HTML, CSS, JavaScript and React interview questions

ReactNode covers strings, numbers, fragments, and elements, which is what most layout components accept.

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

Reading layout after writes in a loop forces the browser to recalculate style repeatedly.

It is the work from HTML and CSS bytes to pixels: parse, CSSOM, render tree, layout, paint.

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

If we merge untrusted objects into a config with a recursive assign, an attacker can set __proto__ fields and change Object.prototype.

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

I profile first: if a cheap parent re-renders an expensive subtree, memo or moving state down can help.

Leading runs immediately, then ignores calls until the window ends

Aria-live polite announces new text without moving focus, which is right for toasts and search result counts.

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

As const on a route name array makes a union of literal strings instead of string.

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

Large HTML snapshots fail on class-name noise and get updated blindly.

I take heap snapshots before and after visiting a route twice and look for detached DOM nodes or growing arrays.

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

On desktop I show a list of links.

Escaping turns angle brackets into text so nothing executes

FOIT is invisible text while the webfont loads

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