React interview questions and answers

When the next state depends on several fields and event types, a reducer documents the transitions in one place.

I virtualize so only visible rows mount, using a library like tanstack virtual.

Debounce delays when the update even starts.

Reselect or createSelector returns the same reference if input slices did not change, so useSelector's equality check skips render.

Context is for rare global UI config.

SSR is always fresh and more expensive per request, good for user-specific HTML.

They run on the server, can read files or databases, and ship zero client JS for that component.

I keep secrets in server-only modules and never pass them as props to client components.

Server and client format Date differently by timezone, or I generate Math.random in render.

I wrap the await in try catch and set error state or show a toast.

The server can flush HTML in chunks as Suspense boundaries resolve instead of waiting for the whole tree.

UseLayoutEffect runs after DOM updates but before the browser paints, so I can measure and adjust without flicker.

Middleware runs on the Edge before the request hits a page, good for redirects, A/B cookies, and auth gates.

Error boundaries still require a class or a small wrapper library because there is no hook for it.

Normalization stores entities by id in a map plus an array of ids for order.

Streaming can send the shell quickly so TTFB and first paint improve even if a slow widget is still pending.

React matches by key, so state lives with the identity, not the position.

I keep a small set of variants, sizes, and an asChild or polymorphic pattern so consumers do not fork the CSS.

useMemo caches an expensive calculated value between renders.

React.memo wraps a component so it skips re-render when props are shallow-equal.