[Next.js](/courses/nextjs) provides `notFound()` and `not-found.js` for cases where the requested resource genuinely does not exist. This is different from an unexpected application error. ### Example Dynamic Page Suppose the route is: ```text app/articles/[slug]/page.tsx ``` A user requests: ```text /articles/unknown-article ``` The page can check whether that article exists: ```tsx import { notFound } from 'next/navigation'; export default async function ArticlePage({ params }) { const { slug } = await params; const article = await getArticle(slug); if (!article) { notFound(); } return

{article.title}

; } ``` ### Create the Fallback UI ```text app/ articles/ [slug]/ page.tsx not-found.tsx ``` ```tsx import Link from 'next/link'; export default function NotFound() { return (

Article not found

The article may have been removed or the URL may be incorrect.

Browse articles

); } ``` ### What Does notFound() Do? Calling: ```javascript notFound(); ``` stops normal rendering for that route segment and tells [Next.js](/courses/nextjs) to render the appropriate `not-found` UI. You do not need: ```javascript return notFound(); ``` because the [function](/javascript/functions-in-javascript) interrupts the normal rendering flow. ### notFound vs error.js Use: ```text notFound() → requested resource does not exist error.tsx → unexpected runtime/rendering failure ``` For example: ```text Article ID not found → notFound() Database driver unexpectedly crashes → error boundary ``` ### SEO Behavior Not-found responses are intended not to be indexed as normal content. Next.js adds appropriate `noindex` behavior for 404 handling. ### Good 404 Experience A useful not-found page should help users recover. Include links such as: ```text Return home Browse all articles Search courses Go to previous section ``` rather than displaying only: ```text 404 ``` ### Interview-Ready Answer Use `notFound()` when a requested Next.js resource does not exist. It stops rendering the current route segment and displays the nearest `not-found.js` UI. Use it for known missing-resource situations rather than treating them as unexpected runtime errors handled by error.tsx.