next.config.js

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

module.exports = phase => {
  const isDev = phase === PHASE_DEVELOPMENT_SERVER;
  const isProd =
    phase === PHASE_PRODUCTION_BUILD && process.env.STAGING !== "1";
  const isStaging = PHASE_PRODUCTION_BUILD && process.env.STAGING === "1";

  const env = {
    URL: (() => {
      if (isDev) return "http://localhost:3000";
      if (isProd) {
        return "https://nextjs-graphcms-podcast-starter.now.sh";
      }
      if (isStaging) return "http://localhost:11639";
    })()
  };

  return {
    env
  };
};