You might not need an Effect in code
const visible = tickets.filter(ticket => ticket.status === status);Read the example from data and control flow to the resulting UI. Keep the component boundary small.
Back to week 2
Topic
You might not need an Effect
Definition
Derived values and event-specific work should usually be calculated during rendering or handled in events instead of synchronized with an Effect.
In simpler words
If no external system is involved, compute it directly or do it in the click handler.
Derived state, memoization only when needed, and event logic.
After this you can
- Remove unnecessary Effects and extra renders.
- Explain the trade-off to a teammate using a small example.
- Name at least one common bug pattern for this topic.
Understand You might not need an Effect
Derived state, memoization only when needed, and event logic.
Start by identifying which value or browser behavior changes. Then describe the UI from that current input instead of editing the DOM as a separate source of truth.
Apply You might not need an Effect
Keep rendering as a calculation. Put user-triggered changes in event handlers, preserve UI memory in state, and reserve external synchronization for Effects or the server-state layer.
Name values by their UI meaning, test the loading and error path when data is remote, and avoid keeping two editable copies of the same value.
Ask before adding code: is this local UI memory, shared client state, or Nest-owned server state?
Where bugs hide
Definition
High-bug areas are places where a small API misuse looks correct but produces stale UI, duplicate work, or silent failures.
In simpler words
Each mistake below shows Wrong vs Right code — compare them side by side.
When something misbehaves, match the symptom to a pattern below before rewriting the feature.
Prefer fixing the ownership or update path over adding another Effect or sync step.
Mistake: Sync props into state with Effect
// Wrong
useEffect(() => setValue(propValue), [propValue]);
// Right
// use propValue directly, or key={id} to resetCopying props to state is usually redundant.
Mistake: Filter list in an Effect
// Wrong
useEffect(() => setVisible(items.filter(...)), [items]);
// Right
const visible = items.filter(...);Derived data belongs in render.
Mistake: Open modal via Effect flag
// Wrong
useEffect(() => { if (shouldOpen) setOpen(true); }, [shouldOpen]);
// Right
// setOpen(true) in the click handler that decided to openUser intent belongs in events.
Live playground
You might not need an Effect sandbox
Change one input at a time and predict the next render.
Derived during render — no Effect mirroring props into state.
fullName = Ada Lovelace
- Auth cookie
- Layouts
- Query keys
- Pulse draft
const fullName = first + " " + last; const filtered = tickets.filter(...)
Keep in mind
- Keep the formal definition in mind; it explains which tool belongs where.
- Prefer one source of truth over synchronized copies of the same value.
- When behavior surprises you, trace: input → update → render → committed UI.
- Study the Wrong vs Right examples in “Where bugs hide” before you merge.
Test
Check your understanding
At least 10 questions — mix of concept, syntax, practical, and logic. Score ≥ 80% (enforced by the API) to save progress.
Checking your session…
10 questions · concept 3 · syntax 3 · practical 2 · logic 2