Hiring a senior React developer is more than just checking technical skills—it’s about finding someone who understands the nuances of modern React, can build scalable applications, and knows how to make smart architectural decisions.

With React’s evolution over the years, the definition of “senior” has shifted. A developer who was experienced in class components and lifecycle methods five years ago might struggle today without a deep understanding of hooks, concurrent rendering, or server-side rendering.

So, how do you separate an experienced React engineer from someone who just follows tutorials? You ask the right questions—not just about syntax, but about design patterns, performance, and modern best practices.

Here are 20 essential questions (with explanations) to help you assess whether a candidate truly knows React inside and out.

1. How has React evolved over the years, and how do modern components differ from older ones?

React has changed dramatically since its early days. The shift from class components to functional components with hooks was a game-changer, making code more readable, maintainable, and efficient.

A senior developer should be able to explain that:

• Before hooks, state and side effects were managed using lifecycle methods (componentDidMount, componentDidUpdate), which often led to messy, hard-to-maintain code.
• With hooks like useState and useEffect, components became more concise and reusable, avoiding pitfalls like this binding.

If a candidate still prefers class components over functional components without a strong reason, that’s a red flag.

2. What frameworks or tools do you prefer when starting a React project, and why?

React itself is a library, not a framework, so it often needs supporting tools. Senior engineers should have strong opinions on whether to use:

• Next.js – for server-side rendering (SSR), static site generation, and improved SEO.
• Vite – a modern alternative to Create React App (CRA) that offers faster build times and better developer experience.
• Remix – for full-stack React applications with built-in data loading and better nested routing.

A solid answer shows they understand trade-offs—for example, CRA is still useful for quick prototypes but falls short for production-ready apps.

3. Why is TypeScript so widely used with React, and what’s its biggest advantage?

Any senior React dev should recognize that TypeScript isn’t just about types—it’s about better developer experience and maintainability.

TypeScript offers:

• Static typing, which catches errors at compile time instead of runtime.
• Stronger prop validation, reducing runtime bugs.
• Better autocompletion and tooling, making large-scale React projects easier to navigate.

While TypeScript has a learning curve, avoiding it in large codebases is rarely a good idea.

4. Styled-components vs. CSS Modules vs. Sass—what’s the best way to style React components?

There’s no one-size-fits-all answer, but an experienced developer should know the strengths of each:

• Styled-components – best for scoping styles inside components and avoiding global CSS conflicts.
• CSS Modules – great for modular CSS with class names, making styles reusable while keeping them separate from JavaScript logic.
• Sass – useful for complex global styles but can become hard to maintain in large React projects.

Their answer should reflect real-world project experience, not just theoretical knowledge.

5. What’s the real benefit of React hooks, and why were they introduced?

Hooks solved a major issue in React: component complexity.

Before hooks, developers relied on class components with lifecycle methods that became hard to manage as components grew. Hooks like useState, useEffect, and useReducer allowed functional components to handle state and side effects without needing classes.

A senior developer should be able to explain not just what hooks do, but why they exist—and when to use or avoid them.

6. What’s the difference between useEffect and useLayoutEffect, and when should you use each?

This question separates surface-level knowledge from deeper understanding.

• useEffect runs asynchronously after the browser has painted the screen—perfect for API calls and data fetching.
• useLayoutEffect runs synchronously before the browser repaints, making it useful for DOM measurements (e.g., calculating element positions).

If a candidate can explain when useLayoutEffect might cause performance issues, they really understand React’s rendering behavior.

7. How do you prevent unnecessary re-renders in React?

Performance optimization is a critical skill. A strong React developer should discuss:

• React.memo – prevents re-renders if props haven’t changed.
• useMemo & useCallback – memoize expensive calculations and function references.
• Keeping state local – avoid unnecessary reactivity at the component level.

Common mistake? Using useMemo everywhere—even when it’s unnecessary. A true expert knows when to optimize and when to keep it simple.

8. What are common mistakes developers make with useEffect?

A few classic pitfalls:

• Forgetting dependencies, leading to stale closures and incorrect data.
• Triggering unnecessary API calls, causing excessive network requests.
• Not cleaning up effects, leading to memory leaks (e.g., missing return () => cleanup() in event listeners).

A senior developer should know how to properly structure effects to avoid performance issues.

9. How would you structure a large-scale React application?

There’s no single “correct” way, but a good architecture includes:

• Feature-based folder structure instead of deeply nested folders.
• Atomic design principles for reusable components.
• Separate concerns – hooks, contexts, and utilities in dedicated directories.

If they mention modularization, lazy loading, and scalability, that’s a sign they’ve worked on large applications.

10. How do you debug performance issues in a React application?

A seasoned developer will rely on:

• React DevTools – to inspect re-renders and component updates.
• Chrome’s Performance tab – to track slow API calls and scripting delays.
• Profiling hooks (useMemo, useCallback) – to optimize computations.

They should also highlight that memory leaks and excessive re-renders cause 90% of performance problems in React.

Final Thoughts

Hiring a senior React developer isn’t just about knowing syntax—it’s about understanding architecture, scalability, and real-world problem-solving.

At Zarego, we specialize in staff augmentation, connecting businesses with top-tier React developers who seamlessly integrate into your team. Our engineers aren’t just skilled coders—they’re problem solvers who bring experience, strategic thinking, and efficiency to every project.

Looking to scale your team with high-caliber React developers? Let’s talk.