intersection

Returns the elements that exist in both arrays, filtering duplicate values.

• Create a Set from b, then use Array.prototype.filter() on a to only keep values contained in b.

const intersection = (a, b) => {
  const s = new Set(b);
  return [...new Set(a)].filter(x => s.has(x));
};

intersection([1, 2, 3], [4, 3, 2]); // [2, 3]