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

`useOptimistic` (React 19) lets you show an optimistic version of state while a Server Action is in flight. useOptimistic shows temp state → Server Action → confirm or rollback <rect x="40" y="40"

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

**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 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

What is `create-react-app` and when would you use it?

Accepted Answer

**Target Audience:** Engineers preparing for React interviews — assumes you have started a React project before. **Difficulty:** Easy > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The deprecation status below was checked against the React team announcement rather than recalled — this is a topic where an out-of-date answer is very visible, so the source is cited inline. ## 1. Why This Even Matters — A Story First <

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

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 5

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 6

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

Question 7

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

Accepted Answer

**Target Audience:** Engineers preparing for senior React interviews — assumes transitions and actions. **Difficulty:** Hard > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The timing behaviour in section 3 was produced by clicking a real button on React 19.2.8 and reading the DOM before and after the action resolved. ## 1. Why This Even Matters — A Story First A bartender takes your order and puts a glass on the co

Question 8

How does React 19 handle document metadata like `` and `<meta>`?

Accepted Answer

**Target Audience:** Engineers preparing for React interviews — assumes JSX basics. **Difficulty:** Hard > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Both behaviours in sections 3 and 4 were **executed against React 19.2.8** — including the deduplication result, which is the part most write-ups get wrong. Related: the resource preloa

Question 9

What is the difference between client-side rendering (CSR) and server-side rendering (SSR) in React?

Accepted Answer

**Target Audience:** Engineers preparing for React interviews — no prior rendering-strategy knowledge assumed. **Difficulty:** Hard > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The byte counts in section 3 come from **actually rendering the same component both ways** on React 19.2.8. For the mechanics of each side see what SSR is and when to use

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