Intermediate

Common interview patterns

• Trace execution order across sync, microtasks, and macrotasks
• Implement debounce and throttle from scratch
• Cancel a fetch with `AbortController` and the take-latest pattern
• Compose `Promise.all`, `allSettled`, `race`, and `any` correctly

Common mistakes

• Confusing microtask and macrotask ordering — Promises drain before setTimeout
• `await`-ing inside a loop when `Promise.all` would parallelize
• Forgetting unhandled rejections still fire even after a `.catch` is attached later
• Treating async/await as a magic sync conversion instead of syntactic sugar over Promises

Common interview patterns

• Pick the right hook: state vs ref vs reducer vs effect
• Use stable keys to keep list reconciliation O(n)
• Lift state up — or colocate down — based on who reads it
• Build controlled forms with validation and field-level errors

Common mistakes

• Using array index as a key for a list that reorders
• Reaching for `useEffect` for derived state instead of computing during render
• Mutating state in place instead of creating a new reference
• Creating new object/array literals in JSX that thrash child memo

Common interview patterns

• Cancel stale requests with `AbortController` and the take-latest pattern
• Compare `Cache-Control`, `ETag`, and `stale-while-revalidate`
• Walk through a CORS preflight and what the browser actually sends
• Pick WebSocket, SSE, or long polling based on directionality and reconnection cost

Common mistakes

• Ignoring CORS until it breaks in prod — credentialed requests have stricter rules
• Not setting `SameSite` on auth cookies
• Polling for data that an SSE stream could push for free
• Treating fetch as a drop-in for Axios without porting interceptors and retry logic

Common interview patterns

• Use discriminated unions to model `loading | success | error` UI state
• Pick between `interface` and `type` based on extension and declaration merging
• Build a generic helper with constraints and inferred return types
• Narrow with `in`, `typeof`, `instanceof`, and user-defined type guards

Common mistakes

• Reaching for `any` instead of `unknown` and narrowing
• Using enums where a string union would be lighter and more inferable
• Forgetting that interface declaration merging affects library typings globally
• Over-engineering generics that don't make any call site safer

Common interview patterns

• Separate server state (React Query/RTK Query) from client state (Zustand/Redux)
• Lift state only as far as the lowest common ancestor needs
• Use context for low-frequency values (theme, auth) and a store for high-frequency
• Model derived state as a selector, not a mirrored copy

Common mistakes

• Putting server response data in Redux when React Query would deduplicate it for free
• Subscribing to the whole store and re-rendering every consumer on any change
• Treating context as a state library — every consumer re-renders on any value change
• Building reducer logic for a value that one component owns