Routing, Loading and Error States Build navigation that accounts for shared layout, pending UI, failures, and not-found states instead of only the happy path.
Routing, Navigation, Loading, Errors and 404s A production frontend is not only successful page rendering. Users navigate, wait, hit missing resources, and encounter failures. The App Router gives these states explicit places in the route tree.
Navigation with Link import Link from "next/link";
<Link href="/projects">Projects</Link>Use Next.js Link for internal application navigation. It integrates with client-side navigation and prefetching behavior instead of forcing a full browser document request for every route transition.
A simple TaskFlow route map / /projects /settings
src/app/ |-- page.tsx |-- projects/page.tsx `-- settings/page.tsx
Shared navigation belongs in layout structure If Home, Projects, and Settings share the same top navigation, render that navigation from a layout rather than copy it into every page. A component handles reusable markup; a layout controls where that reusable markup participates in routing.
loading.tsx - pending UI A loading file provides route-segment fallback UI while content below the segment is waiting. It works with streaming and Suspense semantics. Use it to communicate progress, not to hide slow architecture indefinitely.
export default function Loading() {
return <p aria-live="polite">Loading projects...</p>;
}error.tsx - recoverable route UI An error boundary lets a route segment show fallback UI when a descendant throws during rendering. Error components need client-side behavior because reset/retry interaction happens in the browser.
"use client";
export default function ErrorView({ reset }: { reset: () => void }) {
return (
<section>
<h2>Something went wrong</h2>
<button onClick={() => reset()}>Try again</button>
</section>
);
}Do not swallow errors An error boundary is presentation for failure, not permission to ignore the root cause. Production logging/observability enters later, but even now the developer should inspect the terminal and browser console.
not-found.tsx
Use a not-found UI for missing route resources. Later, a project detail page can call notFound() when the API says the project does not exist. This is different from a server crash or validation error.
Status thinking Situation Frontend concept Later backend meaning
Unexpected render failure error boundary Potential 5xx or application error
Requested resource does not not-found UI Often 404 exist
Dynamic route preview src/app/projects/[projectId]/page.tsx
This will eventually display one project based on route params. We postpone the full implementation until Phase 06+ when a backend exists. The course avoids fake architecture solely to demonstrate syntax.
success failure
next page error / 404
Checkpoint • Internal navigation uses Link rather than raw full-page navigation by default. • Layouts own shared route shell. • loading.tsx models pending UI, error.tsx models rendering failure, and not-found.tsx models missing resources. • You can explain why missing data and unexpected failure are not the same state.
Routing Loading and Errors matters because it changes a real boundary in TaskFlow. The goal is to understand the inputs, owner, observable output, and failure modes before adding more abstraction.
Restate the goal of “Routing Loading and Errors” as one observable behavior. Write the success signal before touching code.
Trace the boundary map: failure → typed/domain error → exception mapping → HTTP envelope → client handling. Mark the one boundary this lesson changes.
Inspect the current repository state and record the exact files, route, table, or runtime involved. Avoid creating an abstraction before you can name the existing owner.
Implement the smallest vertical slice. Add one coherent change, run the narrowest verification command, and read the output before continuing.
Connect the slice to TaskFlow’s phase artifact: A navigable TaskFlow web shell with route groups/layouts, server-first composition, selective client interactivity, and verified environment handling.. Verify behavior from the outside as a user or API client would.
Create one controlled failure that is safe to reproduce. Predict the symptom first, capture evidence, locate the earliest broken boundary, then fix only that cause.
Close the reference and rebuild or explain the core change from memory. Record what you would monitor or test before trusting the same change in production.
{
"error": {
"code": "TASK_NOT_FOUND",
"message": "Task was not found",
"requestId": "req_..."
}
}Do not copy this as a finished answer. Predict what each boundary does, type the smallest useful slice, run it, then compare the result with your prediction.
Return to the nearest observable boundary: input, network, process log, database, or rendered UI. Compare actual evidence with the predicted path.
Change one variable at a time and reproduce the original failure after the fix; otherwise you have correlation, not diagnosis.
Without copying the final source, create a second small scenario that applies “Routing Loading and Errors” to a different TaskFlow resource or input. Write the acceptance criteria first, implement it, and provide the evidence that each criterion is satisfied.
Add one Client Component for interaction and explain what 'use client' changes in the dependency graph.
Use the smallest change that demonstrates this lesson. Stop after it works and capture evidence.Using effects for work that belongs in server rendering or a data layer
Predict the symptom before causing the failure. Then locate the earliest boundary where reality diverges from expectation.Why does this responsibility live here, what input does it trust, what output proves success, and what would go wrong if another layer owned it?
If your explanation is vague, repeat the boundary trace before moving on.Close the reference. Reproduce the core idea from memory or explain the exact request/data flow aloud. Then check your diff or notes for accidental shortcuts that bypass validation, authorization, error handling, or verification earned in earlier phases.