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Junior React Interview Questions

Curated Junior-level React interview questions for developers targeting junior positions. 35 questions available.

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React Interview Questions & Answers

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Welcome to our comprehensive collection of React interview questions and answers. This page contains expertly curated interview questions covering all aspects of React, from fundamental concepts to advanced topics. Whether you're preparing for an entry-level position or a senior role, you'll find questions tailored to your experience level.

Our React interview questions are designed to help you:

  • Understand core concepts and best practices in React
  • Prepare for technical interviews at all experience levels
  • Master both theoretical knowledge and practical application
  • Build confidence for your next React interview

Each question includes detailed answers and explanations to help you understand not just what the answer is, but why it's correct. We cover topics ranging from basic React concepts to advanced scenarios that you might encounter in senior-level interviews.

Use the filters below to find questions by difficulty level (Entry, Junior, Mid, Senior, Expert) or focus specifically on code challenges. Each question is carefully crafted to reflect real-world interview scenarios you'll encounter at top tech companies, startups, and MNCs.

Questions

35 questions
Q1:

How does React reconcile two virtual DOM trees during re-rendering?

Junior

Answer

React uses its diffing algorithm to compare the previous and next virtual DOM nodes. It reuses elements with the same type and key while replacing mismatches, ensuring minimal DOM operations.
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Q2:

Why is the render process pure but the commit phase is not in React?

Junior

Answer

The render phase only calculates changes and contains no side effects, making it pure. The commit phase applies changes to the DOM, causing side effects and making it impure.
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Q3:

What problem do React Server Components solve?

Junior

Answer

They reduce client bundle size by performing data fetching and heavy computation on the server and sending lightweight serialized output to the client.
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Q4:

Why is data fetching in useEffect considered an anti-pattern in React 18+?

Junior

Answer

Fetching in useEffect runs after rendering, causing UI flicker. Modern React encourages server components or framework-level data loading.
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Q5:

What is the difference between batching in React 17 and React 18?

Junior

Answer

React 17 batched updates only inside React events. React 18 batches updates automatically in async operations like promises and timeouts.
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Q6:

Why does React warn when state is updated during rendering?

Junior

Answer

Updating state during render breaks render purity and creates infinite re-render loops.
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Q7:

What is the purpose of the useTransition hook?

Junior

Answer

useTransition marks certain state updates as non-urgent, keeping the UI responsive while React processes expensive updates in the background.
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Q8:

How does React decide which components to re-render?

Junior

Answer

React re-renders a component when its state or props change or when its parent re-renders. Siblings do not re-render unless their props change.
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Q9:

Why must React state remain immutable?

Junior

Answer

React detects state changes using reference equality. Mutating state directly keeps the same reference, preventing React from re-rendering.
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Q10:

What happens when state setters run inside loops outside event handlers?

Junior

Answer

It can trigger multiple renders or infinite loops because render must stay pure and deterministic.
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Q11:

What is the Render Prop pattern?

Junior

Answer

It is a technique where a component receives a function as a child to dynamically render UI, enabling reusable logic.
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Q12:

What problem does React Context solve?

Junior

Answer

Context avoids prop drilling by allowing components to access shared data without passing props through every level.
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Q13:

How does React Context differ from Redux?

Junior

Answer

Context provides simple global state but triggers broad re-renders. Redux offers structured state management, middleware, and optimized performance.
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Q14:

What does useReducer help solve?

Junior

Answer

useReducer manages complex state logic with multiple transitions or nested updates, offering a structured alternative to multiple useState hooks.
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Q15:

Why should objects or arrays not be placed directly in dependency arrays?

Junior

Answer

Objects and arrays create new references every render, causing effects to re-run unnecessarily.
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Q16:

What is memoization in React and when is React.memo useful?

Junior

Answer

React.memo caches rendered output and prevents re-renders when props have not changed, improving performance for stable components.
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Q17:

What is the difference between useMemo and useCallback?

Junior

Answer

useMemo memoizes computed values, while useCallback memoizes function references.
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Q18:

Why is lifting state up used in React?

Junior

Answer

Shared state must be managed in a common parent to keep child components synchronized.
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Q19:

What are controlled inputs and why can they hurt performance?

Junior

Answer

Controlled inputs update React state on each keystroke, causing re-renders that may slow down long forms.
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Q20:

What is Suspense and why was it introduced?

Junior

Answer

Suspense lets components wait for async resources with a fallback UI. It supports streaming and async rendering.
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Q21:

How does Suspense differ in data fetching vs code splitting?

Junior

Answer

For code splitting it waits for dynamic imports; for data fetching it waits for a Promise to resolve.
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Q22:

What is hydration in React?

Junior

Answer

Hydration attaches event listeners and React behavior to server-rendered HTML to make it interactive.
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Q23:

What is partial hydration?

Junior

Answer

Only selected components are hydrated, reducing JavaScript execution and improving performance.
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Q24:

How does React handle synthetic events?

Junior

Answer

React attaches a single root event listener and normalizes events across browsers for consistency and performance.
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Q25:

What is useDeferredValue used for?

Junior

Answer

It returns a deferred version of a rapidly changing value, allowing heavy UI updates to lag behind typing.
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Q26:

How does React 19 improve hydration?

Junior

Answer

React 19 supports partial hydration, reduces long tasks, and hydrates server components more efficiently.
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Q27:

What is useOptimistic in React 19?

Junior

Answer

useOptimistic enables optimistic UI updates before server confirmation, improving perceived performance.
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Q28:

What is the difference between the use hook and await in React 19?

Junior

Answer

The use hook unwraps promises directly during rendering and pauses rendering until resolved, working in server components.
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Q29:

Why shouldn’t data fetching occur in constructors or render methods?

Junior

Answer

Fetching causes side effects. Render and constructors must remain pure, so fetching should occur in effects or server loaders.
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Q30:

What is an error boundary?

Junior

Answer

An error boundary catches runtime errors in child components and displays a fallback UI instead of crashing the entire app.
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Q31:

Why shouldn’t you mutate refs?

Junior

Answer

Refs are not reactive. Mutating them will not trigger re-renders, so using them to drive UI updates is ineffective.
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Q32:

What does useImperativeHandle do?

Junior

Answer

It customizes the instance value exposed to parent components when using refs, often exposing methods like focus().
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Q33:

What are custom hooks and why are they important?

Junior

Answer

Custom hooks extract reusable logic from components, making code cleaner, modular, and easier to maintain.
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Q34:

Why is having too many states in a component problematic?

Junior

Answer

Too many states cause unnecessary re-renders, tightly coupled logic, and reduced component reusability.
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Q35:

Why do React 18+ and React 19 emphasize asynchronous rendering?

Junior

Answer

Async rendering prevents UI blocking during heavy updates, improving responsiveness and reducing jank.
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People Also Ask - Related React Questions

How does React reconcile two virtual DOM trees during re-rendering?
Why is the render process pure but the commit phase is not in React?
What problem do React Server Components solve?
Why is data fetching in useEffect considered an anti-pattern in React 18+?
What is the difference between batching in React 17 and React 18?
Why does React warn when state is updated during rendering?
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