HTML, CSS, JavaScript and React interview questions

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.

React compares the new element tree with the previous one by type and key.

Inserting or reordering makes React reuse the wrong component instance, so inputs keep the wrong text.

The effect closes over stale props or state from the render that created it.

UseMemo memoizes a computed value.

It skips a re-render when props are shallow-equal.

For low-frequency data like theme or auth user, Context is enough.

Context re-renders all consumers when the provider value changes, so putting a rapidly updating cart in a huge provider is costly.

Controlled means every keystroke goes through React state, which is required for instant validation and disabling submit.

Only call hooks at the top level of a React function, not in loops or conditions, so the order stays stable.

React relies on call order to match state cells.