Custom Webpack setting in Next.js

next.config.js

const webpack = require('webpack')

module.exports = {
  webpack: function (config) {

    config.plugins.push(
      new webpack.DefinePlugin({
        'process.env': {
          S3_ACCESS_KEY: JSON.stringify(process.env.S3_ACCESS_KEY),
          S3_SECRET_KEY: JSON.stringify(process.env.S3_SECRET_KEY)
        },
      })
    )

    return config
  }
}