30-seconds-of-code/snippets/symmetricDifferenceBy.md

title, tags, expertise, cover, firstSeen, lastUpdated

title	tags	expertise	cover	firstSeen	lastUpdated
Mapped array symmetric difference	array	intermediate	blog_images/river-houses.jpg	2018-01-24T11:59:02+02:00	2020-10-22T20:24:30+03:00

Returns the symmetric difference between two arrays, after applying the provided function to each array element of both.

• Create a Set from each array to get the unique values of each one after applying fn to them.
• Use Array.prototype.filter() on each of them to only keep values not contained in the other.

const symmetricDifferenceBy = (a, b, fn) => {
  const sA = new Set(a.map(v => fn(v))),
    sB = new Set(b.map(v => fn(v)));
  return [...a.filter(x => !sB.has(fn(x))), ...b.filter(x => !sA.has(fn(x)))];
};

symmetricDifferenceBy([2.1, 1.2], [2.3, 3.4], Math.floor); // [ 1.2, 3.4 ]
symmetricDifferenceBy(
  [{ id: 1 }, { id: 2 }, { id: 3 }],
  [{ id: 1 }, { id: 2 }, { id: 4 }],
  i => i.id
);
// [{ id: 3 }, { id: 4 }]