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 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 3

useOptimistic — how do you build instant UI for Server Actions?

Accepted Answer

`useOptimistic` (React 19) lets you show an optimistic version of state while a Server Action is in flight.

Question 4

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 5

What is the difference between controlled and uncontrolled components?

Accepted Answer

**Target Audience:** Engineers preparing for React interviews — no prior forms knowledge assumed. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The warning texts and the ref reading in sections 4 and 5 were **captured by executing against React 19.2.8**, not quoted from memory. Related: Actions in React 19, which ch

Question 6

How do you handle forms in React?

Accepted Answer

**Target Audience:** Engineers preparing for React interviews — assumes state and events. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The FormData and useActionState results below were produced by submitting real forms on React 19.2.8. ## 1. Why This Even Matters — A Story First Two ways to take an order in a restaurant. The waiter can write each item down as you

Question 7

useFormStatus & useActionState — modern form handling in React 19

Accepted Answer

**Target Audience:** Engineers preparing for React interviews — assumes Actions. **Difficulty:** Medium > **How to read this doc:** this is the **practical assembly** doc. The mechanics of each hook live in useActionState and useFormStatus; here

Question 8

Server Actions — how do mutations work without API routes in Next.js 15?

Accepted Answer

**Target Audience:** Engineers preparing for Next.js interviews — assumes what the directive does. **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 a running Next.js server and there is none here, so the workflow was **not executed**. The React-level pieces it builds

Question 9

Next.js instrumentation hook — observability in Next.js 15

Accepted Answer

**Target Audience:** Engineers preparing for Next.js interviews — assumes basic App Router knowledge. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. **A note on verification:** this could not be executed — there is no instrumentation file in this repo and no OpenTelemetry installed. What section 3 shows is **read directly from the type definitions shipped in the installed Next.js package**, so

Question 10

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

REACT.JSDECODED.

Easy

What are React Hooks? Name a few common ones.

How do you debug React applications?

Medium

useOptimistic — how do you build instant UI for Server Actions?

How do you handle asynchronous operations in React functional components?

What is the difference between controlled and uncontrolled components?

How do you handle forms in React?

useFormStatus & useActionState — modern form handling in React 19

Server Actions — how do mutations work without API routes in Next.js 15?

Next.js instrumentation hook — observability in Next.js 15

Hard

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

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

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

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

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

What are Server Actions and the `"use server"` directive in React 19?