Question 1

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 2

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 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

Observability with OpenTelemetry in Next 15 — how is OTEL wired?

Accepted Answer

Install `@vercel/otel`, call `registerOTel` in `instrumentation.ts register()`, and set `experimental.instrumentationHook: true` — Next auto-traces RSC render, fetch, and middleware.

Question 7

How do you handle authentication in Next.js?

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

NEXT.JSDECODED.

Medium