Calculate the union of two sets. It merges common elements using expression: { ...mapA[b.key], ...b }

union.js

/* it is either in A or B */
function union(arrA, arrB) {
  const mapA = arrA.reduce((res, a) => ({ ...res, [a.key]:a }), {});
  const inter = arrB.reduce((res, b) => (b.key in mapA) ? [ ...res, { ...mapA[b.key], ...b } ] : res, []);
  const mapInter = inter.reduce((res, i) => ({ ...res, [i.key]:i }), {});
  const diffA = arrA.reduce((res, a) => !(a.key in mapInter) ? [ ...res, a ] : res, []);
  const diffB = arrB.reduce((res, b) => !(b.key in mapInter) ? [ ...res, b ] : res, []);
  return [ ...diffA, ...inter, ...diffB ];
}