React interview questions and quick answers

No. Props are read-only; React treats them as immutable snapshots from the parent. To change a value, the parent passes down a callback, and the child calls it so the parent updates its own state.

Only when the list is static: never reordered, filtered, or inserted into. If items can move or be removed, index keys make React reuse the wrong component state and DOM, so use a stable ID from the data.

Pass props when only a few levels need the data. Use Context for values many components read but that change rarely, such as theme, language or the logged-in user. Reach for a state library when shared state is large, updates often, or needs tools like selectors and devtools.

In a controlled component, React state holds the input's value and every change goes through an onChange handler. In an uncontrolled component, the browser DOM keeps the value and you read it when needed, for example with a ref or from the form data on submit.

For a few fields that are only read on submit, uncontrolled inputs with FormData are enough and re-render less. Use controlled inputs when you need live validation, dependent fields, formatting as the user types or to disable the button.

Function components are plain functions that use hooks for state and lifecycle behaviour. Class components use this.state and lifecycle methods. New code should use function components; classes still work, and error boundaries still need a class.

Mutating an object prop, for example props.user.name = x, which breaks memoisation and causes stale UI.

"Props are read-only, so a child must never mutate them. The usual pattern is lifting state up: the parent owns the value and passes a callback like onChange. The child calls it, the parent updates state, and the child re-renders with the new prop."