Question 1

How do you run Turbopack locally — `next dev --turbo` vs default?

Accepted Answer

`next dev --turbo` (now default in Next 15) gives sub-100ms HMR via persistent `.next/cache/turbo` — use `--show-config` or `TURBOPACK=1` to debug.

Question 2

Turbopack — why is it 10x faster than Webpack in Next.js 15?

Accepted Answer

Turbopack is Next.js's Rust-based incremental bundler — 5-10x faster cold start and HMR than Webpack by caching at the module-function level instead of rebuilding whole bundles.

Question 3

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 4

Caching defaults in Next 15 — what is no longer cached by default?

Accepted Answer

Next 15 makes `fetch` and `GET` Route Handlers dynamic by default — add `cache: 'force-cache'` / `'use cache'` or `export const dynamic = 'force-static'` to re-cache.

Question 5

App Router 15 — why are params and searchParams now Promises?

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 does `use cache` unlock in Next.js 15?

Accepted Answer

`'use cache'` marks an async function or component as cacheable — with `cacheLife` and `cacheTag` you control TTL and on-demand revalidation without wrapping fetch.

NEXT.JSDECODED.

Easy