They don't seem to expose the List to be accesible from the instanced Object. This is from the EcmaScript Draft:
23.2.4 Properties of Set Instances
Set instances are ordinary objects that inherit properties from the Set prototype. Set instances also have a [[SetData]] internal slot.
[[SetData]] is the list of Values the Set is holding.
A possible solution (an a somewhat expensive one) is to grab an iterator and then call next() for the first value:
var x = new Set();
x.add(1);
x.add({ a: 2 });
//get iterator:
var it = x.values();
//get first entry:
var first = it.next();
//get value out of the iterator entry:
var value = first.value;
console.log(value); //1
Worth mention too that:
Set.prototype.values === Set.prototype.keys
