Adding gem paths to Webpacker config

test.js

// config/webpack/test.js

const environment = require('./environment');
const webpackConfigWithGems = require('./utils/webpack_config_with_gems');

module.exports = webpackConfigWithGems(environment);

webpack_config_with_gems.js

// config/webpack/utils/webpack_config_with_gems.js

const { execSync } = require('child_process');

function getGemPath(gemName, path) {
  const gemPath = execSync(`bundle show ${gemName}`).toString().trim();

  return `${gemPath}/${path}/assets/javascripts`;
}

module.exports = function(environment) {
  const config = environment.toWebpackConfig();

  config.resolve.modules.push(
    getGemPath('js-routes', 'app'),
    getGemPath('client_side_validations', 'vendor'),
    getGemPath('client_side_validations-simple_form', 'vendor'),
    getGemPath('i18n-js', 'app')
  );

  return config;
};