HTML, CSS, JavaScript and React interview questions

It tells you when an element enters the viewport without scroll listeners.

PushState and replaceState change the URL without a full reload, which SPAs use for routing.

Inline styles beat classes and ids unless a rule uses important.

All fails fast if any promise rejects, which is good when every request is required.

The question mark dot stops the lookup and returns undefined if the left side is null or undefined.

A custom hook is a function starting with use that calls other hooks, so I can reuse stateful logic.

Document.cookie can set cookies the page can also read, so XSS can steal them.

They are variables defined with a double-dash name, often on :root, and read with var.

React 18 batches setState in timeouts, promises, and native events so one event loop turn can become one render.

If I add a window resize listener in useEffect and forget the cleanup, the handler keeps the component closure alive after unmount.

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.