Question 1

What is not-found.js in Next.js and how do you trigger a 404?

Accepted Answer

**Target Audience:** Frontend engineers preparing for Next.js interviews — assumes basic React and App Router familiarity.
**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 real output from a build, a running server or a real browser.

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

Question 2

What is proxy.ts in Next.js 16 and how do you migrate from middleware.ts?

Accepted Answer

**Target Audience:** Frontend engineers preparing for Next.js interviews — assumes you know what an HTTP redirect and rewrite are.
**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 (`file-conventions/proxy`, `middleware`, the version 16 guide) and, in the **Verified** section, against `next build`, `next start` and `curl` in a 16.4.2 lab.

##

Question 3

What breaks when you upgrade to Next.js 16, and how do you migrate?

Accepted Answer

**Target Audience:** Frontend engineers preparing for Next.js interviews — useful for anyone who has to plan a 15 to 16 upgrade.
**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 upgrade guide shipped in `node_modules/next/dist/docs/` and, in the **Verified** section, against real `next build` and `next start` output from a 16.4.2 lab app written i

Question 4

What is Middleware in Next.js and what can it do?

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 5

How do you handle redirects and rewrites 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 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 test a Next.js app (unit tests, Server Components, end-to-end)?

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: every test shown here was run, with vitest 5.0.5, @testing-library/react 16.3.3, jsdom 29.1.

Question 7

What is static export (output: 'export') in Next.js and what does it not support?

Accepted Answer

**Target Audience:** Developers preparing for Next.js interviews — assumes the basics of App Router rendering (static vs dynamic routes).
**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 lab builds with `output: "export"`, a plain static file server and a Chromium browser.

##

Question 8

How do you deploy a Next.js app (Vercel vs self-hosted)?

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 9

What runtimes does Next.js support (Node.js vs Edge runtime)?

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 10

What is after() in Next.js and when should you use it?

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 or a running server.

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

At a good caf

NEXT.JSDECODED.

Easy

What is not-found.js in Next.js and how do you trigger a 404?

Medium

What is proxy.ts in Next.js 16 and how do you migrate from middleware.ts?

What breaks when you upgrade to Next.js 16, and how do you migrate?

What is Middleware in Next.js and what can it do?

How do you handle redirects and rewrites in Next.js?

How do you test a Next.js app (unit tests, Server Components, end-to-end)?

What is static export (output: 'export') in Next.js and what does it not support?

How do you deploy a Next.js app (Vercel vs self-hosted)?

What runtimes does Next.js support (Node.js vs Edge runtime)?

Hard

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

How do you build a multi-tenant Next.js app with subdomain routing?

How do you handle authentication in Next.js?

What are the security considerations for Server Actions in Next.js?

What is the RSC payload and how does Next.js stream it to the browser?

What are Multi-Zones in Next.js and when would you split an app into zones?

How do you internationalize (i18n) a Next.js App Router app?

How do you set security headers and a nonce-based Content Security Policy in Next.js?