HTML, CSS, JavaScript and React interview questions

Hashed asset filenames can be cached immutable for a year while index.html stays no-cache so users get new hashes.

INP looks at interaction latency throughout the page life, not only the first input.

I run the HTML through a sanitizer with an allowlist of tags and no script, iframe, or event handlers.

A malicious site can POST to my API and the browser attaches SameSite-None cookies.

CSP tells the browser which script, style, and connect origins are allowed.

I mock the network, render, assert a loading state, then waitFor the items by role.

MSW intercepts at the network layer so the app code stays realistic.

I use a dialog with aria-modal, label it, and move focus to the first control on open.

Text needs enough contrast against its background so low-vision users can read it, with stricter ratios for small text.

When fetch uses credentials include, the server cannot reply Access-Control-Allow-Origin star.

They can serve shell HTML and assets offline, which is great for PWAs.

Synchronous logs run first.

Class Foo still has Foo.prototype and instances link to it.

Position sticky on th only works if ancestors do not have overflow hidden.

A stacking context is a local z-index world.

I wrap the await in try catch and set error state or show a toast.

The server can flush HTML in chunks as Suspense boundaries resolve instead of waiting for the whole tree.

In the effect I create a controller, pass signal to fetch, and abort in cleanup.

Each route becomes its own JS payload plus shared vendors.

I wrap with a real store preloaded with fixture state, or I render through the same provider the app uses.