Context API Interview Questions — Medium

• Each context object holds a registry of consumers; Providers register themselves on a global stack during render (_currentValue push/pop).
• On change, React marks fibers with dependencies pointing at changed contexts and walks consumers from that provider down — propagation cost scales with consumer count below the provider, not tree size.
• Consumers read via stack discipline: nearest provider wins. Implication: placing providers HIGH multiplies traversal; colocate providers as deep as their audience allows.

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• memo compares PROPS only; useContext reads bypass props entirely, forcing re-render on value identity change. Isolation patterns:

1. Narrow consumption: move useContext into small bridge components that pass plain props into memoized heavy children.
2. Split read/write contexts so action-only components never subscribe to state changes.
3. Multiple domain contexts sized by change frequency. Rule to state plainly: render isolation from context comes ONLY from subscription narrowing.

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const AuthStateCtx = createContext<AuthState|null>(null);
const AuthDispatchCtx = createContext<AuthDispatch|null>(null);

function AuthProvider({children}) {
  const [state, dispatch] = useReducer(authReducer, null, initFromStorage);
  const dispatchStable = useMemo(() => ({
    signIn: u => dispatch({type:'signIn', user:u}),
    signOut: () => dispatch({type:'signOut'}),
  }), []);
  return (
    <AuthStateCtx.Provider value={state}>
      <AuthDispatchCtx.Provider value={dispatchStable}>{children}</AuthDispatchCtx.Provider>
    </AuthStateCtx.Provider>
  );
}

Points to narrate: stable actions object avoids new identities; state changes re-render only State consumers; persistence handled in effects keyed off state transitions.

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Render props expose values as ARGUMENTS: <Auth>{({user}) => ...}</Auth> — explicit dataflow per usage site. Beats hooks when:

• You need MULTIPLE instances of the same logic simultaneously (two independent draft editors) — hooks bind one instance per position in tree.
• TypeScript generics flow naturally per-usage. Costs: nesting pyramids, no mid-component access without restructuring. Modern verdict: default to hooks+context; reach for render props where instance multiplicity matters (virtualizers, drag contexts exposing slots).

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const [value] = useState(() => buildExpensiveTheme());

Lazy initializer runs once per provider mount — not every render. For async bootstrap:

1. Synchronous placeholder + effect upgrade (flash risk).
2. Suspense-integrated resource throwing promise (needs boundary).
3. Gate rendering: provider computes readiness flag; renders splash until ready — simplest robust SSR-friendly choice. Never compute heavy objects inline in value={} — recreates per render even if discarded.

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