Question 1

What happens when you run next dev, next build and next start?

Accepted Answer

**Target Audience:** Beginners preparing for Next.js interviews — no prior Next.js knowledge assumed.
**Difficulty:** Easy

> **How to read this doc:** Each idea is explained in plain language first, then tagged with `📌 Interview term:`. Every behaviour claim was checked against the Next.js 16.4.2 docs shipped with the package and, in the **Verified** section, against a real `create-next-app@16.4.2` project run under all three commands and probed with `curl`.

## 1. Why This Even Matters — A St

Question 2

How do you handle forms with Server Actions in Next.js (progressive enhancement)?

Accepted Answer

**Target Audience:** Frontend engineers preparing for Next.js interviews — assumes basic React and App Router familiarity.
**Difficulty:** Medium

> **How to read this doc:** Each idea is explained in plain language first, then tagged with `📌 Interview term:`. Every behaviour claim was checked against the Next.js docs shipped with the package (written on 16.3.4, re-reviewed against 16.4.2) and, in the **Verified** section, against real output from a build or a running server; each Verified bloc

Question 3

What is error.js in Next.js and how does error handling work in the App Router?

Accepted Answer

**Target Audience:** Frontend engineers preparing for Next.js interviews — assumes basic React and App Router familiarity.
**Difficulty:** Medium

> **How to read this doc:** Each idea is explained in plain language first, then tagged with `📌 Interview term:`. Every behaviour claim was checked against the Next.js 16.4.2 docs shipped with the package and, in the **Verified** section, against real output from a build, a running server or a real browser.

## 1. Why This Even Matters — A Story Firs

Question 4

How do Next.js Devtools MCP and AGENTS.md change AI-assisted debugging?

Accepted Answer

**Target Audience:** Frontend engineers preparing for Next.js interviews — no prior knowledge of MCP needed.
**Difficulty:** Medium

> **How to read this doc:** Each idea is explained in plain language first, then tagged with `📌 Interview term:`. Every behaviour claim was checked against the Next.js 16.4.2 docs (the AI agents and MCP guides, `agentRules`, `agentFeedback`) and, in the **Verified** section, against a 16.4.2 `next dev` server: its terminal, the files it wrote, and JSON-RPC calls t

Question 5

What are the special files in the Next.js App Router (page, layout, loading, error)?

Accepted Answer

**Target Audience:** Frontend engineers preparing for Next.js interviews — assumes basic React and App Router familiarity.
**Difficulty:** Medium

> **How to read this doc:** Each idea is explained in plain language first, then tagged with `📌 Interview term:`. Every behaviour claim was checked against the Next.js docs shipped with the package (written on 16.3.4, re-reviewed against 16.4.2) and, in the **Verified** section, against real output from a build or a running server; each Verified bloc

Question 6

How do you debug Server Functions and hydration mismatches in Next.js 16.2?

Accepted Answer

**Target Audience:** Frontend engineers preparing for Next.js interviews — assumes you know Server Actions and what hydration is.
**Difficulty:** Medium

> **How to read this doc:** Each idea is explained in plain language first, then tagged with `📌 Interview term:`. Every behaviour claim was checked against the Next.js 16.4.2 docs (`logging`, the debugging guide, the hydration guide) and, in the **Verified** section, against real `next dev` and `next start` output from a 16.4.2 lab driven by P

Question 7

What are forbidden() and unauthorized() in Next.js?

Accepted Answer

**Target Audience:** Frontend engineers preparing for Next.js interviews — assumes basic React and App Router familiarity.
**Difficulty:** Medium

> **How to read this doc:** Each idea is explained in plain language first, then tagged with `📌 Interview term:`. Every behaviour claim was checked against the Next.js 16.4.2 docs and typings shipped with the package and, in the **Verified** section, against real output from a build, a running server or a real Chromium browser.

## 1. Why This Even M

Question 8

How do you handle instrumentation and observability in Next.js (instrumentation.ts)?

Accepted Answer

**Target Audience:** Frontend engineers preparing for Next.js interviews — assumes basic React and App Router familiarity.
**Difficulty:** Hard

> **How to read this doc:** Each idea is explained in plain language first, then tagged with `📌 Interview term:`. Every behaviour claim was checked against the Next.js docs shipped with the package (written on 16.3.4, re-reviewed against 16.4.2) and, in the **Verified** section, against real output from a build or a running server; each Verified block

Question 9

What are Cache Components in Next.js 16 and how does the 'use cache' directive work?

Accepted Answer

**Target Audience:** Frontend engineers preparing for Next.js interviews — assumes you know what a Server Component is.
**Difficulty:** Hard

> **How to read this doc:** Each idea is explained in plain language first, then tagged with `📌 Interview term:`. Every behaviour claim was checked against the Next.js 16.4.2 docs shipped in `node_modules/next/dist/docs/` and, in the **Verified** section, against real output from `next build`, `next start` and `curl` in a 16.4.2 lab.

## 1. Why This Even

Question 10

Parallel routes (@slot) — how do you build a dashboard with independent slots?

Accepted Answer

Parallel routes (`@slot` folders) let one layout render several `page.tsx` children at once — each slot has its own loading/error and navigates independently.

NEXT.JSDECODED.

Easy

What happens when you run next dev, next build and next start?

Medium

How do you handle forms with Server Actions in Next.js (progressive enhancement)?

What is error.js in Next.js and how does error handling work in the App Router?

How do Next.js Devtools MCP and AGENTS.md change AI-assisted debugging?

What are the special files in the Next.js App Router (page, layout, loading, error)?

How do you debug Server Functions and hydration mismatches in Next.js 16.2?

What are forbidden() and unauthorized() in Next.js?

Hard

How do you handle instrumentation and observability in Next.js (instrumentation.ts)?

What are Cache Components in Next.js 16 and how does the 'use cache' directive work?

Parallel routes (@slot) — how do you build a dashboard with independent slots?