Question 1

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 2

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 3

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 4

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 5

How do you create a new Next.js app, and what is in the project structure?

Accepted Answer

**Target Audience:** Beginners preparing for Next.js interviews — no prior Next.js knowledge assumed, basic React helps.
**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 run of `create-next-app@16.4.2`, `next build` and `next start` in a lab.

## 1. Why This Even Matters

Question 6

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 7

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

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 next.config.js in Next.js and what are common configurations?

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

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

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

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

What happens when you run next dev, next build and next start?

How do you create a new Next.js app, and what is in the project structure?

Medium

Migrating Webpack config to Turbopack — what breaks?

How do you analyze and reduce bundle size in Next.js?

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

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

What is next.config.js in Next.js and what are common configurations?

How do you enable and evaluate the React Compiler in Next.js 16?