While UMD gives you a way to output a single module that works in both AMD and CommonJS, SystemJS will allow you to use ES6 modules closer to their native semantics without requiring an ES6-compatible browser engine.
SystemJS was introduced by Angular 2.0, which is a popular web application development framework.
Refer to the official SystemJS project website at https://github.com/systemjs/systemjs to learn more about SystemJS.
There is a free list of common module mistakes available online at http://www.typescriptlang.org/Handbook#modules-pitfalls-of-modules.
