I was about to publish a module to NPM, when I thought about rewriting it in ES6, to both future-proof it, and learn ES6. I\'ve used Babel to transpile to ES5, and run tests
The pattern I have seen so far is to keep the es6 files in a src directory and build your stuff in npm's prepublish to the lib directory.
src
lib
You will need an .npmignore file, similar to .gitignore but ignoring src instead of lib.