Question 1

What are React Hooks? Name a few common ones.

Accepted Answer

**Target Audience:** Anyone preparing for a React interview — assumes components and props. **Difficulty:** Easy > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The hook inventory in section 4 was read directly off the installed React 19.2.8 rather than recalled, which matters because several were added recently. ## 1. Why This Even Matters — A Story First Imagine a workshop where only the master craftsmen were allo

Question 2

How do you handle side effects in functional components?

Accepted Answer

**Target Audience:** Frontend engineers preparing for React interviews — assumes basic familiarity with Hooks. **Difficulty:** Medium > **How to read this doc:** Every concept is explained in plain language first, then tagged with 📌 Interview term: — the exact vocabulary an interviewer expects. Every ordering and lifecycle claim below was produced by actually running React 19.2.8 in a real DOM (jsdom) in this repo, not described from documentation. ## 1. Why This Even Matters — A

Question 3

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 4

What are React Components?

Accepted Answer

**Target Audience:** Anyone preparing for a React interview — assumes JavaScript functions, no React required. **Difficulty:** Easy > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The instance and prototype claims in section 4 were produced by inspecting real components on React 19.2.8. ## 1. Why This Even Matters — A Story First Think about LEGO. A brick is small, does one thing, and — crucially — has a standard co

Question 5

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 6

RSC vs Client Components — when do you add 'use client' in Next.js 15?

Accepted Answer

**Target Audience:** Engineers preparing for Next.js interviews — assumes what Server Components are. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. **A note on verification:** this needs an RSC runtime, and react-server-dom-webpack is **not installed here**, so nothing below was executed. **Version n

Question 7

When would you choose a class component over a functional component with hooks?

Accepted Answer

**Target Audience:** Engineers preparing for React interviews — assumes hooks. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every claim below was **executed against React 19.2.8** — the export check, the hook enumeration, the boundary catching a real error, and the setState merge behaviour. ## 1. Why This Even Matters — A Story First A workshop replaces its hand tools with powe

Question 8

Explain the purpose of `useEffect`.

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:. The render counts in section 4 were measured on React 19.2.8. This doc focuses on *when an effect is the wrong tool*; see handling side effects for the broader strategy an

Question 9

What are Error Boundaries in React?

Accepted Answer

**Target Audience:** Engineers preparing for React interviews — no prior error-handling knowledge assumed. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The behaviour in section 3 was **executed against React 19.2.8**. The companion doc, what an Error Boundary can and cannot catch, covers the limits — which

Question 10

What are Effect Events (`useEffectEvent`) and what problem do they solve?

Accepted Answer

**Target Audience:** Engineers preparing for senior React interviews — assumes effects and dependency arrays. **Difficulty:** Hard > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The re-run counts in section 3 were measured on React 19.2.8, where useEffectEvent is a stable export. ## 1. Why This Even Matters — A Story First A courier is told to redeliver whenever the **address** changes. Sensible. They

REACT.JSDECODED.

Easy

What are React Hooks? Name a few common ones.

How do you handle side effects in functional components?

What is the purpose of `React.StrictMode`?

What are React Components?

Medium

Describe the React component lifecycle.

RSC vs Client Components — when do you add 'use client' in Next.js 15?

When would you choose a class component over a functional component with hooks?

Explain the purpose of `useEffect`.

What are Error Boundaries in React?

Hard

What are Effect Events (`useEffectEvent`) and what problem do they solve?

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

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

React Server Components vs SSR — what is the real difference?

useEffectEvent — separating reactive and non-reactive logic in Effects

The Activity API — hiding UI without unmounting in React 19

How do you safely handle async work and race conditions inside effects?

What can and can't an Error Boundary catch?