Question 1

What are React synthetic events?

Accepted Answer

**Target Audience:** Engineers preparing for React interviews — no prior knowledge of the event system assumed. **Difficulty:** Easy > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Everything about the event object below was **executed against React 19.2.8** and the raw output is pasted in section 3. For the delivery mechanism — where the listener actually lives — see

Question 2

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 3

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 4

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 5

What is the purpose of the `key` prop when rendering lists in React?

Accepted Answer

**Target Audience:** Engineers preparing for React interviews — assumes rendering lists. **Difficulty:** Easy to Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The mismatch in section 3 was produced by typing into a real input and then reordering the list on React 19.2.8. ## 1. Why This Even Matters — A Story First A cloakroom numbers its hooks rather than its coats. Hook 1, hook 2, hook 3. It works perfectl

Question 6

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 7

What are the advantages of using React over plain JavaScript?

Accepted Answer

**Target Audience:** Engineers preparing for React interviews — assumes JavaScript and DOM basics. **Difficulty:** Easy to Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. This is a trade-off question, so the answer below deliberately includes the costs — an interviewer is often testing whether you can argue against your own tool. ## 1. Why This Even Matters — A Story First Picture a whiteboard tracking a foot

Question 8

What is the difference between element and component in React?

Accepted Answer

**Target Audience:** Engineers preparing for React interviews — assumes basic JSX familiarity. **Difficulty:** Easy to Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every object-shape claim below was produced by inspecting real values in React 19.2.8, not described from documentation. ## 1. Why This Even Matters — A Story First A **recipe** for a cake is not a cake. It is also not an *order* for a cake. -

Question 9

What are fragments in React?

Accepted Answer

**Target Audience:** Engineers preparing for React interviews — assumes basic JSX. **Difficulty:** Easy > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The DOM output in section 3 was produced by rendering both versions in React 19.2.8 and reading back the real HTML. ## 1. Why This Even Matters — A Story First You need to post three documents. The post office insists on one item, so you put them in an envelope. Fine

Question 10

What is the Virtual DOM in React?

Accepted Answer

**Target Audience:** Anyone preparing for a React interview — assumes basic components. **Difficulty:** Easy > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The reconciliation behaviour in section 5 was produced by actually swapping an element type on React 19.2.8 and observing what happened to the child state. ## 1. Why This Even Matters — A Story First An architect wants three changes to a building. They do not de

REACT.JSDECODED.

Easy

What are React synthetic events?

What are React Hooks? Name a few common ones.

How do you handle side effects in functional components?

What is the role of `react-dom`?

What is the purpose of the `key` prop when rendering lists in React?

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

What are the advantages of using React over plain JavaScript?

What is the difference between element and component in React?

Medium

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

How does React handle security concerns like XSS?

Explain the concept of reconciliation in React.

How does React handle events?

How does React's Batching work?

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

What is the difference between controlled and uncontrolled components?

How do you handle memory leaks in React?

Hard

What is `useFormStatus` in React 19, and why does it exist?

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

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

How can the `key` prop be used to deliberately reset component state?

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

When and why do you use `forwardRef` with `useImperativeHandle`?

The Activity API — hiding UI without unmounting in React 19

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