Scaling state with reducer and context in code
const TasksContext = createContext(null);
const [tasks, dispatch] = useReducer(tasksReducer, []);Read the example from data and control flow to the resulting UI. Keep the component boundary small.
Back to week 2
Topic
Scaling state with reducer and context
Definition
Combining a reducer with context centralizes state transitions and makes state plus dispatch available throughout a component subtree.
In simpler words
Put the state machine in one reducer and let descendants dispatch actions without prop drilling.
Provider composition, dispatch, and predictable actions.
After this you can
- Scale a complex client-only workflow across nested components.
- Explain the trade-off to a teammate using a small example.
- Name at least one common bug pattern for this topic.
Understand Scaling state with reducer and context
Provider composition, dispatch, and predictable actions.
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 Scaling state with reducer and context
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: Mega-context for everything
// Wrong
value={{ theme, tickets, drafts, ui, auth }}
// Right
// separate providers per concernSplit by update frequency and ownership.
Mistake: Dispatch during render
// Wrong
function Page() {
dispatch({ type: "boot" });
return <Main />;
}
// Right
useEffect(() => { dispatch({ type: "boot" }); }, []);
// or initialize state lazilyDo not dispatch as a render side effect.
Mistake: Copy Query into reducer
// Wrong
dispatch({ type: "setTickets", tickets: query.data })
// Right
// read tickets from Query; reducer holds UI-only stateOne source of truth for server entities.
Live playground
Scaling state with reducer and context sandbox
Change one input at a time and predict the next render.
Provider would expose { state, dispatch } to the subtree.
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