Why?

• There are many improvements in es6 which makes it a lot easier to write javascript
• The module system makes it easier to write javascript unit tests.
• All modern js libraries and frameworks switched to es6 (angular, vue, react… and many more). We should at least try to keep up with the js world.
• We already use es6 features in some parts of the project: i.e.: tests, media manager, custom elements.
• Browser Support for es6 is good: https://kangax.github.io/compat-table/es6/ Essentially all browsers except IE11. To support IE11 we need to compile es6 code down to es5. This means there will be no change for the end user.

More information about es6: https://github.com/lukehoban/es6features

How?

• Update the package.json and install dependencies needed to compile ES6 down to ES5. Add/Update scripts to compile es6 to es5: #19650
• Adopt airbnb es6 javascript style guide as javascript coding standard: #19638
• Fix code style errors in build scripts #19717
• Rename all Javascript files to *.es6.js and compile them: https://github.com/joomla-projects/joomla-es6/issues
• check all dependencies in package.json (are the versions up 2 date? Are they still required?)
• Re/Organize joomla namespace
• Refactor libraries, so that they are unit testable.
• Drop javascript test dependencies: jasmine-jquery, requirejs, jasmine-ajax… and reduce the usage of fixture testing (prefer real unit tests, probably with a mocking lib like sinon).
• Refactor the unit tests.
• optional: implement module loading
• Remove all minified scripts from repository. #19637
• add .eslint.es5 and lint old es5 code. Personally i think we should focus on moving things to es6 and enforce cs during refactoring instead of spending time with this.