Sets: Union, Intersection and Difference

sets.js

const union = (...sets) =>
  sets.reduce((acc, set) => {
    set.forEach(val => acc.add(val));
    return acc;
  }, new Set);

const intersection = (...sets) =>
  sets.reduce((acc, set) => {
    set.forEach(val => {
      if (sets.every(set => set.has(val)))
        acc.add(val);
    }); 
    return acc;
  }, new Set);

const difference = (...sets) =>
  sets.reduce((acc, set) => {
    const others = sets.filter(other => set != other);
    set.forEach(val => {
      if (others.every(set => !set.has(val)))
        acc.add(val);
    }); 
    return acc;
  }, new Set);