HTML, CSS, JavaScript and React interview questions

The effect starts after paint, fetch is scheduled, and the function yields at await so React can keep working.

I reproduce by changing filters quickly and logging request ids.

Fiber is a linked list of work units so React can pause, reuse, and prioritize updates instead of one long recursive stack.

It lets React prepare a new tree in the background without blocking typing.

React 18 Strict Mode mounts, unmounts, and remounts to surface missing cleanup.

I include the value in the dependency array, or I read it from a ref that I update every render.

Server data goes in a cache like React Query with keys per widget.

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.

I use a status field like idle, loading, success, error and narrow with a switch.

Interfaces with the same name in the same scope merge their members, which is useful for extending libraries.

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 record a load, find the LCP marker, and see whether the delay is TTFB, render-blocking CSS, or late image discovery.

I use Web Vitals CLS and layout shift regions in Performance.