HTML, CSS, JavaScript and React interview questions

Rem is relative to the root html font size, so spacing stays consistent across the page.

It clips content that sticks outside the box and usually removes scrollbars.

Media queries apply CSS only when the viewport or device matches a condition like max-width.

JavaScript runs on one call stack.

Destructuring pulls values from arrays or objects into named variables in one line.

Spread copies array or object properties into a new literal.

useMemo caches an expensive calculated value between renders.

section means thematic grouping with meaning

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

absolute positions relative to a positioned ancestor.

The function you return from useEffect runs before the next effect and on unmount.

Component composition, context for sparse global values, or lightweight stores for frequent updates.

GET puts fields in the URL query string and is for safe reads you might bookmark.

Loading route or feature bundles on demand.

It is the browser steps from HTML and CSS to first painted pixels: parse DOM, parse CSSOM, render tree, layout, paint.

Keyboard support, aria-expanded, listbox roles, and focus management.

I target LCP with optimized hero images and server or static HTML where possible.

It sets spacing between items without margin hacks.

Promise callbacks are microtasks and run before the next macrotask like setTimeout.

A real button gets keyboard focus, Enter/Space activation, and the right role for free.