FULL_STACK_DEVELOPER_2026_COMPLETE - Phase 06 CORS Preflight and Fetch Error Semantics
CORS Preflight and Fetch Error Semantics OPTIONS, JSON POST, response.ok and network-vs-HTTP failures
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 CORS Preflight and Fetch Error Semantics
Why GET may work while POST suddenly shows OPTIONS A simple cross-origin GET can often proceed without an OPTIONS preflight. A browser POST that sends Content-Type: application/json is different: application/json is not a CORS-safelisted content type, so the browser commonly checks permission first.
Browser wants POST /todos Content-Type: application/json
OPTIONS /todos CORS preflight
NestJS CORS response Access-Control-Allow-*
POST /todos actual JSON request
201 Created Todo representation
Figure - A JSON cross-origin POST commonly triggers an automatic browser preflight. The frontend normally does not hand-code the OPTIONS request.
Create helper Add to apps/web/src/lib/api/todos.ts
export async function createTodo(baseUrl: string, title: string): Promise<Todo> { const response = await fetch(`${baseUrl}/todos`, { method: 'POST', headers: { 'Content-Type': 'application/json' }, body: JSON.stringify({ title }), });
if (!response.ok) { throw new Error(`POST /todos failed: ${response.status}`); }
return response.json() as Promise<Todo>; }
The browser owns the preflight The frontend normally does not write an OPTIONS request manually. The browser decides that a cross-origin request requires preflight, sends OPTIONS with the intended method/headers, inspects the server CORS response, and only then sends the actual request if policy allows it.
FULL_STACK_DEVELOPER_2026_COMPLETE - Phase 06 CORS Preflight and Fetch Error Semantics
Network error vs HTTP error Situation fetch promise Response available? What to inspect
API process stopped Rejects No catch / network error
DNS or malformed URL Rejects No URL/configuration
CORS blocked browser Typically rejects to JS as a No usable response Console + Network + CORS headers access network-style failure
404 Not Found Resolves Yes, ok=false status + response body
500 Server Error Resolves Yes, ok=false status + server logs
A try/catch around fetch is not enough to detect HTTP failures. Check response.ok or response.status because 404 and 500 are still valid HTTP responses.
Preflight debugging checklist ● Was the browser request actually cross-origin? ● Did the Network panel show OPTIONS before the actual method? ● What Origin header did the browser send? ● Did the API return Access-Control-Allow-Origin for that exact origin? ● Did Access-Control-Allow-Methods include the intended method? ● Did Access-Control-Allow-Headers permit non-safelisted headers? ● Did the actual request run after OPTIONS?
Do not "fix" CORS with no-cors Setting fetch mode to no-cors is not a general solution. It produces an opaque response whose useful status/body are not available to application JavaScript. The correct fix is an intentional server CORS policy when the browser should be allowed to call the API.
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.
CORS Preflight and Fetch Error Semantics 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 “CORS Preflight and Fetch Error Semantics” as one observable behavior. Write the success signal before touching code.
Trace the boundary map: User intent → UI boundary → API contract → business rule → persistence → observable result. 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.
OPTIONS /tasks HTTP/1.1
Origin: http://localhost:3000
Access-Control-Request-Method: POST
Access-Control-Request-Headers: authorization,content-type
HTTP/1.1 204 No Content
Access-Control-Allow-Origin: http://localhost:3000Do 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 “CORS Preflight and Fetch Error Semantics” to a different TaskFlow resource or input. Write the acceptance criteria first, implement it, and provide the evidence that each criterion is satisfied.
Centralize response parsing and non-2xx error handling in one API helper.
Use the smallest change that demonstrates this lesson. Stop after it works and capture evidence.Using the browser-only URL from server-to-server code in deployment
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.