Question 1

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 2

The `use()` API with Suspense — how do you unwrap promises in render?

Accepted Answer

`use()` is a React 19 primitive that unwraps a Promise or Context during render.

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 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 5

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 6

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 7

How does streaming SSR with selective hydration work?

Accepted Answer

**Target Audience:** Engineers preparing for senior React interviews — assumes SSR and Suspense. **Difficulty:** Hard > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Section 3 is a **real streamed response captured in Node against React 19.2.8** — the chunk sizes and millisecond timings are measured, not illustrative. The one part **not** execu

Question 8

How do Suspense data-fetching waterfalls happen, and how do you avoid them?

Accepted Answer

**Target Audience:** Engineers preparing for senior React interviews — assumes Suspense basics. **Difficulty:** Hard > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every timing below was measured on React 19.2.8 by rendering and polling until the content actually appeared, rather than by a fixed sleep. ## 1. Why This Even Matters — A Story First Three people need to be phoned before a meeting can start. Call the fi

Question 9

Suspense streaming in React 19 & Next.js 15 — how does HTML stream?

Accepted Answer

With `renderToPipeableStream` (and Next.js App Router), React streams HTML in chunks: the shell (layout, nav) flushes immediately, each `` boundary streams its.

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

How do you handle side effects in functional components?

Medium

The `use()` API with Suspense — how do you unwrap promises in render?

How do you handle asynchronous operations in React functional components?

How do you fetch data in React?

Async Server Components & waterfall prevention — fetching without blocking

Explain React Suspense and its use cases.

Hard

How does streaming SSR with selective hydration work?

How do Suspense data-fetching waterfalls happen, and how do you avoid them?

Suspense streaming in React 19 & Next.js 15 — how does HTML stream?

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

Compare render-as-you-fetch, fetch-on-render, and fetch-then-render.

What does the `use()` hook do, and why is it special?

What do `startTransition` and `useTransition` actually do under the hood?

What is "tearing" in concurrent React?