Load cocoon JS with webpacker

application.js

// app/javascripts/packs/application.js
import 'cocoon'

environment.js

// config/webpack/environment.js
const { environment } = require('@rails/webpacker')

environment.plugins.prepend('GemAssetsPlugin', require('./plugins/gem-assets'))

module.exports = environment

gem-assets.js

// config/webpack/plugins/gem-assets.js
const { exec } = require('child_process')

module.exports = {
  apply(compiler) {
    compiler.hooks.beforeRun.tap('GemAssetsPlugin', (compilation) => {
      exec('mkdir -p vendor/assets/javascripts')
      exec('cp $(env -u DEBUG bundle show cocoon)/app/assets/javascripts/cocoon.js vendor/assets/javascripts')
      return true
    })
  }
}