babel-asset-pipeline transforms ES2015 files into ES5 and can output any module format you want (AMD, UMD, SystemJS).
When the app is packaged for distribution the asset-pipeline config in build.gradle is used.
When the app is run during development with bootRun or run this config is used.
Sources live in: grails-app/assets/javascripts
Tests live in: src/test/javascript
This tracks the NPM dependencies and contains the test command. The test command runs Mocha with the Babel compiler and sets NODE_PATH so tests in src/test/javascript can import modules that live in grails-app/assets/javascripts
Many libraries on NPM assume that projects use Webpack or Browserify to package dependencies for distribution with an app, they won't always provide UMD files in their distribution. There isn't an asset-pipeline plugin for Webpack or Browserify yet, this is my stopgap solution. It is a collection of Webpack commands to copy assets we want from node_modules into grails-app/assets/javascripts/vendor. The output in this directory is committed and as a result is visible to asset-pipeline during buildtime. This script only needs to be run when dependencies in package.json are updated.
The unit tests live in src/test/javascript and are run with Mocha on Node. They hook into gradle tasks with gradle-node-plugin and run when the project's test task is run.
This only controls Babel config for tests, babel-asset-pipeline still uses Babel 5 and doesn't look at this file.