Question 1

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 2

What is React and its main features?

Accepted Answer

**Target Audience:** Anyone preparing for a React interview — assumes no prior React knowledge. **Difficulty:** Easy > **How to read this doc:** Every concept is explained in plain language first. Right after, you will see a callout like 📌 Interview term: — the exact vocabulary an interviewer expects. The object-shape claims below were produced by running React 19.2.8 in this repo. ## 1. Why This Even Matters — A Story First Imagine decorating a room by giving instructions over

Question 3

What are React's design principles?

Accepted Answer

**Target Audience:** Engineers preparing for senior React interviews — assumes broad familiarity with the API. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. This is a discussion question rather than a mechanical one, so instead of a "Verified" section it cites the React team own published principles and ties each to a concrete API you already use. ## 1. Why This Even Matters — A Story First

Question 4

How do you handle asynchronous operations in React functional components?

Accepted Answer

**Target Audience:** Engineers preparing for React interviews — assumes useEffect and promises. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The race-condition outcome in section 4 and React warning text in section 3 were both produced by running the code on React 19.2.8, not described from memory. ## 1. Why This Even Matters — A Story First You order a taxi, change your mind,

Question 5

How do you test React components?

Accepted Answer

**Target Audience:** Engineers preparing for React interviews — assumes basic testing familiarity. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Section 3 is **executed** — the same three assertions run against a component and against a **pure refactor** of it, using the Testing Library version installed in this repo. ## 1. Why This Even Matters — A Story First A restaurant inspector could g

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

How do you fetch data in React?

Accepted Answer

**Target Audience:** Engineers preparing for React interviews — assumes useEffect. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. This doc is the decision landscape; async operations covers the mechanics and race conditions, and

Question 8

How do you handle forms in React?

Accepted Answer

**Target Audience:** Engineers preparing for React interviews — assumes state and events. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The FormData and useActionState results below were produced by submitting real forms on React 19.2.8. ## 1. Why This Even Matters — A Story First Two ways to take an order in a restaurant. The waiter can write each item down as you

Question 9

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 10

How do you interact with external APIs in React?

Accepted Answer

**Target Audience:** Engineers preparing for React interviews — assumes fetch and hooks. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. This doc is about **integration architecture**; how to fetch data in React covers choosing a fetching strategy, and

REACT.JSDECODED.

Easy

How do you pass data between components in React?

What is React and its main features?

Medium

What are React's design principles?

How do you handle asynchronous operations in React functional components?

How do you test React components?

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

How do you fetch data in React?

How do you handle forms in React?

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

How do you interact with external APIs in React?

Hard

What problem does `useSyncExternalStore` solve?

What are Portals in React and when would you use them?

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

Why must React state updates be immutable?

Compare render-as-you-fetch, fetch-on-render, and fetch-then-render.

What does the `use()` hook do, and why is it special?

cache() and fetch memoization — request deduplication in RSC

How does React 19 handle document metadata like `<title>` and `<meta>`?