Separating events from Effects in code
function handleSubmit() {
createTicketMutation.mutate(draft);
}Read the example from data and control flow to the resulting UI. Keep the component boundary small.
Back to week 2
Topic
Separating events from Effects
Definition
Event handlers respond to specific user actions, while Effects synchronize because rendered state and props require external work.
In simpler words
Ask whether work happened because the user clicked or because the screen now has a value.
Causal reasoning, handlers, and reactive synchronization.
After this you can
- Place side effects in the right part of a component.
- Explain the trade-off to a teammate using a small example.
- Name at least one common bug pattern for this topic.
Understand Separating events from Effects
Causal reasoning, handlers, and reactive synchronization.
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 Separating events from Effects
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: Navigate from Effect watching a flag
// Wrong
const [go, setGo] = useState(false);
useEffect(() => { if (go) router.push("/x"); }, [go]);
<button onClick={() => setGo(true)} />
// Right
<button onClick={() => router.push("/x")} />Put the navigation in the click handler.
Mistake: Mutation from Effect on unrelated deps
// Wrong
useEffect(() => { createTicket(draft); }, [draft, theme]);
// Right
function onSubmit() { createTicket(draft); }Writes belong to explicit user actions.
Mistake: Effect as event substitute
// Wrong
useEffect(() => { if (clicked) doThing(); }, [clicked]);
// Right
onClick={() => doThing()}Effects synchronize external systems, not clicks.
Live playground
Separating events from Effects sandbox
Change one input at a time and predict the next render.
path = /course/frontend
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