const people = ["chris", "chris", "cindy"];
const peopleSet = new Set(people);
peopleSet.add("chris");
console.log(peopleSet);
// {"chris","cindy"}

Now they are even more useful with recently universally available methods, as Brian Smith summarizes:

• intersection() returns a new set with elements in both this set and the given set.
• union() returns a new set with all elements in this set and the given set.
• difference() returns a new set with elements in this set but not in the given set.
• symmetricDifference() returns a new set with elements in either set, but not in both.
• isSubsetOf() returns a boolean indicating if all elements of a set are in a specific set.
• isSupersetOf() returns a boolean indicating if all elements of a set are in a specific set.
• isDisjointFrom() returns a boolean indicating if this set has no elements in common with a specific set.