Question 1

What is `PropTypes` and when would you use it?

Accepted Answer

**Target Audience:** Engineers preparing for React interviews — assumes basic component knowledge. **Difficulty:** Easy > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. This topic has a genuine breaking change behind it, so the central claim below was verified by actually rendering a component with propTypes on React 19.2.8 and by checking the React upgrade guide — not recalled. ## 1. Why This Even Matter

Question 2

What are React Developer Tools and how do you use them?

Accepted Answer

**Target Audience:** Engineers preparing for React interviews — assumes you have built a React app. **Difficulty:** Easy > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The React 19 API surface referenced in section 6 was read off the installed React 19.2.8 in this repo. ## 1. Why This Even Matters — A Story First The browser Elements panel shows you the finished building: walls, doors, paint. What it cannot show yo

Question 3

Explain the concept of 'unidirectional data flow' in React.

Accepted Answer

**Target Audience:** Anyone preparing for a React interview — assumes props and state. **Difficulty:** Easy > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The prop-mutation result in section 4 was produced by actually assigning to a prop on React 19.2.8 and catching what happened. ## 1. Why This Even Matters — A Story First A newspaper has one editor per section. Reporters file copy upward; the editor decides what

Question 4

How do you debug React applications?

Accepted Answer

**Target Audience:** Engineers preparing for React interviews — assumes hooks and component basics. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The React 19 debugging API named in section 5 was read off the installed React 19.2.8 in this repo rather than recalled. ## 1. Why This Even Matters — A Story First A doctor presented with "I feel unwell" does not start by ordering every test in th

Question 5

What is the purpose of `React.StrictMode`?

Accepted Answer

**Target Audience:** Engineers preparing for React interviews — assumes familiarity with useEffect. **Difficulty:** Easy to Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The render and effect counts below were produced by actually mounting components with and without StrictMode on React 19.2.8 in this repo. ## 1. Why This Even Matters — A Story First A flight instructor does not wa

Question 6

What is the role of a bundler like Webpack in a React project?

Accepted Answer

**Target Audience:** Engineers preparing for React interviews — assumes you have run a build. **Difficulty:** Easy to Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The compiled output in section 3 was produced by running a real JSX file through this project own compiler toolchain, not copied from documentation. ## 1. Why This Even Matters — A Story First A publisher does not hand a reader a box of loose man

Question 7

What is the purpose of the cleanup function in useEffect?

Accepted Answer

**Target Audience:** Engineers preparing for React interviews — assumes useEffect basics. **Difficulty:** Easy to Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The exact call sequence in section 3 was produced by rendering a component through three dependency changes and an unmount on React 19.2.8. ## 1. Why This Even Matters — A Story First Every time you check into a hotel you get a key. Ever

Question 8

Describe the React component lifecycle.

Accepted Answer

**Target Audience:** Engineers preparing for React interviews — assumes hooks basics. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every ordering below is **executed output from React 19.2.8**, not a description — the raw sequences are in sections 3 and 5. ## 1. Why This Even Matters — A Story First A market stall has three moments: setting up before opening, adjusting through the day as st

Question 9

How do you handle memory leaks in React?

Accepted Answer

**Target Audience:** Engineers preparing for React interviews — assumes useEffect familiarity. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The listener counts in section 3 and the unmount behaviour in section 6 were produced by executing real mount and unmount cycles against React 19.2.8 in this repo. ## 1. Why This Even Matters — A Story First You rent a meeting room and book

Question 10

Describe the benefits of using TypeScript with React.

Accepted Answer

**Target Audience:** Engineers preparing for React interviews — assumes basic TypeScript familiarity. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Section 3 is **five real compiler diagnostics**, produced by running the **TypeScript 7.0.2 installed in this repo** over a deliberately wrong component usage — exact codes and messages, not paraphrases. ## 1. Why This Even Matters — A Story First

REACT.JSDECODED.

Easy

What is `PropTypes` and when would you use it?

What are React Developer Tools and how do you use them?

Explain the concept of 'unidirectional data flow' in React.

How do you debug React applications?

What is the purpose of `React.StrictMode`?

What is the role of a bundler like Webpack in a React project?

What is the purpose of the cleanup function in useEffect?

Medium

Describe the React component lifecycle.

How do you handle memory leaks in React?

Describe the benefits of using TypeScript with React.

Hard

How do you optimize React component rendering?

Why does React 18 StrictMode double-invoke your components and effects?

Why must React state updates be immutable?