React interview questions and answers

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.

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.