Question 1

What triggers a React re-render?

Accepted Answer

State/context updates in that component (or parents re-rendering children), and hooks that schedule updates. Re-render ≠ DOM update. Reconciliation decides DOM mutations. Children re-render by default when parents do.

Question 2

Render vs commit phase?

Accepted Answer

Render calculates the next element tree; commit applies DOM changes and runs layout/effects. Understanding phases helps reason about Suspense and concurrent features.

Question 3

What is concurrent rendering in React 18+?

Accepted Answer

React can interrupt, prioritize, and reuse work so urgent updates stay responsive. Features: startTransition, Suspense, selective hydration. Write pure renders.

Question 4

How does Suspense change loading UX?

Accepted Answer

Declaratively show fallbacks while lazy components or data resolve. Coordinate boundaries. Avoid waterfalls. Frameworks integrate data fetching with Suspense differently.

Question 5

Why did my child re-render when props look the same?

Accepted Answer

Parent re-rendered and child isn't memoized; or props are new references each time. Use React DevTools highlight updates. Stabilize props or memoize carefully.

Question 6

Automatic batching — what changed in React 18?

Accepted Answer

Updates inside timeouts/promises batch too, not only React event handlers. Fewer intermediate renders. Use flushSync sparingly when you must read DOM immediately.

Question 7

Architecture tip for render performance?

Accepted Answer

State down, components up: keep state close; split contexts; virtualize lists; profile before memo everywhere. Structure beats micro-memoization.

Rendering React Interview Questions

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