Question 1

How do you debug React applications?

Accepted Answer

**Target Audience:** Engineers preparing for React interviews — assumes hooks and component basics. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The React 19 debugging API named in section 5 was read off the installed React 19.2.8 in this repo rather than recalled. ## 1. Why This Even Matters — A Story First A doctor presented with "I feel unwell" does not start by ordering every test in th

Question 2

What is the purpose of the cleanup function in useEffect?

Accepted Answer

**Target Audience:** Engineers preparing for React interviews — assumes useEffect basics. **Difficulty:** Easy to Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The exact call sequence in section 3 was produced by rendering a component through three dependency changes and an unmount on React 19.2.8. ## 1. Why This Even Matters — A Story First Every time you check into a hotel you get a key. Ever

Question 3

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 4

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 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

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 7

Async Server Components & waterfall prevention — fetching without blocking

Accepted Answer

**Target Audience:** Engineers preparing for React interviews — assumes Server Components basics. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. **A note on verification:** unlike most docs in this set, the behaviour here cannot be executed in a unit environment — Server Components need a server runtime and a bundler that understands the RSC protocol. Everything below is reasoned from the docum

Question 8

Explain React Suspense and its use cases.

Accepted Answer

**Target Audience:** Engineers preparing for React interviews — assumes components and promises. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The suspension behaviour in section 3 was produced by rendering a pending promise through use() on React 19.2.8 and reading the DOM before and after it resolved. ## 1. Why This Even Matters — A Story First In a restaurant, individual dish

Question 9

What does the `useActionState` hook do in React 19?

Accepted Answer

**Target Audience:** Engineers preparing for React interviews — assumes what Actions are. **Difficulty:** Hard > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The tuple shape and the render sequence in section 3 were **executed against React 19.2.8** by submitting a real form in jsdom. Its sibling is

Question 10

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

Accepted Answer

**Target Audience:** Engineers preparing for senior React interviews — assumes SSR. **Difficulty:** Hard > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. **A note on verification:** Server Components need a bundler-integrated server runtime, and react-server-dom-webpack is **not installed in this repo**, so nothing about RSC below w

REACT.JSDECODED.

Easy

How do you debug React applications?

What is the purpose of the cleanup function in useEffect?

Medium

How do you handle asynchronous operations in React functional components?

How does React's Batching work?

How do you test React components?

How do you fetch data in React?

Async Server Components & waterfall prevention — fetching without blocking

Explain React Suspense and its use cases.

Hard

What does the `useActionState` hook do in React 19?

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

When would you use `useLayoutEffect` instead of `useEffect`?

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

What can and can't an Error Boundary catch?