FULL_STACK_DEVELOPER_2026_COMPLETE - Phase 06 Loading, Error, Empty and Success States
Loading, Error, Empty and Success States Make network dependency visible in the user experience
Phase 06 turns two working applications into one full-stack system.
http://localhost:3000 apps/web http://localhost:4000
Later phases replace in-memory Todos with Prisma + PostgreSQL.
Figure - TaskFlow Phase 06 architecture. The browser sees Next.js on port 3000 while the independent NestJS API listens on port 4000.
FULL_STACK_DEVELOPER_2026_COMPLETE - Phase 06 Loading, Error, Empty and Success States
A real network creates more than data Once the frontend depends on another process, the UI must represent time and failure. A professional interface does not assume data appears instantly. Even before TanStack Query, the learner should recognize the four basic remote-data states.
Loading Request in progress Communicate activity without pretending data exists
Error Request failed or HTTP outcome rejected Show actionable feedback; preserve diagnostics
Server Component states For server rendering, route-level loading.tsx and error.tsx can provide structured loading/error behavior around segments. Phase 03 introduced those files; Phase 06 gives them a real network reason to exist.
Client Component states State rendering mental model
if (loading) return <p>Loading Todos...</p>; if (error) return <p role="alert">{error}</p>; if (todos.length === 0) return <p>No Todos yet.</p>;
return <TodoList todos={todos} />;
Avoid false success Do not initialize an empty array and immediately render "No Todos" while a request is still pending. That tells the user the server returned an empty collection when the server has not answered yet. Loading and empty are different states.
Minimal browser mutation flow User clicks Create demo Todo Client event
createTodo() POST JSON
NestJS 201 Created
Client refreshes Todos GET /todos
UI shows new item success state
Figure - Phase 06 keeps mutation synchronization manual so the learner sees the work that TanStack Query will later automate.
FULL_STACK_DEVELOPER_2026_COMPLETE - Phase 06 Loading, Error, Empty and Success States
When TanStack Query appears, query status, caching, invalidation, retries and mutations will replace much of this manual browser server-state code. Understanding the manual lifecycle first prevents the library from becoming magic.
Official source notes Primary framework references: Next.js App Router Fetching Data and fetch API documentation; Next.js Environment Variables documentation; NestJS CORS documentation. Browser protocol references: MDN CORS, preflight request, and Fetch API documentation.
The course remains version-locked to the versions established in Phase 00. These sources are used to verify concepts and current API behavior, not to silently upgrade the course midstream.
Loading Error Empty and Success States 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 “Loading Error Empty and Success States” as one observable behavior. Write the success signal before touching code.
Trace the boundary map: value → owner → lifetime → source of truth → consumer. 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 typed API client boundary that can call the NestJS health/tasks endpoints from the appropriate Next.js execution context.. 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.
API task rows -> TanStack Query
Search/filter URL -> URL/search params
Create form fields -> React Hook Form
Dialog open/closed -> local UI state
Authenticated user -> auth/session stateDo 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 “Loading Error Empty and Success States” to a different TaskFlow resource or input. Write the acceptance criteria first, implement it, and provide the evidence that each criterion is satisfied.
Define API base URLs per environment instead of hard-coding localhost throughout components.
Use the smallest change that demonstrates this lesson. Stop after it works and capture evidence.Ignoring non-2xx fetch responses because fetch did not throw
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.