Set difference keeps the elements in one set that are absent from another. It’s handy when you want the ninjas who aren’t also samurai. section 9.3.4
Listing 9.23. Difference of sets
const ninjas = new Set(["Kuma", "Hattori", "Yagyu"]);
const samurai = new Set(["Hattori", "Oda", "Tomoe"]);
const pureNinjas = new Set(
[...ninjas].filter(ninja => !samurai.has(ninja)) #A
);
assert(pureNinjas.size === 2, "There's only one ninja samurai");
assert(pureNinjas.has("Kuma"), "Kuma is a true ninja");
assert(pureNinjas.has("Yagyu"), "Yagyu is a true ninja");
Spread the first set into an array, filter out elements found in the second set with has, then wrap the survivors in a new Set. section 9.3.4
