Q: What is the Data Cache in Next.js and how do you opt out of it?

The **Data Cache** stores fetch (and cached function) results on the **server across requests and deployments** to avoid refetching. Opt out per fetch with cache: 'no-store' , per segment with export const dynamic = 'force-dynamic' or revalidate = 0 — so data is always fresh. <rect class='d-box-accent' x='2

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

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 3

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 4

What is the Data Cache in Next.js and how do you opt out of it?

Accepted Answer

The **Data Cache** stores fetch (and cached function) results on the **server across requests and deployments** to avoid refetching. Opt out per fetch with cache: 'no-store', per segment with export const dynamic = 'force-dynamic' or revalidate = 0 — so data is always fresh.

Question 5

What is the difference between getServerSideProps/getStaticProps and App Router data fetching?

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

How does fetch caching work in the Next.js App Router (force-cache, no-store, revalidate)?

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 7

How do you fetch data in Next.js Client Components (SWR, React Query, use)?

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

How do you test a Next.js app (unit tests, Server Components, end-to-end)?

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 16.4.2 docs shipped with the package and, in the **Verified** section, against real output: every test shown here was run, with vitest 5.0.5, @testing-library/react 16.3.3, jsdom 29.1.

Question 9

What is the difference between revalidateTag(), updateTag(), and refresh() in Next.js 16?

Accepted Answer

**Target Audience:** Frontend engineers preparing for Next.js interviews — assumes you know what a Server Action and a cache tag 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 shipped in `node_modules/next/dist/docs/` and, in the **Verified** section, against a 16.4.2 lab measured with `curl` and with Playwright clicking real buttons i

Question 10

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

NEXT.JSDECODED.

Easy

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

Medium

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

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

What is the Data Cache in Next.js and how do you opt out of it?

What is the difference between getServerSideProps/getStaticProps and App Router data fetching?

How does fetch caching work in the Next.js App Router (force-cache, no-store, revalidate)?

How do you fetch data in Next.js Client Components (SWR, React Query, use)?

How do you test a Next.js app (unit tests, Server Components, end-to-end)?

What is the difference between revalidateTag(), updateTag(), and refresh() in Next.js 16?

Hard

What changed about caching defaults in Next.js 15?

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

What are Multi-Zones in Next.js and when would you split an app into zones?

What is the Router Cache in Next.js and what are its implications?

What are the caching layers in Next.js (Request Memoization, Data Cache, Full Route Cache, Router Cache)?

unstable_cache vs `cache` (React) — which dedupes what?

What is the Full Route Cache in Next.js?

How does prefetching and navigation caching work in Next.js 16?