next.js/examples/with-polyfills/next.config.js

module.exports = {
  webpack: function (cfg) {
    const originalEntry = cfg.entry
    cfg.entry = async () => {
      const entries = await originalEntry()

      if (entries['main.js']) {
        entries['main.js'].unshift('./client/polyfills.js')
      }

      return entries
    }

    return cfg
  }
}