HTML, CSS, JavaScript and React interview questions

I select the node and call addEventListener with click and a function.

After a handler on the target runs, the event travels up through parents to document.

A Promise represents a value that will arrive later: pending, then fulfilled or rejected.

Async functions always return a Promise.

Debounce waits until the user stops firing an event for N milliseconds, then runs once.

Throttle runs at most once per interval while the event keeps firing, like scroll or mousemove.

Browsers block a page on one origin from reading responses from another origin unless the server sends Access-Control-Allow-Origin.

LocalStorage is for non-sensitive UI preferences, larger, and never sent automatically with requests.

SessionStorage is like localStorage but tied to the tab and cleared when that tab closes.

It is React's in-memory description of the UI as a tree of elements.

Keys tell React which item is which across renders so it can reuse DOM and state.

The input value comes from state and onChange updates that state, so React is the source of truth.

It stores a value for this component and queues a re-render when you call the setter.

JSX is HTML-like syntax that compiles to React.createElement or the new jsx runtime.

Props are inputs passed from parent to child and should be treated as read-only.

Context avoids threading the same prop through many layers, like theme or the current user.

Redux puts shared client state in one store with a strict update path: dispatch an action, reducer returns new state.

It catches wrong props, missing fields, and typo-level bugs before the browser.

Both describe object shapes

A union means a value can be one of several types, like string or number, or loading or success or error.