next.config.js

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

// This uses phases as outlined here: https://nextjs.org/docs/#custom-configuration
module.exports = (phase) => {
  console.log(phase)
  // when started in development mode `next dev` or `npm run dev` regardless of the value of STAGING environmental variable
  const isDev = phase === PHASE_DEVELOPMENT_SERVER
  // when `next build` or `npm run build` is used
  const isProd = phase === PHASE_PRODUCTION_BUILD && process.env.STAGING !== '1'
  // when `next build` or `npm run build` is used
  const isStaging = phase === PHASE_PRODUCTION_BUILD && process.env.STAGING === '1'

  console.log(`isDev:${isDev}  isProd:${isProd}   isStaging:${isStaging}`)

  // next.config.js object
  return {
    publicRuntimeConfig: {
      // can add static folder instead public folder.
      staticFolder: '/public'
    },
    serverRuntimeConfig: {
      API_URL: process.env.API_URL,
      JWT_SECRET: process.env.JWT_SECRET
    }
  }
}