Set seems like a nice way to create Arrays with guaranteed unique elements, but it does not expose any good way to get properties, except for generator [Set
Perhaps to late to the party, but you could just do the following:
const set = new Set(['a', 'b']);
const values = set.values();
const array = Array.from(values);
This should work without problems in browsers that have support for ES6 or if you have a shim that correctly polyfills the above functionality.
Edit: Today you can just use what @c69 suggests:
const set = new Set(['a', 'b']);
const array = [...set]; // or Array.from(set)