next.js/server/config.js

import { join } from 'path'
import { existsSync } from 'fs'

const cache = new Map()

const defaultConfig = {
  webpack: null,
  poweredByHeader: true,
  distDir: '.next',
  assetPrefix: ''
}

export default function getConfig (dir) {
  if (!cache.has(dir)) {
    cache.set(dir, loadConfig(dir))
  }
  return cache.get(dir)
}

function loadConfig (dir) {
  const path = join(dir, 'next.config.js')

  let userConfig = {}

  const userHasConfig = existsSync(path)
  if (userHasConfig) {
    const userConfigModule = require(path)
    userConfig = userConfigModule.default || userConfigModule
  }

  return Object.assign({}, defaultConfig, userConfig)
}