FULL_STACK_DEVELOPER_2026_COMPLETE - Phase 06 DevTools and End-to-End Debugging
DevTools and End-to-End Debugging Trace browser, Next.js and NestJS failures with observable evidence
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 DevTools and End-to-End Debugging
Debug the path, not the symptom "The page is blank" is not a diagnosis. End-to-end development becomes manageable when every failure is located on the actual request path.
Is Next.js running? port 3000
Is NestJS running? port 4000
What exact URL did fetch use? environment variable
Did the request leave the browser? Network tab
Was there OPTIONS first? preflight
What status + headers returned? HTTP evidence
Did fetch code check response.ok? application handling
Figure - Phase 06 debugging follows the network path instead of guessing from the UI symptom.
Browser Network panel workflow 1 Open DevTools -> Network. 2 Clear previous entries. 3 Trigger exactly one operation. 4 Select the request to port 4000. 5 Inspect Request URL and Request Method. 6 Inspect Status Code. 7 Inspect Request Headers, especially Origin and Content-Type. 8 Inspect Response Headers, especially Access-Control-Allow-* when cross-origin. 9 Inspect Response/Preview body. 10 If OPTIONS exists, inspect it separately from the actual request.
Terminal evidence still matters A Server Component fetch runs in the Next.js server process, so its stack trace and fetch error appear in the Next.js terminal. A NestJS handler failure appears in the API terminal. A browser Client Component failure can appear in browser Console/Network. Good debugging follows the process that executed the failing code.
FULL_STACK_DEVELOPER_2026_COMPLETE - Phase 06 DevTools and End-to-End Debugging
Failure matrix Symptom Likely layer to inspect first
fetch catch runs with Failed to fetch Network/CORS rather than HTTP 4xx/5xx
response.ok is false with 500 Nest logs / application failure
Public API URL changed but browser still uses old NEXT_PUBLIC_ build-time inlining / restart-build context value
Do not change CORS, fetch, controllers, service logic and environment files simultaneously. Preserve one failing request, make one hypothesis, change one layer, rerun the same request, then observe whether the evidence changed.
A powerful test: bypass the browser If a browser call fails, repeat the equivalent request with PowerShell. If PowerShell succeeds while browser JavaScript fails, that strongly narrows the problem toward browser-only policy/configuration such as CORS. It does not prove CORS automatically, but it prevents pointless database or controller rewrites.
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.
DevTools and End to End Debugging 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 “DevTools and End to End Debugging” as one observable behavior. Write the success signal before touching code.
Trace the boundary map: Next.js UI → typed request → NestJS endpoint → service → response state. 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.
Input -> owner -> transformation -> observable output -> failure evidenceDo 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 “DevTools and End to End Debugging” to a different TaskFlow resource or input. Write the acceptance criteria first, implement it, and provide the evidence that each criterion is satisfied.
Call a public endpoint from a Server Component and inspect where the request originates.
Use the smallest change that demonstrates this lesson. Stop after it works and capture evidence.Duplicating fetch/error logic in every component
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.