ES6 is fully available in Node 4. I was wondering whether it includes a concept of interface to define method contracts as in MyClass implements MyInterface
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Given that ECMA is a 'class-free' language, implementing classical composition doesn't - in my eyes - make a lot of sense. The danger is that, in so doing, you are effectively attempting to re-engineer the language (and, if one feels strongly about that, there are excellent holistic solutions such as the aforementioned TypeScript that mitigate reinventing the wheel)
Now that isn't to say that composition is out of the question however in Plain Old JS. I researched this at length some time ago. The strongest candidate I have seen for handling composition within the object prototypal paradigm is stampit, which I now use across a wide range of projects. And, importantly, it adheres to a well articulated specification.
more information on stamps here