Derived State, Selectors and Computation Volume 06 - Professional Frontend | Phase 23
If it can be computed, prefer computing it Derived state is information that can be calculated from authoritative state during render or through a memoized selector when the computation is actually expensive.
Examples const tasks = tasksQuery.data?.data ?? []
const completedCount = tasks.filter( task => task.status === "DONE", ).length
const hasSelection = selectedIds.size > 0const canGoNext = Boolean(tasksQuery.data?.meta.hasNextPage)Avoid synchronized counters // Avoid const [completedCount, setCompletedCount] = useState(0)
useEffect(() => { setCompletedCount( tasks.filter(task => task.status === "DONE").length ) }, [tasks])
Memoization rule Use useMemo for measured or structurally useful expensive computation, not as a ritual around every derived value. Correct ownership matters more than premature memoization.
Derived State Selectors and Computation 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 “Derived State Selectors and Computation” 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 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.
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 “Derived State Selectors and Computation” to a different TaskFlow resource or input. Write the acceptance criteria first, implement it, and provide the evidence that each criterion is satisfied.
Move filter/search/page state that should survive/share navigation into URL search params.
Use the smallest change that demonstrates this lesson. Stop after it works and capture evidence.Storing derived values that can be computed
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.