaticParams must return an array. dynamicParams = false makes unknown params 404, but it isn't available with Cache Components, where an empty params array is a build error.

Common traps

notFound() and forbidden() work by throwing. A broad try/catch swallows them unless you rethrow, and once streaming has started the status stays 200.
Choosing a parallel slot in a layout (role === 'admin' ? admin : user) still renders both slots on the server, so authorize inside each slot.
A modal slot stays visible after a <Link> to another route unless the slot matches that route with a component that returns null, for example a catch-all.

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