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

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01

Understand the problem

The framework configuration file.

nextjsnext-configconfigurationbuild
02

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03

Study the solution

next.config.js (or .ts/.mjs) configures the build and runtime: redirects(), rewrites(), headers(), images (remote patterns/loaders), output mode, experimental flags, env, transpilePackages, and webpack/turbopack tweaks. It's evaluated at build/start, not per request.

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04

Read the code

A typical next.config
// next.config.ts
import type { NextConfig } from 'next';
const config: NextConfig = {
  output: 'standalone',
  images: { remotePatterns: [{ protocol: 'https', hostname: 'cdn.example.com' }] },
  async redirects() { return [{ source: '/old', destination: '/new', permanent: true }]; },
  async headers() { return [{ source: '/(.*)', headers: [{ key: 'X-Frame-Options', value: 'DENY' }] }]; },
  experimental: { optimizePackageImports: ['lodash'] },
};
export default config;
05

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