What is React?

React Basics Interview Questions

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❓ WHAT IS REACT?

(INTERVIEW-READY ANSWER)

React is an open-source JavaScript library used to build fast, scalable, and interactive user interfaces, especially for single-page applications.

But that answer alone is not enough in interviews.

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🧠 WHY REACT WAS CREATED? (VERY IMPORTANT)

Before React:

• Websites were mostly server-rendered
• Every user action caused a full page reload
• UI updates were slow and hard to manage

Facebook faced a huge problem:
👉 UI was getting complex and slow to update

So React was created to:

• Update only what changes
• Make UI predictable
• Improve performance

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⚙️ HOW REACT WORKS (INTERVIEW EXPLANATION)

React works using three core ideas:

1️⃣ Component-Based Architecture

• UI is divided into small reusable components
• Each component manages its own state

Example:

• Header
• Sidebar
• Dashboard
• Footer

This makes applications:
✔ Reusable
✔ Maintainable
✔ Testable

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2️⃣ Virtual DOM (MOST ASKED)

Instead of updating the real DOM directly:

• React creates a Virtual DOM
• Compares old vs new Virtual DOM (diffing)
• Updates only the changed part

👉 This is why React is fast.

Interview tip:
React does not update the entire DOM — only the minimal required changes.

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3️⃣ Declarative UI

In React:

• You describe what UI should look like
• React handles how to update it

Example:

“If state changes, UI should update automatically.”

This makes code:
✔ Predictable
✔ Easy to debug

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🔄 REACT IS A LIBRARY, NOT A FRAMEWORK

⚠️ Very common interview trap

React is a library, not a framework because:

• It focuses only on UI
• You choose routing, state management, HTTP libraries

Frameworks like Angular:

• Provide everything out of the box

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📌 REACT VS JAVASCRIPT

React:

• Uses JavaScript + JSX
• Makes UI easier to manage
• Handles re-rendering efficiently

JavaScript alone:

• Manual DOM manipulation
• Hard to scale large applications

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💡 REAL INTERVIEW QUESTION

❓ Why do companies prefer React?

✅ Answer:

• Reusable components
• Better performance
• Large ecosystem
• Strong community
• Backed by Meta (Facebook)

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🚫 COMMON MISTAKES CANDIDATES MAKE

❌ Saying React is a framework
❌ Not explaining Virtual DOM properly
❌ No real-world reasoning

Avoid these in interviews.

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🎯 WHEN SHOULD YOU USE REACT?

• Single Page Applications
• Dashboards
• Real-time applications
• Large-scale frontend systems

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🧠 ONE-LINE INTERVIEW SUMMARY

“React is a JavaScript library that builds fast and scalable UIs using components, Virtual DOM, and declarative rendering.”

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Read other awesome articles in Medium.com or in akcoding’s posts.

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