Topic
Definition
React Context lets a provider make a value available to descendant components without manually passing it through every intermediate prop.
In simpler words
Context is a shared channel for a subtree, useful when many levels need the same value.
createContext, providers, consumers, and focused context values.
createContext, providers, consumers, and focused context values.
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.
const ThemeContext = createContext("light");
<ThemeContext.Provider value="dark"><Page /></ThemeContext.Provider>;Read the example from data and control flow to the resulting UI. Keep the component boundary small.
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?
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.
// Wrong
<AppContext.Provider value={{ tickets, draft, theme, user }}>
// Right
// Split contexts; keep frequently changing values narrowWide, hot providers re-render large trees.
// Wrong
const TicketsContext = createContext([])
// Right
useQuery({ queryKey: ["tickets"], queryFn: list })Server cache belongs in Query.
// Wrong
const v = useContext(ThemeContext); // undefined in prod route
// Right
<ThemeContext.Provider value="dark">{children}</ThemeContext.Provider>Wrap the subtree that consumes the context.
Live playground
Change one input at a time and predict the next render.
Deep child reads theme = light (no prop drilling)
Test
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