Question 1

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 2

What is the purpose of the `key` prop when rendering lists in React?

Accepted Answer

**Target Audience:** Engineers preparing for React interviews — assumes rendering lists. **Difficulty:** Easy to Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The mismatch in section 3 was produced by typing into a real input and then reordering the list on React 19.2.8. ## 1. Why This Even Matters — A Story First A cloakroom numbers its hooks rather than its coats. Hook 1, hook 2, hook 3. It works perfectl

Question 3

How do you pass data between components in React?

Accepted Answer

**Target Audience:** Engineers preparing for React interviews — assumes props and state basics. **Difficulty:** Easy to Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The composition measurement referenced in section 6 was produced by counting real renders on React 19.2.8; see composition for the full figures. ## 1. Why This Ev

Question 4

What is the Virtual DOM in React?

Accepted Answer

**Target Audience:** Anyone preparing for a React interview — assumes basic components. **Difficulty:** Easy > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The reconciliation behaviour in section 5 was produced by actually swapping an element type on React 19.2.8 and observing what happened to the child state. ## 1. Why This Even Matters — A Story First An architect wants three changes to a building. They do not de

Question 5

Describe the `useRef` hook and its typical use cases.

Accepted Answer

**Target Audience:** Engineers preparing for React interviews — assumes state and effects. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The render counts in section 3 were produced by mutating a ref repeatedly on React 19.2.8 and counting component invocations. ## 1. Why This Even Matters — A Story First A kitchen has a whiteboard facing the room and a notepad in a drawer. Change the whiteb

Question 6

Explain the concept of 'composition' in React components.

Accepted Answer

**Target Audience:** Engineers preparing for React interviews — assumes props and components. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The render counts in section 4 were produced by actually counting renders on React 19.2.8 in this repo — this is the part of the answer that most candidates only assert. ## 1. Why This Even Matters — A Story First Consider two ways to build a range of co

Question 7

How does React's Batching work?

Accepted Answer

**Target Audience:** Engineers preparing for React interviews — assumes useState. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every render count below was measured on React 19.2.8 by clicking real buttons and counting component invocations. ## 1. Why This Even Matters — A Story First A waiter takes your order: main, side, drink. They do not walk to the kitchen three times. The

Question 8

What is the difference between useMemo and useCallback?

Accepted Answer

**Target Audience:** Engineers preparing for React interviews — assumes hooks and re-renders. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The recompute and identity counts in section 3 were measured across three real renders on React 19.2.8. ## 1. Why This Even Matters — A Story First Two different things get cached in a kitchen. The **stock** — simmered for hours — is made once and kept,

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

How do you prevent unnecessary re-renders in functional components?

Accepted Answer

**Target Audience:** Engineers preparing for React interviews — assumes memo and hooks. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The render counts quoted below were measured on React 19.2.8 across this collection; see avoiding re-renders with composition and

REACT.JSDECODED.

Easy

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

What is the purpose of the `key` prop when rendering lists in React?

How do you pass data between components in React?

What is the Virtual DOM in React?

Medium

Describe the `useRef` hook and its typical use cases.

Explain the concept of 'composition' in React components.

How does React's Batching work?

What is the difference between useMemo and useCallback?

How do you handle memory leaks in React?

How do you prevent unnecessary re-renders in functional components?

What is 'prop drilling' and how can it be avoided?

How do you optimize performance in a React application?

Hard

How do you optimize React component rendering?

How do you avoid re-renders with composition instead of memoization?

What is `useDeferredValue` and how does it differ from debouncing?

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

How do Suspense data-fetching waterfalls happen, and how do you avoid them?

What is server-side rendering (SSR) and when would you use it with React?

React Compiler — how does auto-memoization work in React 19?

Why does updating Context re-render all its consumers, and how do you fix it?