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

      return entries
    }

    return cfg
  }
}

polyfills.js

/* eslint no-extend-native: 0 */
// core-js comes with Next.js. So, you can import it like below
import includes from 'core-js/library/fn/string/virtual/includes'
import repeat from 'core-js/library/fn/string/virtual/repeat'
import assign from 'core-js/library/fn/object/assign'

// Add your polyfills
// This files runs at the very beginning (even before React and Next.js core)
console.log('Load your polyfills')

String.prototype.includes = includes
String.prototype.repeat = repeat
Object.assign = assign