HTML, CSS, JavaScript and React interview questions

Bundlers drop unused ES module exports when they can prove they are unused.

I render the form, find fields by label text, type with userEvent, and click the submit button by role.

Classes change during restyles even when the user experience did not.

Aria-label is a string on the element itself.

Native input type checkbox is best.

If you assign a string that contains a script or an onerror handler from the user, the browser will execute it.

HttpOnly stops document.cookie from reading the session, which reduces XSS token theft.

GetStaticProps builds HTML at build time, great for marketing pages.

The HTML from the server does not equal what React renders on the client for the first pass.

They catch errors during render, lifecycle, and constructors of children via componentDidCatch.

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.