Q: React 19 ref as prop & ref cleanup — the new cleanup return from ref callbacks

In React 19 a ref callback may return a cleanup function. ref callback now returns cleanup → called on unmount or ref change <tex

Question 1

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 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

forwardRef removal — how do refs work as props in React 19?

Accepted Answer

In React 19 `ref` is a normal prop. Function components receive `ref` like any other prop, so `forwardRef` is no longer needed and is deprecated.

Question 4

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 5

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 6

React 19 ref as prop & ref cleanup — the new cleanup return from ref callbacks

Accepted Answer

In React 19 a ref callback may return a cleanup function.

Question 7

What are Higher-Order Components (HOCs) in React?

Accepted Answer

**Target Audience:** Engineers preparing for React interviews — assumes components and props. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The losses documented in section 4 were produced by actually building a naive HOC on React 19.2.8 and inspecting what survived. ## 1. Why This Even Matters — A Story First Think of a gift-wrapping service. You hand over a boxed appliance and get back the

Question 8

What are Actions in React 19, and how do they change form handling?

Accepted Answer

**Target Audience:** Engineers preparing for React interviews — assumes forms and hooks basics. **Difficulty:** Hard > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every behaviour below was **executed against React 19.2.8** by rendering a form into jsdom and submitting it; the raw output is in sections 3 and 5. The focused companions are

Question 9

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

Accepted Answer

**Target Audience:** Engineers preparing for React interviews — assumes reconciliation. **Difficulty:** Hard > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The state and lifecycle transitions in section 3 were **executed against React 19.2.8**. ## 1. Why This Even Matters — A Story First A hotel room is cleaned between guests. Nob

Question 10

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

Accepted Answer

**Target Audience:** Engineers preparing for React interviews — no prior SSR experience assumed. **Difficulty:** Hard > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every server-render output below was **executed against React 19.2.8** using react-dom/server; the raw output is in sections 3 and 5. The companion docs are CSR vs

REACT.JSDECODED.

Easy

What are React Hooks? Name a few common ones.

How do you pass data between components in React?

forwardRef removal — how do refs work as props in React 19?

Medium

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

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

React 19 ref as prop & ref cleanup — the new cleanup return from ref callbacks

What are Higher-Order Components (HOCs) in React?

Hard

What are Actions in React 19, and how do they change form handling?

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`?

What is hydration in React?

How did ref handling change in React 19 (ref as a prop, forwardRef, ref cleanup)?