Nuxt/packages/webpack/src/config/modern.js

import WebpackClientConfig from './client'

export default class WebpackModernConfig extends WebpackClientConfig {
  constructor(...args) {
    super(...args)
    this.name = 'modern'
    this.isModern = true
  }

  env() {
    return Object.assign(super.env(), {
      'process.modern': true
    })
  }

  getBabelOptions() {
    return {
      ...this.buildContext.buildOptions.babel,
      envName: this.name,
      presets: [
        [
          require.resolve('@nuxt/babel-preset-app'),
          {
            modern: true
          }
        ]
      ]
    }
  }
}