Potential config...

config.js

import {canUseDOM} from 'exenv';

function getWhitelistedVars(global) {
  return Object.keys(global).reduce((obj, key) => {
    if (defaults.hasOwnProperty(key)) {
      obj[key] = global[key];
    }
    return obj;
  }, {});
}

const defaults = {
  API_BASE: 'https://api.pactcoffee.com',
  STRIPE_PUBLISHABLE_KEY: null,
  NODE_ENV: 'development',
  DISABLE_SSR: false,
  PORT: 3000,
}

const globalConfig = canUseDOM ? window.CONFIG : process.env;

const config = {
  ...defaults,
  ...getWhitelistedVars(globalConfig),
};

usage.js

import {API_BASE} from 'config';

// do stuff with API_BASE