React interview questions and answers

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.

The UI dispatches an action object.

A reducer is a function of previous state and action that returns the next state.

Classic Redux needed hand-written action types, switch statements, and spread copies.

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.

A custom hook is a function starting with use that calls other hooks, so I can reuse stateful logic.

React 18 batches setState in timeouts, promises, and native events so one event loop turn can become one render.

Middleware wraps dispatch so I can log, crash-report, or run async.

A layout wraps child routes and does not remount on navigation inside that segment, so shared chrome stays mounted.

Fiber is a linked list of work units so React can pause, reuse, and prioritize updates instead of one long recursive stack.

It lets React prepare a new tree in the background without blocking typing.

React 18 Strict Mode mounts, unmounts, and remounts to surface missing cleanup.

I include the value in the dependency array, or I read it from a ref that I update every render.

Server data goes in a cache like React Query with keys per widget.