Question 1

What are React's design principles?

Accepted Answer

**Target Audience:** Engineers preparing for senior React interviews — assumes broad familiarity with the API. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. This is a discussion question rather than a mechanical one, so instead of a "Verified" section it cites the React team own published principles and ties each to a concrete API you already use. ## 1. Why This Even Matters — A Story First

Question 2

useTransition vs startTransition — when to use which?

Accepted Answer

Both mark an update as a low-priority transition. `useTransition` is a hook that returns `[isPending.

Question 3

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 4

How do you optimize performance in a React application?

Accepted Answer

**Target Audience:** Engineers preparing for React interviews — assumes broad React familiarity. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. This is a breadth question, so it leans on measurements taken elsewhere in this collection rather than introducing new ones — the render-cost figures come from preventi

Question 5

What problem does `useSyncExternalStore` solve?

Accepted Answer

**Target Audience:** Engineers preparing for senior React interviews — assumes hooks and rendering basics. **Difficulty:** Hard > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The multi-reader result in section 3 was measured on React 19.2.8; the failure modes in section 5 were verified separately and are quoted from tearing in concurrent React

Question 6

What is `useDeferredValue` and how does it differ from debouncing?

Accepted Answer

**Target Audience:** Engineers preparing for senior React interviews — assumes hooks and memoisation basics. **Difficulty:** Hard > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every render count below was **executed against React 19.2.8**, including the head-to-head against a real debounce — the raw numbers are in section 4. Related: w

Question 7

Why does React 18 StrictMode double-invoke your components and effects?

Accepted Answer

**Target Audience:** Engineers preparing for React interviews — assumes effects and cleanup. **Difficulty:** Hard > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The counts in section 3 were **executed against React 19.2.8** by rendering the same component with and without StrictMode. **A production build was not measured here** — the development-only claim comes from the documented behaviour, and this do

Question 8

Why must React state updates be immutable?

Accepted Answer

**Target Audience:** Engineers preparing for senior React interviews — assumes state basics. **Difficulty:** Hard > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The render counts and the delayed corruption in section 3 were produced by actually mutating state on React 19.2.8. ## 1. Why This Even Matters — A Story First A librarian checks whether a shelf has changed by looking at the label on its end, not by reading

Question 9

Concurrent Rendering — useDeferredValue and time slicing in React 19

Accepted Answer

Concurrent Rendering (React 18+) lets React render low-priority updates interruptibly in time slices.

Question 10

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

Accepted Answer

**Target Audience:** Engineers preparing for senior React interviews — assumes hooks and re-render basics. **Difficulty:** Hard > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every render count and ordering below was **executed against React 19.2.8** in a jsdom renderer, and the raw output is pasted in section 3. ## 1. Why This Even Matters — A Story First A newsroom gets two jobs at once: fix a typo in tomorrow's

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