Question 1

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 2

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 3

Migrating Webpack config to Turbopack — what breaks?

Accepted Answer

Most `webpack:` customizations map to `experimental.turbo` or `turbopack` config; Turbopack handles SVGR, Sass, MDX natively but custom plugins that tap Webpack's compiler API still need Webpack.

Question 4

How do you analyze and reduce bundle size 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 5

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 6

What is Turbopack in Next.js and how does it compare to Webpack?

Accepted Answer