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 is the role of `react-dom`?

Accepted Answer

**Target Audience:** Frontend engineers preparing for React interviews — assumes basic familiarity with components. **Difficulty:** Easy to Medium > **How to read this doc:** Every concept is explained in plain language first. Right after, you will see a callout like 📌 Interview term: — that is the exact vocabulary a senior engineer would use for the same idea. Every package-export and behavioural claim below was produced by actually running React 19.2.8 in this repo, not describe

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 JSX?

Accepted Answer

**Target Audience:** Anyone preparing for a React interview — assumes JavaScript basics, no React required. **Difficulty:** Easy > **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 actually running a JSX source file through this project bundler toolchain, and the escaping behaviour in section 5 by rendering it in React 19.2.8. ## 1. Why This Even Matters — A Story First M

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 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 7

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 8

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 9

How does React handle security concerns like XSS?

Accepted Answer

**Target Audience:** Engineers preparing for React interviews — assumes basic JSX. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every security behaviour below was produced by actually rendering the hostile input in React 19.2.8 and reading back the resulting DOM — including one result that contradicts a lot of older writing on this topic. ## 1. Why This Even Matters — A Story First A good l

Question 10

The `use()` API with Suspense — how do you unwrap promises in render?

Accepted Answer

`use()` is a React 19 primitive that unwraps a Promise or Context during render.

REACT.JSDECODED.

Easy

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

What is the role of `react-dom`?

How do you pass data between components in React?

What is JSX?

What is the purpose of `React.StrictMode`?

What are React Components?

Medium

What are React's design principles?

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

How does React handle security concerns like XSS?

The `use()` API with Suspense — how do you unwrap promises in render?

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?

How do you fetch data in React?

Hard

What causes stale closures in React hooks, and how do you fix them correctly?

What problem does `useSyncExternalStore` solve?

How does streaming SSR with selective hydration work?

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

How does `useOptimistic` enable optimistic UI in React 19?

The Activity API — hiding UI without unmounting in React 19

What is "tearing" in concurrent React?

What are the resource preloading APIs in React 19 (`preload`, `preinit`, etc.)?