The intersection of two sets. It merges objects in common.

intersection.js

/* it is in both A & B */
/* merge common objects using expression: { ...map[b.key], ...b } */
function intersection(arrA, arrB) {
  const mapA = arrA.reduce((res, a) => ({ ...res, [a.key]:a }), {});
  return arrB.reduce((res, b) => (b.key in mapA) ? [ ...res, { ...mapA[b.key], ...b } ] : res, []);
}