State Testing, Debugging and Common Failures Volume 06 - Professional Frontend | Phase 23
Debug ownership before debugging values When a screen shows stale or contradictory information, ask which subsystem owns each value and whether a second copy exists. Many "React bugs" are synchronization bugs created by unclear ownership.
Failure patterns Filter changes You likely made local filter state canonical. Move navigation state back to the URL. but URL does not
Query refetches Look for a copied local tasks array that no longer follows the Query cache. but table stays old
Edit form resets The form is being continuously controlled by query data. Initialize/reset only at while typing intentional boundaries.
Logout shows Verify Phase 22 cache clearing and that no second client store holds protected server previous user data.
data briefly
Workspace The canonical workspace may live only in Context. Prefer reconstructable route scope for changes but business navigation.
route still points elsewhere
Test layers • Pure reducer tests for local transition logic.
• Component tests for local interaction state.
• Navigation tests for URL-owned filters.
• Form tests for draft/reset/dirty behavior.
• TanStack Query integration tests for server-cache behavior.
• End-to-end tests for cross-boundary flows such as workspace -> filters -> edit -> logout.
Tests are executable behavioral contracts. Good tests protect meaningful invariants and failure cases, not implementation trivia.
Restate the goal of “State Testing Debugging and Common Failures” as one observable behavior. Write the success signal before touching code.
Trace the boundary map: behavior contract → arrange → act → assert → regression signal. 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 state ownership map for TaskFlow plus refactored examples removing redundant synchronized state.. 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.
it("rejects deleting another member's task", async () => {
const response = await asUser(bob).delete(`/tasks/${aliceTask.id}`);
expect(response.status).toBe(403);
expect(await taskStillExists(aliceTask.id)).toBe(true);
});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.
Look for shared database state, timers, globals, unordered execution, or missing cleanup before adding retries.
Move the highest-risk boundary to an integration test with the real adapter/database.
Without copying the final source, create a second small scenario that applies “State Testing Debugging and Common Failures” to a different TaskFlow resource or input. Write the acceptance criteria first, implement it, and provide the evidence that each criterion is satisfied.
Evaluate whether any global store is actually needed; write the requirement before adding one.
Use the smallest change that demonstrates this lesson. Stop after it works and capture evidence.URL and React state disagreeing about filters
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.