Question 1

What is the purpose of the cleanup function in useEffect?

Accepted Answer

**Target Audience:** Engineers preparing for React interviews — assumes useEffect basics. **Difficulty:** Easy to Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The exact call sequence in section 3 was produced by rendering a component through three dependency changes and an unmount on React 19.2.8. ## 1. Why This Even Matters — A Story First Every time you check into a hotel you get a key. Ever

Question 2

Describe the React component lifecycle.

Accepted Answer

**Target Audience:** Engineers preparing for React interviews — assumes hooks basics. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every ordering below is **executed output from React 19.2.8**, not a description — the raw sequences are in sections 3 and 5. ## 1. Why This Even Matters — A Story First A market stall has three moments: setting up before opening, adjusting through the day as st

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

Explain the purpose of `useEffect`.

Accepted Answer

**Target Audience:** Engineers preparing for React interviews — assumes useState. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The render counts in section 4 were measured on React 19.2.8. This doc focuses on *when an effect is the wrong tool*; see handling side effects for the broader strategy an

Question 5

What are Error Boundaries in React?

Accepted Answer

**Target Audience:** Engineers preparing for React interviews — no prior error-handling knowledge assumed. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The behaviour in section 3 was **executed against React 19.2.8**. The companion doc, what an Error Boundary can and cannot catch, covers the limits — which

Question 6

Error Boundaries with RSC — why can't Server Components catch errors?

Accepted Answer

**Target Audience:** Engineers preparing for React interviews — assumes error boundaries and Server Components. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. **A note on verification:** this repo has **no RSC runtime** — r

Question 7

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 8

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

Accepted Answer

**Target Audience:** Engineers preparing for senior React interviews — assumes hooks and Suspense. **Difficulty:** Hard > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The conditional-call comparison in section 3 was **executed against React 19.2.8** with a hook deliberately placed after the conditional one, so the hook order genuinely shifts. Related:

Question 9

What can and can't an Error Boundary catch?

Accepted Answer

**Target Audience:** Engineers preparing for React interviews — assumes what an Error Boundary is. **Difficulty:** Hard > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every row of the table in section 3 was **executed against React 19.2.8** — each throw site was actually triggered and the boundary checked. ## 1. Why This Even Matte

REACT.JSDECODED.

Easy