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React Refs: Memory Without Re-renders
A React ref is like a secret pocket for your component to remember information without triggering re-renders. Use it to hold values not needed for rendering, like a timer ID. The footgun: changing a ref's value won't update the UI; use state for that.
What will count be after three setCount calls in a row?
Tests React state batching and stale closures. Answer: count is 1 because all three calls read the same closed-over value of 0. Fix: pass an updater function setCount(c => c + 1) three times so React queues each update.
React's Rule: Favor Composition Over Inheritance
Instead of extending a base component, build complex UIs by wrapping and configuring smaller ones. Use this for generic containers like dialogs or for creating specialized versions of a component.
How do you fetch data with useEffect and prevent memory leaks?
This tests useEffect async lifecycle and cleanup. A good answer places fetch inside useEffect, uses an ignore flag or AbortController to block state updates after unmount, and includes deps. A red flag is skipping cleanup and letting a late response set state.
What is the purpose of the useEffect cleanup function?
Explain cleanup prevents memory leaks by clearing timers or unsubscribing when dependencies change or on unmount.
Why pass a function to useState for expensive initial values?
This tests your understanding of React's render-phase behavior. Without the function, expensive work reruns on every render and is thrown away; the initializer runs once only during mount.
When would you choose useLayoutEffect over useEffect?
Use useLayoutEffect to measure or mutate DOM before paint to stop flicker; useEffect runs after paint.
Why is exhaustive-deps critical and when can you disable it?
This tests closure and effect timing intuition. A strong answer explains missing deps let effects read stale render values, gives a concrete override like a ref or stable callback, and warns that silencing the rule hides refactor hazards.
What is prop drilling in React?
Define it as threading props through middle layers; give a three-layer example like a theme toggle; cite useContext.
Custom Hooks: Package Component Logic for Reuse
A custom hook packages stateful logic (like useState and useEffect) into a reusable function. Use one when multiple components need the same logic, like tracking online status for a status bar and a save button. Footgun: Names must start with use.
How does children work, and why is it fundamental to composition?
Tests React composition knowledge. Good answer: children is a prop holding nested JSX; a Card wrapper renders {children} to let parents inject markup; contrast with prop-driven text. Red flag: calling children magic syntax instead of a standard prop.
The useReducer Hook: Predictable State Updates
useReducer is a state machine for your components, managing complex state changes predictably. Use it when state logic is complex or the next state depends on the previous one, like in a shopping cart.
What is a custom hook? Write a simple useToggle example.
What it tests: Whether you see hooks as stateful logic extraction. Outline: Define a use-prefixed function calling hooks; write useToggle with useState and callback; explain extraction vs duplication. Red flag: Helpers without hooks or missing use prefix.
React's useCallback: Cache Functions, Not Just Values
useCallback gives you the same function instance across re-renders, as long as its dependencies are stable. This is key for optimizing child components with React.memo or preventing useEffect from re-running.
Explain useReducer and when to prefer it over useState
Tests if you know useReducer centralizes complex state transitions better than scattered useState. Answer: define reducer/dispatch, then contrast with useState via interdependent fields or deep updates. Red flag: calling it only for global state or Redux.
useMemo: Cache Expensive Calculations in React
The useMemo hook caches a function's return value, preventing slow calculations from running on every render. Use it to skip heavy data filtering unless its inputs change. The main footgun is a missing dependency, which leads to stale, cached data.
Use useContext to provide theme state without prop drilling
This tests global state sharing without prop drilling. A strong answer uses createContext and a Provider with useState at the app root, then calls useContext in nested components. Red flag: ignoring re-render costs or placing the Provider too deep.
React's Stale Closure Problem in Hooks
A useEffect closure captures props and state from its render. If the effect doesn't re-run, its functions can operate on old, "stale" data. This surfaces in timers or socket listeners. The footgun is omitting dependencies, which causes these subtle bugs.
Explain the difference between useMemo and useCallback
This tests whether useMemo caches values while useCallback caches functions, and why that matters for React.memo. A strong answer notes React.memo uses Object.is, so a new inline function prop triggers a child re-render. Red flag: claiming they are the same.
useLayoutEffect: Synchronous Effects Before Browser Paint
useLayoutEffect is useEffect's synchronous twin, running its code after DOM mutations but before the browser repaints. Use it to read DOM layout and re-render to prevent visual flickers, like positioning a tooltip. Its main footgun: it blocks rendering.