React Interview Questions for 5 Years Experience

useEffect runs asynchronously after the browser paints — used for data fetching, subscriptions, and side effects that don't need to block rendering. useLayoutEffect runs synchronously after DOM mutations but before browser paint — use only when you need to measure DOM elements or apply CSS changes that would cause visible flicker. In most cases, useEffect is correct. useLayoutEffect is for rare cases like tooltip positioning.

Reconciliation is React's algorithm to determine what changed between renders and update only the necessary DOM nodes. React compares virtual DOM trees using the diffing algorithm: (1) Elements of different types → rebuild entire subtree, (2) Elements of same type → update attributes, (3) Lists → use key prop to identify which items moved/added/removed. Always provide stable, unique keys for list items — using index as key breaks reordering.