Question 1

What is the difference between `useState` and `useReducer`?

Accepted Answer

**Target Audience:** Frontend engineers preparing for React interviews — assumes familiarity with useState. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term: — the exact vocabulary an interviewer expects. Every behavioural claim below was produced by actually running React 19.2.8 in this repo, including the render counts. ## 1. Why This Even Matters — A Story First Imagine a shop wit

Question 2

What are render props and how do they work?

Accepted Answer

**Target Audience:** Engineers preparing for React interviews — assumes props and state. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The memoisation measurement in section 5 was produced by counting real renders on React 19.2.8. ## 1. Why This Even Matters — A Story First A weather station measures temperature. It has no opinion about whether you want a number, a dial, or a chart — so inst

Question 3

What is the difference between useMemo and useCallback?

Accepted Answer

**Target Audience:** Engineers preparing for React interviews — assumes hooks and re-renders. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The recompute and identity counts in section 3 were measured across three real renders on React 19.2.8. ## 1. Why This Even Matters — A Story First Two different things get cached in a kitchen. The **stock** — simmered for hours — is made once and kept,

Question 4

How do you prevent unnecessary re-renders in functional components?

Accepted Answer

**Target Audience:** Engineers preparing for React interviews — assumes memo and hooks. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The render counts quoted below were measured on React 19.2.8 across this collection; see avoiding re-renders with composition and

Question 5

What is `React.memo` and when would you use it?

Accepted Answer

**Target Audience:** Engineers preparing for React interviews — assumes props and re-render basics. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every render count below was measured by actually mounting the components and re-rendering their parent on React 19.2.8. ## 1. Why This Even Matters — A Story First A translator is handed a document and asked to translate it. Then handed it again.

Question 6

How do you optimize React component rendering?

Accepted Answer

**Target Audience:** Engineers preparing for senior React interviews — assumes profiling basics. **Difficulty:** Hard > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The durations in section 3 were measured with React's own Profiler component on React 19.2.8. This doc is about **cost per render**; preventing unnecessary r

Question 7

React Compiler — how does auto-memoization work in React 19?

Accepted Answer

**Target Audience:** Frontend engineers preparing for senior React interviews — assumes familiarity with useMemo and React.memo. **Difficulty:** Hard > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The runtime inspection below was produced by reading the installed React 19.2.8 in this repo. Release-date and version claims were checked against the React team announcement rather than recalled —

Question 8

Why does updating Context re-render all its consumers, and how do you fix it?

Accepted Answer

**Target Audience:** Engineers preparing for senior React interviews — assumes Context and memo. **Difficulty:** Hard > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every render count below was measured on React 19.2.8 with memo-wrapped consumers, so an ordinary parent re-render was already blocked — anything that still rendered got through via context. ## 1. Why This Even Matters — A Story First A mai

Question 9

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 10

When does `React.memo` do nothing — or actually hurt performance?

Accepted Answer

**Target Audience:** Engineers preparing for senior React interviews — assumes what React.memo is. **Difficulty:** Hard > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every render count and timing below was **executed against React 19.2.8** in a jsdom renderer; the raw output is in sections 3 and 6. This doc is the failure-mode companio

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