Question 1

How do environment variables work in Next.js (the NEXT_PUBLIC_ prefix)?

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 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 2

What is Next.js and what problems does it solve?

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 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 3

What is the route segment config in Next.js (dynamic, revalidate, 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 4

What is generateStaticParams in Next.js and how does it enable SSG for dynamic routes?

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

Server Components and external packages — what does `serverComponentsExternalPackages` 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 6

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.

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Question 7

Middleware in Next 15 — what breaks from the 14 upgrade?

Accepted Answer

Next 15 middleware keeps Edge by default but now supports `runtime: 'nodejs'`, async `NextRequest` helpers, and `instrumentation` sharing — update matchers and await `request` APIs.

Question 8

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 9

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 10

How does next/font work and why does it improve performance?

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

NEXT.JSDECODED.

Easy

How do environment variables work in Next.js (the NEXT_PUBLIC_ prefix)?

What is Next.js and what problems does it solve?

Medium

What is the route segment config in Next.js (dynamic, revalidate, runtime)?

What is generateStaticParams in Next.js and how does it enable SSG for dynamic routes?

Server Components and external packages — what does `serverComponentsExternalPackages` do?

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

Middleware in Next 15 — what breaks from the 14 upgrade?

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 does next/font work and why does it improve performance?

Hard

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

How do you stream responses and Server-Sent Events from a Route Handler?