next.config.js

const { PHASE_DEVELOPMENT_SERVER } = require('next/constants')

module.exports = (phase, { defaultConfig }) => {
  if (phase === PHASE_DEVELOPMENT_SERVER) {
    return {
      /* development only config options here */
      basePath: '',
      serverRuntimeConfig: {
        // Will only be available on the server side
        mySecret: 'secret',
        secondSecret: process.env.SECOND_SECRET, // Pass through env variables
      },
      publicRuntimeConfig: {
        // Will be available on both server and client
        rootFolder: '',
      },
    }
  }

  return {
    /* config options for all phases except development here */
    basePath: '/nextjs-blog',
    serverRuntimeConfig: {
      // Will only be available on the server side
      mySecret: 'secret',
      secondSecret: process.env.SECOND_SECRET, // Pass through env variables
    },
    publicRuntimeConfig: {
      // Will be available on both server and client
      rootFolder: '/nextjs-blog',
    },
  }
}