next.js/examples/with-now-env/next.config.js

const webpack = require('webpack')
/**
 * After the next require you can use process.env to get your secrets
 */
require('now-env')

console.log({
  SECRET: process.env.SECRET,
  ANOTHER_SECRET: process.env.ANOTHER_SECRET,
  SECRET_FAIL: process.env.SECRET_FAIL
})

/**
 * If some of the envs are public, like a google maps key, but you still
 * want to keep them secret from the repo, the following code will allow you
 * to share some variables with the client, configured at compile time.
 */
module.exports = {
  webpack: config => {
    config.plugins.push(
      new webpack.DefinePlugin({
        'process.env.SECRET': JSON.stringify(process.env.SECRET)
      })
      // Same as above
      // new webpack.EnvironmentPlugin(['SECRET'])
    )
    return config
  }
}