How do you handle loading and error states in the App Router?

Answer

Add loading.tsx for a route-level loading UI and error.tsx as a client error boundary for rendering failures. Keep the fallback close to the affected segment so unaffected layouts remain responsive.

Context

Next.js lets a route choose a server-first rendering and caching strategy while preserving React’s component model. Keep client boundaries small and make cache invalidation part of every mutation design.

Example

// app/products/[id]/page.tsx
export default async function ProductPage({ params }: { params: { id: string } }) {
  const product = await getProduct(params.id)
  return <h1>{product.name}</h1>
}

A Server Component can fetch data close to the route without shipping that data-access code to the browser.

Practical considerations

1. Choose the approach from the requirement and constraints, not from habit.
2. Include validation, error handling, and cleanup where the boundary requires them.
3. Verify the observable result with focused tests or measurement.

Follow-up prompts

• What failure mode would you expect if this were implemented incorrectly?
• How would you test this behaviour?
• What changes when the feature must scale to a larger application or team?

In practice

For this medium-level topic, make assumptions explicit, choose the smallest safe implementation, and verify the behavior at the relevant boundary.