Next.js 16+ · Interface

NextConfig

NextConfig is the TypeScript type for a Next.js configuration object. Export a root-level configuration module by default so Next.js can apply its server- and build-time options.

How it works

NextConfig is the TypeScript type for the object exported from a Next.js configuration file. Define the configuration at the project root and export it as the default export; Next.js reads it during server and build phases, while the file is excluded from the browser bundle.

Syntax

const nextConfig: NextConfig = { /* options */ }
export default nextConfig

Returns

A NextConfig configuration object exported as the default export of the project's Next.js configuration module.

Key points

Place the configuration file at the project root so Next.js can discover it.
Export one configuration object as the module's default export.
NextConfig acts as the typed entry point for the documented configuration options.
In Next.js 16, reactCompiler is configured as a stable option rather than under experimental.

Common mistakes

Treat the configuration as executable JavaScript or TypeScript rather than JSON.
Do not expect configuration values to be available directly in client-side browser code.

Example: next.config.ts

import type { NextConfig } from 'next'

const nextConfig: NextConfig = {
  reactCompiler: true,
}

export default nextConfig

This typed configuration enables the stable React Compiler option in a Next.js 16 application.

When to use it

Declare supported Next.js options with TypeScript checking.

Type-safe project configuration

Configure behavior consumed during Next.js server and build phases.

Build and server customization

Enable current options such as stable reactCompiler or top-level turbopack settings.

Next.js 16 feature adoption

Interview questions

QWhat is NextConfig and where is it used?

NextConfig is the type used to describe the configuration object exported by a Next.js config file. The file belongs at the project root and uses a default export.

Follow-up: How would you avoid exposing a secret placed in the config? Keep it server-side: configuration is consumed by the server and build phases and is not included in the browser build, but client-exposed values still require separate care.

QIs next.config.js just a JSON file?

A Next.js configuration file is a regular Node.js module, so it can contain module syntax and executable configuration logic. It is not parsed as a JSON document.

Follow-up: What practical benefit does this provide? It permits typed imports, conditional values, and other Node-side composition that JSON cannot express.

QHow do you declare a Next.js configuration?

Export the configuration object as the file's default export, typically from a root-level next.config.ts or next.config.js file. Next.js then applies the options during server and build phases.

Follow-up: Which current Next.js 16 option could be enabled in this object? The documentation's Next.js 16 guidance shows top-level options such as turbopack, while reactCompiler is also promoted to stable.

turbopack
reactCompiler
next.config.ts
next.config.js

Source: next.config.js. Checked 2026-10-15.