Question 1

Explain the concept of 'lifting state up' in React.

Accepted Answer

**Target Audience:** Engineers preparing for React interviews — assumes props and state. **Difficulty:** Easy > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. This is the focused technique; passing data between components surveys all the mechanisms, and unidi

Question 2

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 3

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 4

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 5

What are render props and how do they work?

Accepted Answer

**Target Audience:** Engineers preparing for React interviews — assumes props and state. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The memoisation measurement in section 5 was produced by counting real renders on React 19.2.8. ## 1. Why This Even Matters — A Story First A weather station measures temperature. It has no opinion about whether you want a number, a dial, or a chart — so inst

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

What is the Context API in React and when would you use it?

Accepted Answer

**Target Audience:** Engineers preparing for React interviews — assumes props and state. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The default-value and re-render behaviour below were measured on React 19.2.8. This doc covers *what Context is and when to use it*; why it re-renders every consumer covers

Question 8

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 9

What are React custom hooks and when should you use them?

Accepted Answer

**Target Audience:** Engineers preparing for React interviews — assumes useState and useEffect. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The state-independence result in section 3 was produced by mounting two components that use the same hook and clicking one of them on React 19.2.8. ## 1. Why This Even Matters — A Story First A recipe for bread is not a loaf o

Question 10

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

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 re-render figures quoted in section 4 were measured on React 19.2.8; see avoiding re-renders with composition for the full run. ## 1. Why This Even Matters — A

REACT.JSDECODED.

Easy

Explain the concept of 'lifting state up' in React.

How do you pass data between components in React?

What are React Components?

Medium

What are React's design principles?

What are render props and how do they work?

Explain the concept of 'composition' in React components.

What is the Context API in React and when would you use it?

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

What are React custom hooks and when should you use them?

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

How do you manage global state in a large React application?

Hard

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

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

When does `React.memo` do nothing — or actually hurt performance?