cypress-react-unit-test/plugins/react-scripts/index.js

const filePreprocessor = require('../cra-v3/file-preprocessor')

module.exports = (on, config) => {
  require('@cypress/code-coverage/task')(on, config)
  on('file:preprocessor', filePreprocessor(config))
  // IMPORTANT to return the config object
  // with the any changed environment variables
  return config
}