HTML, CSS, JavaScript and React interview questions

The modern answer is flex on the parent with justify-content and align-items set to center, or grid with place-items center.

Flex-grow is how leftover space is shared

I use a grid with two columns on min-width 768 and one column in the base styles.

Z-index is compared only inside the same stacking context.

Fixed is relative to the viewport and stays put even when you scroll the page.

I start with a button that has aria-expanded and aria-controls pointing at the list.

Inside a form, the default type is submit, so a random button can post the form by accident.

Synchronous code runs on the call stack until it is empty.

Microtasks include Promise callbacks and queueMicrotask and they flush completely before the next macrotask.

Objects have an internal prototype link used when a property is missing.

It is the walk from an object to Object.prototype and then null while looking up a property.

I keep a timeout id, clear it on every call, and set a new timeout that fires the original function after wait ms.

Call and apply invoke the function now with a chosen this

I put one listener on a parent and use event.target to see which child was clicked.

StructuredClone is the modern built-in for many values.

Let and const exist in the scope from the start of the block but cannot be read until the declaration line.

Listeners can run on the way down from window to the target, then on the way up.

Cookies are small, sent with matching requests, and can be HttpOnly so JS cannot steal them.

A non-simple request triggers OPTIONS first.

A page can freely read data only from the same scheme, host, and port.