HTML, CSS, JavaScript and React interview questions

Required, type email, minlength, and pattern give native validation even if JS fails.

Lazy loading waits until the image is near the viewport, saving bandwidth.

A generic is a type parameter so a function or component works with many shapes while keeping the relationship.

Middleware wraps dispatch so I can log, crash-report, or run async.

A layout wraps child routes and does not remount on navigation inside that segment, so shared chrome stays mounted.

Unit tests cover a pure function or a small component with mocked children.

It is the first focusable link that jumps to main content so keyboard users skip the nav.

Any XSS on the origin can read localStorage and send the token to an attacker.

I name cells and draw the layout as ASCII in CSS, which is easier to review than long line and column numbers.

Slice returns a shallow copy of a portion and does not change the original, which is what I want in React.

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.

absolute positions relative to a positioned ancestor.

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

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.

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

I lift state carefully, keep props stable, and split components so heavy UI does not sit under chatty parents.