Question 1

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 2

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 3

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 4

How do you avoid re-renders with composition instead of memoization?

Accepted Answer

**Target Audience:** Engineers preparing for senior React interviews — assumes memo and re-render basics. **Difficulty:** Hard > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The render counts in section 3 were measured on React 19.2.8 in this repo. This is the performance-focused companion to composition in React components

Question 5

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 6

cache() and fetch memoization — request deduplication in RSC

Accepted Answer

**Target Audience:** Engineers preparing for senior React interviews — assumes Server Components basics. **Difficulty:** Hard > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. **A note on verification:** cache()'s deduplication only operates inside a React server render, which cannot be exercised in a unit environment. What *was* verified here is that the API exists and that outside a server render it delib

Question 7

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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